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Conditions Involving Tuples

A tuple in SQL is represented by a parenthesized list of scalar values. Examples are (123, foo) and (name, address, networth). The first of these has constants as components; the second has attributes as components. Mixing of constants and attributes is permitted.

Subqueries

In SQL, one query can be used in many ways to help in the evaluation of another. A query that is part of another is called a subquery. Subqueries can have subqueries, and so on, down as many levels as we desire. We already saw one example of the use of subqueries; in

Union, Intersection, and Difference of Queries

Sometimes we wish to combine relations using the set operations of relational algebra: union, intersection and difference. SQL provides corresponding operators that apply to the results of queries, provided those queries produce relations with the same list of attributes and

Interpreting Multirelation Queries

There are many ways to define the meaning of the select-from-where expressions that we have just studied. All are equivalent, in the sense that they each give the same answer for each query applied to the same relation instances. We shall examine each in turn.

Tuple Variables

Disambiguating attributes by prefixing the relation name works as long as the query involves combining many different relations. On the other hand, sometimes we need to ask a query that involves two or more tuples from the same relation,

Disambiguating Attributes

Sometimes we ask a query involving many relations, and among these relations are two or more attributes with the same name. If so, we need a way to indicate which of these attributes is meant by a use of their shared name. SQL solves this problem by allowing us to place a

Queries Involving More Than One Relation

Much of the power of relational algebra comes from its ability to combine two or more relations through joins, products, unions, intersections, and differences. We get all of these operations in SQL. The set-theoretic operations - union, intersection, and difference - appear directly in

The Truth-Value UNKNOWN

In Selection in SQL we assumed that the result of a comparison was either TRUE or FALSE, and these truth-values were combined in the obvious way using the logical operators AND, OR and NOT. We have just seen that when NULL values occur, comparisons can yield a third

Null Values and Comparisons Involving NULL

SQL allows attributes to have a special value NULL, which is called the null value. There are many different interpretations that can be put on null values. Here are some of the most common:

Dates and Times

Implementations of SQL normally support dates and times as special data types. These values are often representable in a variety of formats such as 5/14/1948 or 14 May 1948. Here we shall explain only the SQL standard notation, which is very specific about format.