Difference between revisions of "SELECT .. SELECT"

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<h3>The Derived Table (SELECT FROM SELECT)</h3>
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__NOTOC__
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==Subquery Table, Derived Tables, Nested Queries==
 
<p>You can use the results from one query in another query</p>
 
<p>You can use the results from one query in another query</p>
 +
==Subquery with FROM==
 
<div class='qu'>
 
<div class='qu'>
 
You may use a SELECT statement in the FROM line.
 
You may use a SELECT statement in the FROM line.
 +
 
In this case the derived table X has columns <code>name</code> and <code>gdp_per_capita</code>.
 
In this case the derived table X 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.
 
The calculated values in the inner SELECT can be used in the outer SELECT.
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           gdp/population AS gdp_per_capita
 
           gdp/population AS gdp_per_capita
 
     FROM bbc) X
 
     FROM bbc) X
  WHERE region='North America'
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  WHERE gdp_per_capita>20000
 
</source>
 
</source>
 +
Notice that
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*the inner table is given an alias X
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*the first column in the inner query keeps its name
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*the second column in the inner query has an alias
 
</div>
 
</div>
  
<h2>What about ties?</h2>
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==Subquery with IN==
<p>If one region has two countries with the same, highest population then the query will
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<div class='qu'>
produce the right answers but there will be duplication- both countries will be shown.
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<p class='imper'>Find the countries in the same region as Bhutan</p>
</p>
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You may use a SELECT statement in the WHERE line - this returns a list of regions.
 +
 
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<source lang='sql' class='def'>
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SELECT name
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  FROM bbc
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WHERE region IN
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  (SELECT region FROM bbc
 +
    WHERE name='Bhutan')
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</source>
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</div>
 +
 
 +
==Correlated Subquery==
 +
<div class='qu'>
 +
<p>If a value from the outer query appears in the inner query this is "correlated subquery".</p>
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<p class='imper'>Show the countries where the population is greater than 5 times the average for its region</p>
 +
 
 +
<source lang='sql' class='def'>
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SELECT name
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  FROM bbc b1
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WHERE population>
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  5*(SELECT AVG(population) FROM bbc
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    WHERE region=b1.region)
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</source>
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</div>
 +
 
 +
 
 
<div>See also:</div>
 
<div>See also:</div>
 
<ul>
 
<ul>

Revision as of 01:33, 9 August 2012

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)
  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

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')

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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