Difference between revisions of "SELECT .. SELECT"

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==Subquery with FROM==
 
==Subquery with FROM==
 
<div class='qu'>
 
<div class='qu'>
You may use a SELECT statement in the FROM line.
+
You may use a <code>SELECT</code> statement in the <code>FROM</code> line.
  
In this case the derived table X has columns <code>name</code> and <code>gdp_per_capita</code>.
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In this case the derived table <code>X</code> has columns <code>name</code> and <code>gdp_per_capita</code>.
The calculated values in the inner SELECT can be used in the outer SELECT.
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The calculated values in the inner <code>SELECT</code> can be used in the outer <code>SELECT</code>.
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
SELECT name, ROUND(gdp_per_capita)
 
SELECT name, ROUND(gdp_per_capita)
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</source>
 
</source>
 
Notice that
 
Notice that
*the inner table is given an alias X
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*the inner table is given an alias <code>X</code>
 
*the first column in the inner query keeps its name
 
*the first column in the inner query keeps its name
 
*the second column in the inner query has an alias
 
*the second column in the inner query has an alias
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<div class='qu'>
 
<div class='qu'>
 
<p class='imper'>Find the countries in the same region as Bhutan</p>
 
<p class='imper'>Find the countries in the same region as Bhutan</p>
You may use a SELECT statement in the WHERE line - this returns a list of regions.
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You may use a <code>SELECT</code> statement in the <code>WHERE</code> line - this returns a list of regions.
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<div>See also:</div>
 
<div>See also:</div>
 
<ul>
 
<ul>
   <li>[http://sqlzoo.net/w/index.php/SELECT_within_SELECT_Tutorial SELECT in SELECT Tutorial]</li>
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   <li>[[SELECT within SELECT Tutorial]]</li>
 
   <li>[[SELECT_.._JOIN |SELECT JOIN]]</li>
 
   <li>[[SELECT_.._JOIN |SELECT JOIN]]</li>
 
</ul>
 
</ul>

Latest revision as of 05:44, 29 December 2012

[edit] Subquery Table, Derived Tables, Nested Queries

You can use the results from one query in another query

[edit] 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)
  FROM
  (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

[edit] 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.

SELECT name
  FROM bbc
 WHERE region IN
  (SELECT region FROM bbc
    WHERE name='Bhutan')

[edit] 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)


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