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It is very powerful and flexible feature of SQL. Through
subqueries, we can embed a query within an update, delete
or select statements. Sub Queries are useful where the desired
output is based on some query or when we do not have the predefined
list of values.
The syntax of the select clause is to retrieve based
on output of a subquery :-
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SELECT column_name [,column_name...]
FROM table_name
WHERE column_name = (SELECT column_name FROM table_name
where column_name = <value> )
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Table Store Information :-
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Store Name
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Sales
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Date
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The Trend
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1500
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Jan-05-2006
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Su Ru
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250
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Jan-07-2006
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The Trend
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300
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Jan-08-2006
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Positive
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700
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Jan-08-2006
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Table Geography :-
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Region Name
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Store Name
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North
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Positive
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North
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Tantra
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South
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The Trend
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South
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SU RU
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We want to use a subquery to find the sales of all stores
in the South region. To do so, we use the following SQL statement
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SELECT SUM(Sales) FROM Store Information
WHERE Store_name IN
(SELECT store_name FROM Geography
WHERE region_name = 'South') |
RESULT:-
In this example, instead of joining the two tables directly
and then adding up only the sales amount for stores in the South
region. We first use the subquery to find out which stores are in
the South region, and then we sum up the sales amount for these.
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