Questions and answers taken from real job interviews.
Find interview questions and answers on this website:
An outer join includes the records that match and those that do not have a matching value in another table. Outer joins can be a LEFT outer join (includes all records from the first table listed) or a RIGHT outer join (includes all records from the second table listed). Outer joins are not easily used with more than two tables.
Type 1 drivers provide a bridge between Java and ODBC. Types 2-4 drivers are written entirely in Java, but differ as to how they connect to the DBMS. Type 2 drivers rely on the DBMS product for intermachine communication, if any. Type 3 drivers translate JDBC calls into a DBMS-independent network protocol. Type 4 drivers translate JDBC calls into a DBMS-dependent network protocol.
The enterprise modeling that analyzes the current data processing. Conceptual data modeling that identifies entities, relationships, and attributes. The logical database design that identifies data integrity and security requirements. The physical database design and definition that defines the database to a DBMS. The database implementation that installs and converts data from prior systems. Database maintenance that fixes errors in the database and database applications.
Data definition language commands are used to create, alter, and drop tables. Data manipulation commands are used to insert, modify, update, and query data in the database. Data control language commands help the DBA to control the database.
An ERD is a detailed logical representation of the data for an organization. The ERD includes entities, attributes, relationships, and cardinalities. An ERD is the mechanism where an entity-relationship model is displayed.
Processes that manipulate the data. Networks that transport the data. People who process and use the data. Events and point in time when processes need to be performed with the data. Reasons for events and rules to govern the processing of the data.
The hierarchical model is a top-down structure where each parent may have many children but each child can have only one parent. This model supports one-to-one and one-to-many relationships. The network model can be much more flexible than the hierarchical model since each parent can have multiple children but each child can also have multiple parents. This model supports one-to-one, one-to-many, and many-to-many relationships.
A fat client is a PC that is responsible where most processing occurs on the client rather than the server. Some of the fat client activities include processing, presentation logic, and business rules logic. A thin client is a PC where only limited processing occurs. In these cases, more processing should occur on the application server.
The ideal primary key is short, numeric and fixed. A surrogate key is a unique, DBMS-supplied identifier intended to be used as the primary key of a table. Further, the DBMS will not allow the value of a surrogate key to be changed. The values of a surrogate key have no meaning to the users and are usually hidden on forms and reports. By design, they are short, numeric and fixed and thus meet the definition of the ideal primary key
Data constraints on a column are the limits put on the values the data can have. There are four types of data constraints: (1) domain constraints, which define a limited set of values for the column, (2) range constraints, which specify that the values must fall within a certain range, (3) intrarelation constraints, which define what values the column can have based on values of other columns in the same table, and (4) interrelation constraints, which define values the column can have based on values of columns in other tables.
A data warehouse is organized around specific subjects. The data is defined using consistent naming, format, and encoding structure standards. The data contains a time element, so that the data can be studied for trends. No data in a data warehouse can be updated by end users.
A subquery is a query that is composed of two queries. The first query (inner query) is within the WHERE clause of the other query (outer query). In some cases the inner query provides results for the outer query to process. In other cases, the outer query results provide results for the inner query (correlated subquery).
Triggers are stored and controlled in the DBMS. A trigger is executed automatically when a condition is met (INSERT, UPDATE, or DELETE). A procedure is also stored in a database. A procedure is not executed automatically.
QBE is a direct-manipulation database language that uses a graphical approach to query construction. Some database systems translate QBE queries into SQL. QBE does not adhere to a standard but SQL does. Both SQL and QBE are relational database languages.
Concurrency control is the process managing simultaneous operations against a database so that database integrity is not compromised. There are two approaches to concurrency control. The pessimistic approach involves locking and the optimistic approach involves versioning.
Domain constraints include entity integrity and referential integrity. The domain is a set of values that may be assigned to an attribute. The entity integrity rule states that no part of a primary key cannot be null. Referential integrity states that each foreign key value must match a primary key value or be null.
A separate relation is created for each supertype and subtype. The attributes common for all of the subtypes are assigned to the supertype. Each subtype has the primary key from the supertype assigned to it. A subtype discriminator is added to the supertype.
Both ASP and ColdFusion are middleware that ease the connection of databases to Web applications. Both require the use of several files. Both use tags and are executed on the server. Both Internet Explorer and Netscape can access these files. The programmer does not need to be concerned with the client when they work with this middleware.
ODBC is a standard that contains an interface that provides a common language for application programs to access and process SQL databases. In order to use ODBC, a driver, server name, database name, user id, and password are required. ODBC is important for Internet applications and has gained wide acceptance.
You can perform more complex functions and error handling can be accommodated by VBA. VBA code will execute faster since code runs faster than macros and maintenance is easier because VBA modules are stored with the forms and reports. Reading VBA is easier because you can see the entire code. Finally, you can pass parameters and use OLE automation better.
Data administration is responsible for the overall management of data resources. Some of the core roles include the creation of data polices, procedures and standards, resolve data ownership issues, and manage the information repository. Database administration is physical database oversight. Some of the core duties include the selection of the DBMS and software tools, the installation and upgrade of the DBMS, and database performance tuning.
Embedded SQL is the process of including hard coded SQL statements. These statements do not change unless the source code is modified. Dynamic SQL is the process of generating SQL on the fly. The statements generated do not have to be the same each time.
A homogenous database is one that uses the same DBMS at each node. Either each node can work independently or a central DBMS may coordinate database activities. A heterogeneous database is one that may have a different DBMS at each node. It may support some or all of the functionality of one logical database. It may support full Distributed DBMS functionality or partial Distributed DBMS functionality.
A data warehouse is for the enterprise and contains multiple subjects. A data mart is for a specific functional area and focuses on one subject. A data warehouse is flexible and data-oriented and contains detailed historical data. A data mart is restrictive and project-oriented and contains some historical data.
Horizontal portioning is where the rows in a relation are separated by some criteria and placed into a new relation or file with the same layout as the original relation (in this case only the records in each file differ). Vertical portioning is where the columns in a relation are separated by some criteria and placed into a new relation or file with a different layout as the original relation.
A unique primary index is unique and is used to find and store a row. A nonunique primary index is not unique and is used to find a row but also where to store a row (based on its unique primary index). A unique secondary index is unique for each row and used to find table rows. A nonunique secondary index is not unique and used to find table rows.
Relationships are verb phrases and represent actions usually in the present tense. The name should exclude vague names such as "has". The name should explain what action is being taken and possibly why it is important.
Web Services are improving the ability of computers to communicate over the Internet. These services use XML programs and usually run in the background. Easier integration of applications may be possible because developers do not need to be familiar with the technical details with applications that are being integrated. UDDI is a technical specification for creating a distributed registry of Web services and businesses that are open to communicating through Web services.
Website security issues include unauthorized access to the several aspects of one's Website. Security measures should include all aspects of the system such as the network, operating level, database, and Web server. Regular monitoring and security testing by a company should help to avoid intrusion into one's system.
Operational systems are used to run the business in real time on a daily basis and contain current data. Non-managers and non-analysts work in this type of system. Operational systems must process large volumes of data. Informational systems are used to support decisions and contain historical data. Managers and analysts work in this type of system. Informational systems have periodic batch updates.
Choosing primary and secondary keys can increase the speed of row selection, joining, and row ordering. Selecting the appropriate file organization for base tables and indexes can also improve performance. Clustering related rows together and maintaining statistics about tables and indexes can lead to increased efficiency.
Relations are also called tables, and sometimes by the older data processing term files. A row is known as a tuple in the relational model, but may also be referred to as a record. Finally, relational model attributes are known as table columns and sometimes as fields.
A join allows tables to be linked to other tables when a relationship between the tables exists. The relationships are established by using a common column in the tables and often uses the primary/foreign key relationship.
Concurrency transparency is where each transaction in a distributed database is treated as if it is the only one in the system. Therefore if several transactions are running at one time, the results will be the same as if each transaction was run in serial order. The transaction manager helps to provide concurrency control. The three methods that may be used are locking, versioning, and timestamping.
A scenario is used to test business rules. It is a short script that describes how a business reacts to certain situations.
First, add the column as NULL. Then use UPDATE to add data to every row. Finally use an ALTER TABLE . . . ALTER COLUMN statement to change the column constraint to NOT NULL.
The installation of the database is an important issue since the better the installation, the better the performance could be. Memory usage is learning how the DBMS uses main memory and then using that knowledge to enable better performance. I/O is usually very intense in a DB, so understanding how users will use the data will help to prepare the database better. CPU usage and application tuning are also important considerations.
Locking is denying others the ability to update a record until someone completes the update or releases the record. Locking can occur on many different levels in a database. It can occur at the database, table, record, or field level. A lock can be shared (another can read the record while an update is in progress) or exclusive (no one can read the record while an update is in progress).
Minimum cardinality is the minimum number of instances of an entity that can be associated with each instance of another entity. Maximum cardinality is the maximum number of instances of an entity that can be associated with each instance of another entity.
Snapshot replication can be used when an application does not require that the data always be current. These applications can be updated by periodic table copying or snapshots. As part of the snapshot effort, all of the data to be included in the snapshot is collected at a primary point. Then a read-only snapshot is taken and the snapshot is sent to each site so that the update can be made.
Normalization should minimize data redundancy. It should also simplify referential integrity constraints. Normalization will also make it easier to insert, update, and delete data. And finally, it provides better design.
SQL statements return a set of rows, while an application program works on one row at a time. To resolve this mismatch the results of SQL statements are processed as pseudofiles, using a cursor or pointer to specify which row is being processed.
A dynamic view may be created every time that a specific view is requested by a user. A materialized view is created and or updated infrequently and it must be synchronized with its associated base table(s).
An EER includes everything in an ERD and an EER allows for more complex relationships than an ERD. An EER allows for object-oriented data modeling and include supertypes and subtypes entities and inheritance.
An exclusive lock prohibits other users from reading the locked resource; a shared lock allows other users to read the locked resource, but they cannot update it.
Entities have attributes. Attributes are properties that describe the entity's characteristics. Entity instances have identifiers. Identifiers are attributes that name, or identify, entity instances.
Optimistic locking assumes no transaction conflict will occur and deals with the consequences if it does. Pessimistic locking assumes that conflict will occur and so prevents it ahead of time with locks. In general, optimistic locking is preferred for the Internet and for many intranet applications.
The disjoint rule states an entity instance of a supertype can only be a member of one subtype. The overlap rule states an entity instance of a supertype can be a member of multiple subtypes.
With the simple recovery model, no logging is done. The only way to recover a database is to restore the database to the last backup. With full recovery, all database changes are logged. With bulk-logged database recovery, all changes are logged except those that cause large log entries.
Total specialization exists when every instance of a supertype must also be an instance of a subtype. Partial specialization exists when every instance of a supertype does not have to be an instance of a subtype.
A three-tier architecture includes a client and two server layers. The application code is stored on the application server and the database is stored on the database server. A two-tier architecture includes a client and one server layer. The database is stored on the database server.
An Internet database is accessible by everyone who has access to a Web site. An intranet database limits access to only people within a given organization. An extranet database limits access to only people within a company and a company's customers and suppliers.
Universal data models are common to many organizations. These models may be useful for similar functions that are used across companies or organizations such as purchasing and accounting. Industry-specific data models are used by specific industries.
The strictest isolation level is SERIALIZABLE. With it, SQL Server places a range lock on the rows that have been read. This level is the most expensive to use and should only be used when absolutely required. The next most restrictive level is REPEATABLE READ, which means SQL Server places and holds locks on all rows that are read. It is possible to make dirty reads by setting the isolation level to READ UNCOMMITTED, which is the least restrictive level. READ COMMITTED is the default isolation level.
An entity is something that can be identified in the users' work environment, something that the users want to track. Entities of a given type are grouped into entity classes. An entity instance is the representation of a particular entity.
Each of the three layers has a metadata layer linked with it. The metadata layer describes the properties or characteristics of the data. The operational metadata describe the data used in the various operational and external systems. The enterprise data warehouse metadata describe the reconciled data layer. The data mart metadata describes the derived data layer.
This is the traditional method to develop, maintain, and replace information systems that includes the project identification and selection, project initiation and planning, analysis, logical design, physical design, implementation, and maintenance steps. The process is a series of steps that are ideally completed in a linear fashion. In reality, the process is not linear and the process requires steps to be revisited and an overlap of steps.
Oracle supports read committed, serializable, and read-only transaction isolation levels. Because of the way Oracle System Change Number (SCN) values are processed, Oracle never reads dirty data. Serializable isolation is possible, but the application program must be written to process the "Cannot serialize" exception. Applications can place locks explicitly using SELECT FOR UPDATE commands but this is not recommended.
First you must verify that a relation is in both first normal form and second normal form. If the relation is not, you must convert into second normal form. After a relation is in second normal form, you must remove all transitive dependencies.
An ACID transaction is one that is atomic, consistent, isolated, and durable. Durable means that database changes are permanent. Consistency can mean either statement level or transaction level consistency. With transaction level consistency, a transaction may not see its own changes. There are four transaction isolation levels: read committed, read uncommitted, repeatable read and serialization. Atomic means it is performed as a unit.
One reason that an information gap exists is the fact that systems have been developed in separate, segmented efforts. This has helped the data from being stored in an integrated database and thus the data is in an inconsistent structure. The other reason for the gap is that most systems are created to support the operational aspect of an organization. The systems were not developed for decision making.
MySQL uses integer data types combined with the property AUTO_INCREMENT to create surrogate keys. This creates a sequence that starts at one (1) and increases by one (1) for each new record. MySql maintain its metadata in a database named mysql. For example, this database maintains two tables named user and db.
A recursive relationship is a relationship among entities of the same class, and is represented in the same way as other relationships are. The rows of the tables can take two different roles, however. Some are parent rows, and others are child rows. Further, the table will contain both its own primary key and the foreign key that links back to the table itself. If a row has no parent, then the value of the foreign key column in that row will be null. If the row has a parent, then there must be a foreign key value in that row that corresponds to the primary key value of another row in the table.
An OLE DB interface is specified by a set of objects, and the properties and methods that they expose, and OLE DB defines standardized interfaces. An object need not expose all of its properties and methods in a given interface. An OLE DB implementation defines how the object supports the interface. The implementation is completely hidden from the user. Thus developers of an object are free to change the implementation whenever they want, but they should not change the interface without consulting their users.
Definitions are gathered from the same sources and should be accompanied diagrams. A definition will include special conditions, examples, how the data is created, whether the data can change, who owns the data, whether the data is optional, and whether the data can be broken into something more atomic.
Relations in a database have a unique name and no multivalued attributes exist. Each row is unique and each attribute within a relation has a unique name. The sequence of both columns and rows is irrelevant.
A derivation is a statement that is derived from other knowledge. A structured assertion is a statement that expresses some aspect of the static structure of an organization. An action assertion is a statement of a constraint on the actions of an organization.
COUNT: computes the number of rows in a table. SUM: totals numeric columns. AVG: computes the average value. MAX: obtains the maximum value of a column in a table. MIN: obtains the minimum value of a column in a table.
1:1 - a single entity instance of one type is related to a single-entity instance of another type. 1:N - a single entity instance of one type is related to many-entity instances of another type. M:N - many-entity instances of one type relate to many-entity instances of another type.
(1) providing default values, (2) enforcing data constraints, (3) updating views and (4) enforcing referential integrity
XML is used to structure and manipulate data involved with a browser and is becoming the standard for e-commerce. XML uses tags that are similar to HTML in that they use the angle brackets, but XML describes the content whereas HTML describes the appearance. The XML schema standard was published in May 2001 by W3C.
Databases can support from a single user (personal database) up to supporting the requests of the world (internet database). In between, a database can support a workgroup (a relatively small group of people), department database (a functional unit in an organization such as marketing), or an enterprise database (entire organization).
Indexes can be created to enforce uniqueness, to facilitate sorting, and to enable fast retrieval by column values. A good candidate for an index is a column that is frequently used with equal conditions in WHERE clauses.
Data is recorded facts and figures, and information is knowledge derived from data. A database stores data in such a way that information can be created.
A deletion anomaly occurs when, by deleting the facts about one entity, we inadvertently delete facts about another entity; with one deletion, we lose facts about two entities. For example, if we delete the tuple for Student 001289 from a table, we may lose not only the fact that Student 001289 is in Pierce Hall, but also the fact that he has $200 left in his security deposit. An insertion anomaly happens when we encounter the restriction that we cannot insert a fact about one entity until we have an additional fact about another entity. For example, we want to store the fact that the security deposit for Pierce Hall is $300, but we cannot enter this data into the Student relation until a student registers for Pierce Hall.
A sequence is an object that generates a sequential series of unique numbers. Sequences are most often used to provide values for surrogate keys. However, there are three problems with using sequences. First, a developer can use a defined sequence for any purpose; and if a sequence is used for purposes other than the surrogate key, some values will be missing. A second problem is that there is nothing in the schema that prevents someone from issuing an INSERT statement that adds data to the table but that does not use the sequence. Finally, it is possible to use the wrong sequence when putting data into a table.
A standard language can hinder the effort to create a new language. One standard is never enough to meet all of the business needs. A standard can be a compromise among interested parties which can cause the standard to not be ideal. If a standard is altered by some, than portability between platforms could be hurt.
One of the features includes the use of views which allows the presentation of only data needed by someone and limits the capability of database updates. The use of integrity controls includes such things as domains, assertions, and checks. Also authorization rules, user-defined procedures, encryption, authentication schemes, and backups are important.
A stored procedure is a program that is stored within the database and is compiled when used. They can receive input parameters and they can return results. Unlike triggers, their scope is database-wide; they can be used by any process that has permission to use the database stored procedure.
The advantages of stored procedures are (1) greater security, (2) decreased network traffic, (3) the fact that SQL can be optimized and (4) code sharing which leads to less work, standardized processing, and specialization among developers.
A data type should be selected so that all possible values are represented using minimal storage space. The data type should help to ensure data integrity and support all possible data manipulations (i.e., cannot place a letter in a field such as salary where a number is required).
Managing database structure, controlling concurrent processing, managing processing rights and responsibilities, developing database security, providing for database recovery, managing the DBMS and maintaining the data repository.
The steps are: (1) specify the primary key, (2) specify candidate keys, (3) specify column properties including null status, data type, default value (if any), and data constraints (if any), and (4) verifying normalization.
1. Identify the data type, length, and precision for each attribute. 2. Identify the columns that can accept a null value. 3. Identify the columns that need to be unique. 4. Identify primary and related foreign keys with the parent table being created before the child. 5. Determine default values. 6. Determine where the domain values are that need to be constrained. 7. Create the indexes.
An ODBC file data source is a file that can be shared among database users. A ODBC system data source is one that is local to a single computer. A ODBC user data source is only available to the user who created it.
DTD (Document Type Declarations) and XML Schemas. An XML document that conforms to its DTD is called type-valid. A document can be well-formed and not be type-valid, either because it violates the structure of its DTD or because it has no DTD. However, DTDs have limitations and to overcome these limits XML Schemas were created. XML Schemas are XML documents that are the preferred method for defining document structure.
Datafiles, control files and two types of ReDo log files: OnLine Redo and Offline ReDo (which is also known as Archive ReDo).
There are three (3) ways to create an Oracle database. You can create a database using the Database Configuration Assistant, using the Oracle-supplied database creation procedures or using the SQL CREATE DATABASE command.
The major goals of OLE DB are to: (1) Create object interfaces for DBMS functionality pieces; (2) Increase flexibility for developers and users; (3) provide an object interface over any type of data; and (4) do not force data to be converted or moved from where it is.
By breaking the features and the functions of a DBMS into COM objects, OLE DB characteristic overcomes a major disadvantage of ODBC. With ODBC, a vendor must create an ODBC driver for almost all DBMS features and functions in order to participate in ODBC at all. This is a large task that requires a substantial initial investment. With OLE DB, however, a DBMS vendor can implement portions of their product.
A relation is in BCNF when every determinant in the relation is a candidate key. This means that any possible primary key can determine all other attributes in the relation. Attributes may not be determined by non-candidate key attributes or part of a composite candidate key. Thus it is said "I swear to construct my tables so that all nonkey columns are dependent on the key, the whole key and nothing but the key, so help me Codd!"
Referential integrity constraints require that foreign key values in one table correspond to primary key values in another. If the value of the primary key is changed -- that is, updated -- the value of the foreign key must immediately be changed to match it. Cascading updates will set this change to be done automatically by the DBMS whenever necessary.
A data mart is a collection of data smaller in scope and size than a data warehouse. It is dedicated to data from a particular business component of business functional area. A data mart may function as a subset of a larger data warehouse. Users of a data mart are usually knowledgeable analysts in the business area using the data mart.
A dataset is an in-memory database that is disconnected from any regular database, but has all the important characteristics of a regular database. Datasets can have multiple tables, relationships, referential integrity rules, referential integrity actions, views and the equivalent of triggers. Datasets are needed to provide a standardized, non-proprietary means to process database views. They are especially important for the processing of views with multiple multivalued paths.
DBMS stands for Database Management System. A DBMS receives requests from applications and translates those requests into actions on a specific database. A DBMS processes SQL statements or uses other functionality to create, process and administer databases.
A dependency graph is a diagram that is used to portray the connections between database elements. For example, a change in a table can potentially impact relationships, views, indexes, triggers, stored procedures, and application programs. These impacts need to be known and accounted for before making database changes.
A distributed database is a single logical database that is spread across more than one node or locations that are all connected via some communication link. It requires multiple DBMSs, running at each remote site. A distributed database can be either homogenous (same DBMS at each node) or heterogeneous (different DBMS at some nodes).
A foreign key is used to establish relationships among relations in the relational model. Technically, a foreign key is a column (or columns) appearing in one relation that is (are) the primary key of another table. Although there may be exceptions, the values in the foreign key columns usually must correspond to values existing in the set of primary key values. This correspondence requirement is created in a database using a referential integrity constraint on the foreign key.
A Java bean is a Java class that has the following three properties. (1) There are no public instance variables. (2) All persistent values are accessed using methods named getxxx and setxxx. (3) The bean class must either have no constructors or it must have one explicitly defined zero-argument constructor.
A recursive relationship is a relationship between an entity and itself. For example, given the entity PERSON, a recursive relationship could be used to show a PERSON and his or her SIBLINGs (brothers and sisters).
A SQL view is a virtual table built from other tables or views. Views are used to (1) hide columns or rows, (2) show the results of computed columns, (3) hide complicated SQL syntax, (4) layer built-in functions, (5) provide a level of indirection between application programs and tables, (6) assign different sets of processing permissions to tables, and (7) to assign different sets of triggers to the same table.
ADO.NET is a new, improved, and greatly expanded version of ADO that was developed for the Microsoft .NET initiative. ADO.NET incorporates all of the functionality of ADO, but adds much more. In particular, ADO.NET facilitates the transformation of XML documents to and from database data. Most importantly, ADO.NET introduces the concept of datasets, which are in-memory, full-function, independent databases.
An SQL subquery is a means of querying two or more tables at the same time. The subquery itself is an SQL SELECT statement contained within the WHERE clause of another SQL SELECT statement, and separated by being enclosed in parenthesis. Some subqueries have equivalent SQL join structures, but correlated subqueries cannot be duplicated by a join..
Deadlock occurs when two transactions are each waiting on a resource that the other transaction holds. Deadlock can be prevented by requiring transactions to acquire all locks at the same time; once it occurs, the only way to cure it is to abort one of the transactions and back out of partially completed work.
Denormalization is the process of taking normalized relations and changing them so that they are not longer normalized. This process may lead to anomalies and create data redundancy as negative consequences. However, the revised relations should improve database performance.
Enterprise Resource Planning (ERP) is an information system used in manufacturing companies and includes sales, inventory, production planning, purchasing and other business functions. An ERP system typically uses a multiuser database.
RFM analysis is a Business Intelligence (BI) reporting system that analyzes and ranks customers based on their purchasing patterns. R refers to "how recently" a customer placed an order, F refers to "how frequently" the customer orders, and M refers to "how much money" the customer spends. Typically, the customers are ranked into "20%" groups and assigned a number to represent their ranking. Thus 1 means top 20%, 2 the next 20% and so on. In this system a score of 1 is best and a score of 5 is worst. Thus a customer with an RFM score = 1 5 1 would be one who has ordered recently, does not order frequently, and who makes large purchases.
SQL stands for Structured Query Language, and is the most important data processing language in use today. It is not a complete programming language like Java or C#, but a data sublanguage used for creating and processing database data and metadata. All DBMS products today use SQL.
The archetype/instance pattern occurs when one entity tracks occurrences of another entity. A common example is the relationship between CLASS and SECTION, where the actual occurrence of a class happens when students register for a SECTION of that CLASS. The archetype/instance pattern is implemented as an ID-dependent relationship.
1. Load the driver. 2. Establish a connection to the database. 3. Create a statement. 4. Do something with the statement.
A correlated subquery appears deceptively similar to a regular subquery. The difference is that a regular subquery can be processed from the bottom up. In a regular subquery, results from the lowest query can be determined and used to evaluate the upper-level query. In contrast, in a correlated subquery, the processing is nested; that is, a row from an upper query statement is used in comparison with rows in a lower level query. The key distinction of a correlated subquery is that the lower-level select statements use columns from upper-level statements.
An applet is a compiled Java bytecode program that is transmitted to a browser via HTTP and is invoked using the HTTP protocol on the user's workstation. A servlet is a Java program that is invoked on the server to respond to HTTP requests.
Horizontal partitioning is where some rows of a table are placed into the base relations at one site and other rows are placed at another site. Vertical partitioning is where some columns of a table are placed into the base relations at one site and other columns are placed at another site but each all of these relations must share a common domain.
Simple elements have only one data value. ComplexType elements can have multiple elements nested within them. ComplexTypes may also have attributes. The elements contained in a complexType may be simple or other complexTypes. ComplexTypes may also define element sequences.
With a clustered index, the data are stored in the bottom level of the index and in the same order as that index. With a nonclustered index, the bottom level of an index does not contain data; it contains pointers to the data. For data retrieval, clustered indexes are faster than nonclustered indexes.
A complete backup makes a copy of the entire database. A differential backup makes a copy of the changes that have been made to the database since the last complete backup. A complete backup must be made before the first differential backup. Because differential backups are faster, they can be taken more frequently and the chance of data loss is reduced. Complete backups take longer but are slightly simpler to use for recovery.
The inconsistent values problem occurs when different users or data sources use slightly different forms of the same data value. One example is where automobiles are specified as "Ford, 2-door, Red" in one cell and "Red Ford 2-door" in another.
The multivalue, multicolumn problem occurs when a table is designed to include multiple columns that hold variations of one type of attribute data. One example is where boat owners have the names of their boats stored as BOAT_01, BOAT_02 and BOAT_03.
Developed first, the ODBC standard is for relational databases; while the OLE DB standard provides functionality for both relational and other databases. Finally, ADO was developed to provide easier access to OLE DB data for the non-object-oriented programmer.
SQL Server 2000 supports INSTEAD OF and AFTER triggers only. There is no SQL Server support for BEFORE triggers. A table may have one or more AFTER triggers for insert, update and delete actions; AFTER triggers may not be assigned to views. A view or table may have at most one INSTEAD OF trigger for each triggering action of insert, update or delete.
The team includes a system analyst who identifies the need for information services to meet opportunities of the business, database analysts who design the database, users who monitor that the system will meet their needs, programmers who write computer programs, database and data administrators who have responsibility for current and future databases and other technical experts.
While working at IBM, E.F. Codd created the relational database model. A paper he published in 1970 presented his ideas to the world at large. His work is the foundation for most of the DBMSs currently in use, and thus forms the basis for database systems as we know and use them today.
Equations deal with numerical relationships. A functional dependency deals with the existence of a determinant relationship between attributes, regardless of whether or not there is a numerical relationship between them. Thus, if we know that there is no hot water every Wednesday, No-Hot-Water is functionally dependent on Wednesday. So, if we know it is Wednesday, then we know we will have No-Hot-Water. This is a functional dependency, but not an equation.
Tables that are normalized contain data that has been distributed among the tables, but which may need to be recombined to answer queries from an application. To recombine the data, the programmer will have to use subqueries and/or joins. These SQL structures are more complex to write than a simple SELECT statement.
In addition to the users' data, a database contains a description of its own structure. This descriptive data is called "metadata."
Database redesign is necessary for two reasons. First, redesign is necessary both to fix mistakes made during the initial database design. Second, redesign is necessary to adapt the database to changes in system requirements. Such changes are common because information systems and organizations do not just influence each other they create each other. Thus, new information systems cause changes in systems requirements.
The values of data may change. A time stamp helps to ensure that the previous value of the data stays in the database after it has changed so that you can see the before and after values through time. Without a time stamp, you will most likely lose some of the history.
Both problems try to store multiple values on an attribute in a table. In the multivalue, multiplecolumn problem, the multiple values are stored in different columns. In the mutlivalued dependency problem the multiple values are stored in different rows. In both cases, the solution is the same: store the multiple values in a separate table.
XML is a better markup language than HTML, primarily because XML provides a clear separation between document structure, content, and materialization. Symbols cannot be used ambiguously with XML.
SELECT COUNT(*) AS NumStudents FROM STUDENT;
SELECT * FROM STUDENT WHERE Grade >= 90;
(1) Multivalued dependencies, (2) Functional dependencies, (3) Candidate keys, (4) Primary keys and (5) Foreign keys.
In the one-to-one relationship, there will be a constraint on EmpNumber as a foreign key in COMPUTER stating that EmpNumber must be unique. To convert the relationship to a one-to-many relationship, just drop this constraint.