在COUNT里面选择

Question

我想在COUNT中嵌入一个SELECT,但我找不到任何例子.

#pseudosql
SELECT a AS current_a, COUNT(*) AS b,
   COUNT( SELECT FROM t WHERE a = current_a AND c = 'const' ) as d,
   from t group by a order by b desc

Answer 1

你真的不需要一个子选择:

SELECT a, COUNT(*) AS b,
   SUM( CASE WHEN c = 'const' THEN 1 ELSE 0 END ) as d,
   from t group by a order by b desc

Answer 2

您可以在子选择内移动count():

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