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 resultset. 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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