You can move the count() inside your sub-select:
SELECT a AS current_a, COUNT(*) AS b,
( SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const' ) as d,
from t group by a order by b desc
Use SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const' ) as d
.
SELECT a AS current_a, COUNT(*) AS b,
(SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const' ) as d
from t group by a order by b desc
Source: Stackoverflow.com