AREA ans: In DB2, a named section of permanent storage space that is reserved to store the database.
associative entity ans: See composite entity.
bridge entity ans: See composite entity.
candidate key ans: A mi
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AREA ans: In DB2, a named section of permanent storage space that is reserved to store the database.
associative entity ans: See composite entity.
bridge entity ans: See composite entity.
candidate key ans: A minimal superkey; that is, a key that does not contain a subset of attributes that is itself a superkey. See key.
closure ans: A property of relational operators that permits the use of relational algebra operators on existing tables (relations) to produce new relations.
composite entity ans: An entity designed to transform an M:N relationship into two 1:M relationships. The composite entity's primary key comprises at least the primary keys of the entities that it connects. Also known as a bridge entity or associative entity. See also linking table.
composite key ans: A multiple-attribute key.
dependent ans: An attribute whose value is determined by another attribute.
determination ans: The role of a key. In the context of a database table, the statement "A determines B" indicates that knowing the value of attribute A means that the value of attribute B can be looked up.
DIFFERENCE ans: In relational algebra, an operator used to yield all rows from one table that are not found in another union-compatible table.
DIVIDE ans: In relational algebra, an operator that answers queries about one set of data being associated with all values of data in another set of data.
domain ans: In data modeling, the construct used to organize and describe an attribute's set of possible values.
entity integrity ans: The property of a relational table that guarantees each entity has a unique value in a primary key and that the key has no null values.
equijoin ans: A join operator that links tables based on an equality condition that compares specified columns of the tables.
flags ans: Special codes implemented by designers to trigger a required response, alert end users to specified conditions, or encode values. Flags may be used to prevent nulls by bringing attention to the absence of a value in a table.
foreign key (FK) ans: An attribute or attributes in one table whose values must match the primary key in another table or whose values must be null. See key.
full functional dependence ans: A condition in which an attribute is functionally dependent on a composite key but not on any subset of the key.
functional dependence ans: Within a relation R, an attribute B is functionally dependent on an attribute A if and only if a given value of attribute A determines exactly one value of attribute B. The relationship "B is dependent on A" is equivalent to "A determines B," and is written as A->B.
homonyms ans: The use of the same name to label different attributes. Homonyms generally should be avoided. Some relational software automatically checks for homonyms and either alerts the user to their existence or automatically makes the appropriate adjustments. See also synonym.
index ans: An ordered array of key values and row ID values (pointers). Indexes are generally used to speed up and facilitate data retrieval. Also known as an index key.
index key ans: See index.
inner join ans: A join operation in which only rows that meet a given criterion are selected. The join criterion can be an equality condition (natural join or equijoin) or an inequality condition (theta join). The inner join is the most commonly used type of join. Contrast with outer join.
INTERSECT ans: In relational algebra, an operator used to yield only the rows that are common to two union-compatible tables.
JOIN ans: In relational algebra, a type of operator used to yield rows from two tables based on criteria. There are many types of joins, such as natural join, theta join, equijoin, and outer join.
join columns ans: Columns that are used in the criteria of join operations. The join columns generally share similar values (have a compatible domain).
key ans: One or more attributes that determine other attributes. See also superkey, candidate key, primary key (PK), secondary key, and foreign key.
key attribute ans: The attributes that form a primary key. See also prime attribute.
left outer join ans: In a pair of tables to be joined, a join that yields all the rows in the left table, including those that have no matching values in the other table. For example, a left outer join of CUSTOMER with AGENT will yield all of the CUSTOMER rows, including the ones that do not have a matching AGENT row. See also outer join and right outer join.
linking table ans: In the relational model, a table that implements an M:M relationship. See also composite entity.
natural join ans: A relational operation that yields a new table composed of only the rows with common values in their common attribute(s).
null ans: The absence of an attribute value. Note that a null is not a blank.
outer join ans: Returns a set of records that include what an inner join would return but also includes other rows for which no corresponding match is found in the other table. Contrast with inner join. See also left outer join and right outer join.
predicate logic ans: Used extensively in mathematics to provide a framework in which an assertion (statement of fact) can be verified as either true or false.
primary key (PK) ans: In the relational model, an identifier composed of one or more attributes that uniquely identifies a row. Also, a candidate key selected as a unique entity identifier. See also key.
prime attribute ans: A key attribute; that is, an attribute that is part of a key or is the whole key. See also key attributes.
PRODUCT ans: In relational algebra, an operator used to yield all possible pairs of rows from two tables. Also known as the Cartesian product.
PROJECT ans: In relational algebra, an operator used to select a subset of columns.
referential integrity ans: A condition by which a dependent table's foreign key must have either a null entry or a matching entry in the related table.
relational algebra ans: A set of mathematical principles that form the basis for manipulating relational table contents; the eight main functions are SELECT, PROJECT, JOIN, INTERSECT, UNION, DIFFERENCE, PRODUCT, and DIVIDE.
relvar ans: Short for relation variable, a variable that holds a relation. A relvar is a container (variable) for holding relation data, not the relation itself.
RESTRICT ans: See SELECT.
right outer join ans: In a pair of tables to be joined, a join that yields all of the rows in the right table, including the ones with no matching values in the other table. For example, a right outer join of CUSTOMER with AGENT will yield all of the AGENT rows, including the ones that do not have a matching CUSTOMER row. See also left outer join and outer join.
secondary key ans: A key used strictly for data retrieval purposes. For example, customers are not likely to know their customer number (primary key), but the combination of last name, first name, middle initial, and telephone number will probably match the appropriate table row. See also key.
SELECT ans: A SQL command that yields the values of all rows or a subset of rows in a table. The SELECT statement is used to retrieve data from tables. In relational algebra, an operator to select a subset of rows. Also known as RESTRICT.
set theory ans: A part of mathematical science that deals with sets, or groups of things, and is used as the basis for data manipulation in the relational model.
superkey ans: An attribute or attributes that uniquely identify each entity in a table. See key.
synonym ans: The use of different names to identify the same object, such as an entity, an attribute, or a relationship; synonyms should generally be avoided. See also homonym.
system catalog ans: A detailed system data dictionary that describes all objects in a database.
theta join ans: A join operator that links tables using an inequality comparison operator (<, >, <=, >=) in the join condition.
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