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

您实际上不需要子选择：

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

您可以在子选择中移动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

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