在COUNT里面选择

Question

I would like to embed a SELECT inside a COUNT, but I can't find any examples.

#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

You don't really need a sub-select:

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

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

Answer 3

使用SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const' ) as d 。

Answer 4

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