Sub Queries

A sub-query is a nested SELECT statement inside of another parent SELECT statement.

A usage of this is to select a record set from which you then want to manipulate and operate on after that initial pool is generated:

SELECT P.Age
FROM (
	SELECT P1.*
	FROM PERSON P1
	WHERE P1.Name = 'Paul'
)

In this example the first operation performed is the gathering of all records from the P1 table where the Name field contains the string value Paul. Then from that result set we are selecting the Age field from that initial result set.

With sub-queries:

A Required SELECT statement list A Required FROM clause An Optional WHERE clause An Optional GROUP BY clause An Optional HAVING clause But NO ORDER BY clause this will throw an error

Sub queries can be used anywhere an expression is allowed. This includes the SELECT portion of a query, the FROM clause, the WHERE clause, JOIN's, GROUP BY's.