What is sub-query? Explain properties of sub-query?

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 sub-query is a SELECT statement that is nested within another T-SQL statement. A sub-query SELECT statement if executed independently of the T-SQL statement, in which it is nested, will return a result-set. Meaning a sub-query SELECT statement can standalone and is not depended on the statement in which it is nested. A sub-query 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 Sub-query can also be used as a parameter to a function call. Basically a sub-query can be used anywhere an expression can be used.