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==Subquery Table, Derived Tables, Nested Queries==
==Subquery Table, Derived Tables, Nested Queries==

Latest revision as of 15:28, 18 April 2018

Language:Project:Language policy English  • 日本語

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, ROUND(gdp_per_capita)
  (SELECT name,
          gdp/population AS gdp_per_capita
     FROM bbc) 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 region as Bhutan

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

  FROM bbc
 WHERE region IN
  (SELECT region FROM bbc
    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

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

See also: