Difference between revisions of "Using nested SELECT"

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(Using a subquery on the SELECT line)
(Using binary operators over a set)
 
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<h2>Notes</h2>
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==Alias==
 
<p>Some versions of SQL insist that you give the subquery an <i>alias</i>. Simply put <code>AS somename</code> after the closing bracket:
 
<p>Some versions of SQL insist that you give the subquery an <i>alias</i>. Simply put <code>AS somename</code> after the closing bracket:
 
<pre>
 
<pre>
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</p>
 
</p>
  
==Multiple results from the subquery==
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==Multiple Results==
  
 
The subquery may return more than one result - if this happens the query above will fail as you are testing one value against more than one value.
 
The subquery may return more than one result - if this happens the query above will fail as you are testing one value against more than one value.
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==Using a subquery on the SELECT line==
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==Subquery on the SELECT line==
 
If you are certain that only one value will be returned you can use that query on the SELECT line.
 
If you are certain that only one value will be returned you can use that query on the SELECT line.
 
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==Using binary operators over a set==
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==Operators over a set==
 
These operators are ''binary'' - they normally take two parameters:
 
These operators are ''binary'' - they normally take two parameters:
 
  =    equals
 
  =    equals

Latest revision as of 22:19, 8 December 2012

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Contents

[edit] Using SELECT in SELECT

See SELECT FROM SELECT for how to use a derived table.

The result of a SELECT statement may be used as a value in another statement. For example the statement SELECT continent FROM world WHERE name = 'Brazil' evaluates to 'South America' so we can use this value to obtain a list of all countries in the same continent as 'Brazil'

List each country in the same continent as 'Brazil'.

SELECT name FROM world WHERE continent = 
(SELECT continent 
FROM world WHERE name = 'Brazil')
SELECT name 
FROM world WHERE continent = 
  (SELECT continent FROM world WHERE name = 'Brazil')

[edit] Alias

Some versions of SQL insist that you give the subquery an alias. Simply put AS somename after the closing bracket:

SELECT name FROM world WHERE continent = 
  (SELECT continent FROM world WHERE name='Brazil') AS brazil_continent

[edit] Multiple Results

The subquery may return more than one result - if this happens the query above will fail as you are testing one value against more than one value. It is safer to use IN to cope with this possibility.

The phrase (SELECT continent FROM world WHERE name = 'Brazil' OR name='Mexico') will return two values ('North America' and 'South America'). You should use:

SELECT name, continent FROM world
WHERE continent IN
 (SELECT continent FROM world WHERE name='Brazil'
                                 OR name='Mexico')

List each country and its continent in the same continent as 'Brazil' or 'Mexico'.

SELECT name, continent FROM world
WHERE continent IN
  (SELECT continent 
     FROM world WHERE name='Brazil'
                   OR name='Mexico')

[edit] Subquery on the SELECT line

If you are certain that only one value will be returned you can use that query on the SELECT line.

Show the population of China as a multiple of the population of the United Kingdom

SELECT
 population/(SELECT population FROM world
             WHERE name='United Kingdom')
  FROM world
WHERE name = 'China'

[edit] Operators over a set

These operators are binary - they normally take two parameters:

=     equals
>     greater than
<     less than
>=    greater or equal
<=    less or equal

You can use the words ALL or ANY where the right side of the operator might have multiple values.

Show each country that has a population greater than the population of ALL countries in Europe.

Note that we mean greater than every single country in Europe; not the combined population of Europe.

SELECT name FROM world
 WHERE population > ALL
      (SELECT population FROM world
        WHERE continent='Europe')

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