Refresher -CodeIdol

A subquery is a SELECT statement with another SELECT inside it. It looks like this:


SELECT ...         /* outer or parent query */

  (SELECT ...)     /* subquery, inner query or subselect */

...

There are three possible plans to process a subquery:

When processing is out-to-in, the outer query is the driver. When processing is in-to-out, the inner query is the driver.

Figure shows the SQL Standard requirements and the level of support the Big Eight have for subqueries.

Notes on Figure:

Basic Support column
This column is "Yes" if the DBMS fully supports the [NOT] IN, [NOT] EXISTS, <comparison operator> ANY, and <comparison operator> ALL predicates, and correctly handles subquery situations where the number of rows is zero.

ANSI/DBMS Subquery Support
	Basic Support	Row Subquery	Table Subquery	Max Depth	Allow UNION	Types Converted
ANSI SQL	Yes	Yes	Yes	N/S	Yes	Yes
IBM	Yes	Yes	Yes	22	Yes	Yes
Informix	Yes	No	No	23	No	Yes
Ingres	Yes	No	No	11	No	Yes
InterBase	Yes	No	No	>=32	No	Yes
Microsoft	Yes	No	Yes	>=32	Yes	Yes
MySQL	No	No	No	N/A	N/A	Yes
Oracle	Yes	No	No	>=32	Yes	Yes
Sybase	Yes	No	No	16	No	Yes

Row Subquery column

This column is "Yes" if the DBMS supports row subqueries, for example:


SELECT * FROM Table1

  WHERE (column1, column2) =

    (SELECT column1, column2 FROM Table2)

Table Subquery column
This column is "Yes" if the DBMS supports table subqueries, for example:
```
SELECT *

  FROM (SELECT * FROM Table1) AS TableX
```
Max Depth column
Shows the maximum number of subquery levels supported.
Allow UNION column
This column is "Yes" if the DBMS allows UNION in a subquery.
Types Converted column
This column is "Yes" if the DBMS automatically casts similar data types during comparisons.

Refresher