sql - Merge two SELECT queries into one -

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i have 2 queries need count of total records difference in queries 1 field value.

example;

select count(*) group_a tbl category = 'value_a'  select count(*) group_b tbl category = 'value_b'

how can this: (pseudo) 

select count(*) group_a, count(*) group_b tbl category in ('value_a', 'value_b')

but results this

group_a , group_b 56, 101

i thinking case statement in query filter 2 how implement it? or there better way?

i'm doing union right wanted know if return 1 record 2 results

select sum(case when category = 'value_a' 1 else 0 end) group_a,        sum(case when category = 'value_b' 1 else 0 end) group_b     tbl     category in ('value_a', 'value_b')

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