SQL计数列

Question

创建用于计算表中数据出现次数的列的最佳方法是什么？该表需要按一列分组.

我见过

SELECT
    sum(CASE WHEN question1 = 0 THEN 1 ELSE 0 END) AS ZERO,
    sum(CASE WHEN question1 = 1 THEN 1 ELSE 0 END) AS ONE,
    sum(CASE WHEN question1 = 2 THEN 1 ELSE 0 END) AS TWO,
    category
FROM reviews
    GROUP BY category

其中question1的值可以是0,1或2.

我也看过使用它的一个版本 count(CASE WHEN question1 = 0 THEN 1)

然而,随着question1的可能值的数量增加,这变得更加麻烦.有没有一种方便的方法来编写此查询,可能会优化性能？

PS.我的数据库是PostgreSQL

Answer 1

在Postgres 9.4中有一个新的,更清晰的聚合FILTER选项:

SELECT category
     , count(*) FILTER (WHERE question1 = 0) AS zero
     , count(*) FILTER (WHERE question1 = 1) AS one
     , count(*) FILTER (WHERE question1 = 2) AS two
FROM   reviews
GROUP  BY 1;

新FILTER条款的细节:

• 如何简化此游戏统计查询？

如果你想要它简短:

SELECT category
     , count(question1 = 0 OR NULL) AS zero
     , count(question1 = 1 OR NULL) AS one
     , count(question1 = 2 OR NULL) AS two
FROM   reviews
GROUP  BY 1;

可能的变体概述:

• 对于绝对性能,SUM更快还是COUNT？

正确的交叉表查询

crosstab() 产生最佳性能,并且对于更长的选项列表更短:

SELECT * FROM crosstab(
     'SELECT category, question1, count(*)::int AS ct
      FROM   reviews
      GROUP  BY 1, 2
      ORDER  BY 1, 2'
   , 'VALUES (0), (1), (2)'
   ) AS ct (category text, zero int, one int, two int);

详细说明:

• PostgreSQL交叉表查询