SELECT .. SELECT

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Subquery Table, Derived Tables, Nested Queries

You can use the results from one query in another query

Subquery with FROM

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, gdp_per_capita
  FROM
  (SELECT name,
          gdp/population AS gdp_per_capita
     FROM world) X
 WHERE gdp_per_capita>20000

Notice that

• the inner table is given an alias X
• the first column in the inner query keeps its name
• the second column in the inner query has an alias

Subquery with IN

Find the countries in the same continent as Bhutan

You may use a SELECT statement in the WHERE line - this returns a list of continent.

SELECT name
  FROM world
 WHERE continent IN
  (SELECT continent FROM world
    WHERE name='Bhutan')

Correlated Subquery

If a value from the outer query appears in the inner query this is "correlated subquery".

Show the countries where the population is greater than 5 times the average for its region

SELECT name
  FROM bbc b1
 WHERE population> 
  5*(SELECT AVG(population) FROM bbc
     WHERE region=b1.region)

See also:

• SELECT within SELECT Tutorial
• SELECT JOIN