Difference between revisions of "Using nested SELECT"

Revision as of 19:22, 22 August 2012

Using SELECT in SELECT

See SELECT FROM SELECT for how to use a derived table.

The result of a SELECT statement may be used as a value in another statement. For example the statement SELECT region FROM bbc WHERE name = 'Brazil' evaluates to 'South America' so we can use this value to obtain a list of all countries in the same region as 'Brazil'

SELECT name FROM bbc WHERE region = 
(SELECT region 
FROM bbc WHERE name = 'Brazil')

SELECT name 
FROM bbc WHERE region = 
  (SELECT region FROM bbc WHERE name = 'Brazil')

Notes

Some versions of SQL insist that you give the subquery an alias. Simply put AS somename after the closing bracket:

SELECT name FROM bbc WHERE region = 
  (SELECT region FROM bbc WHERE name='Brazil') AS brazil_region

Multiple results from the subquery

The subquery may return more than one result - if this happens the query above will fail as you are testing one value against more than one value. It is safer to use IN to cope with this possibility.

The phrase (SELECT region FROM bbc WHERE name = 'Brazil' OR name='Mexico') will return two values ('North America' and 'South America'). You should use:

SELECT name, region FROM bbc
WHERE region IN (SELECT region FROM bbc WHERE name='Brazil'
                                           OR name='Mexico')

SELECT name, region FROM bbc
WHERE region IN (SELECT region 
                   FROM bbc WHERE name='Brazil'
                               OR name ='Mexico')

Using a query on the SELECT line

If you are certain that only one value will be returned you can use that query on the SELECT line.

SELECT population/(SELECT population FROM bbc
                    WHERE name='United Kingdom')
  FROM bbc
WHERE name = 'China'

Using binary operators over a set

These operators are binary - they normally take two parameters:

=     equals
>     greater than
<     less than
>=    greater or equal
<=    less or equal

You can use the words ALL or ANY where the right side of the operator might have multiple values.

SELECT name FROM bbc
 WHERE population > ALL
      (SELECT population FROM bbc
        WHERE region='Europe')