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SELECT count(*)
FROM t_test t
WHERE EXISTS (
SELECT 1
FROM t_test m
WHERE m.a = t.a
AND m.b = 'M20004'
)
select sum(t.a) from your_table t where t.b='M20004';
select count(*) from (
select distinct(a) from your_table t where t.b='M20004';
)