Tuesday, July 8, 2008
Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be executed
arbitrarily within the body of another SQL statement. A sub-query is executed by enclosing it in a set of
parentheses. Sub-queries are generally used to return a single row as an atomic value, though they
may be used to compare values against multiple rows with the IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECT
statement if executed independently of the T-SQL statement, in which it is nested, will return a result
set. Meaning a subquery SELECT statement can standalone and is not depended on the statement in
which it is nested. A subquery SELECT statement can return any number of values, and can be found
in, the column list of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a
T-SQL statement. A Subquery can also be used as a parameter to a function call. Basically a subquery
can be used anywhere an expression can be used.
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