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The Derived Table (SELECT FROM SELECT)

You can use the results from one query in another query

You may use a SELECT statement in the FROM line. In this case the derived table X has columns name and gdp_per_capita. The calculated values in the inner SELECT can be used in the outer SELECT.

SELECT name, ROUND(gdp_per_capita)
  (SELECT name,
          gdp/population AS gdp_per_capita
     FROM bbc) X
 WHERE region='North America'

What about ties?

If one region has two countries with the same, highest population then the query will produce the right answers but there will be duplication- both countries will be shown.

See also: