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combine 2 sql select count statements

I ahave 2 simple count queries: 

select count (*) from t_object
select count (*) from t_diagram

How is the simplest way to combine their result (sum)?

Solution

  • Use UNION ALL to get two different count:

    select count (*), 't_object count' from t_object
union all
select count (*), 't_diagram count' from t_diagram

    To get the sum of the counts, use a derived table:

    select sum(dt.cnt) from
(
 select count(*) as cnt from t_object
 union all
 select count(*) as cnt from t_diagram
) dt

    Or, use a sub-query:

    select count(*) + (select count(*) from t_diagram) from t_object