SQLzoo.net

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')

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 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')