mysql - 我可以在同一个查询中计算两个不同的事物吗？

Question

我有一个这样的表:

// question_and_answers
+----+---------+---------------+--------+------+
| id |  title  |      body     | amount | type |
+----+---------+---------------+--------+------+
| 1  | t1      | b1            | NULL   | 0    |
| 2  | t2      | b2            | NULL   | 1    |
| 3  | t3      | b3            | NULL   | 1    |
| 4  | t4      | b4            | 100    | 0    |
| 5  | t5      | b5            | NULL   | 0    |
| 6  | t6      | b6            | NULL   | 1    |
| 7  | t7      | b7            | 50     | 0    |
+----+---------+---------------+--------+------+

我有两个疑问:

1:问题数量:

SELECT count(1) FROM question_and_answers WHERE type = 0

2:付费问题数量:

SELECT count(1) FROM question_and_answers WHERE type = 0 AND amount IS NOT NULL

---

我可以合并这两个查询吗？我的意思是我可以编写一个查询来代替它们吗？

Answer 1

您可以使用条件聚合:

SELECT sum(type = 0 AND amount IS NOT NULL),
       count(*) 
FROM question_and_answers 
WHERE type = 0

在 MySQL 中，比较的结果是 0 或 1。您可以像上面的查询一样对这些结果进行总结。

要使其适用于其他数据库引擎，您可以使用以下通用 ANSI SQL 方法:

SELECT sum(case when type = 0 AND amount IS NOT NULL then 1 else 0 end),
       count(*)
FROM question_and_answers 
WHERE type = 0

或使用count():

SELECT count(case when type = 0 AND amount IS NOT NULL then 1 else null end),
       count(*) 
FROM question_and_answers 
WHERE type = 0