How to add count of different query using a query in mysql5?

Hi I need a query like this

    select count(id) from employee where.. => gives count1=10

    select count(id) from emplyer where..  => gives count2=5

    select count(id) from users where..    => gives count3=20

I can add these result like this

    count = count1+count2+count3;

But I want to know whether t开发者_如何学JAVAhere is any option get count using a single query instead of add these result separatly. Also I want to know which one is the best way in case of application perfomance.

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You can do

SELECT ((select count(id) from employee where..) + (select count(id) from emplyer where..) + (select count(id) from users where..))

There will hardly be a performance difference - adding three integers is neither a challenge for the MySql nor for the php engine.

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If you use group by clause you can add option WITH ROLLUP which will return total sum of all counts in group. See http://dev.mysql.com/doc/refman/5.0/en/group-by-modifiers.html