C170 Data Management Application

AREA In DB2, a named section of permanent storage space that is reserved to store the database. associative entity See composite entity. bridge entity See composite entity. candidate key A minimal superkey; that is, a key that does not contain a subset of attributes that is itself a superkey. See key. closure A property of relational operators that permits the use of relational algebra operators on existing tables (relations) to produce new relations. composite entity 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 A multiple-attribute key. dependent An attribute whose value is determined by another attribute. determination 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 In relational algebra, an operator used to yield all rows from one table that are not found in another union-compatible table. DIVIDE 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 In data modeling, the construct used to organize and describe an attribute's set of possible values. entity integrity 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 A join operator that links tables based on an equality condition that compares specified columns of the tables. flags 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) 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 A condition in which an attribute is functionally dependent on a composite key but not on any subset of the key. functional dependence 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 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 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 See index. inner join 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 In relational algebra, an operator used to yield only the rows that are common to two union-compatible tables. JOIN 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 Columns that are used in the criteria of join operations. The join columns generally share similar values (have a compatible domain). key One or more attributes that determine other attributes. See also superkey, candidate key, primary key (PK), secondary key, and foreign key. key attribute The attributes that form a primary key. See also prime attribute. left outer join 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 In the relational model, a table that implements an M:M relationship. See also composite entity. natural join A relational operation that yields a new table composed of only the rows with common values in their common attribute(s).