MySQL 多连接/组查询

Question

id我有一个这样的表

|----|-----------|--------|
| ID | Ethnicity | Gender |
| 1  |    2      |   1    |
| 2  |    3      |   1    |
| 3  |    4      |   2    |
|----|-----------|--------|
etc ....

我正在尝试获取一组结果，这些结果按男性(1)和女性(2)显示种族组

因此本例中的结果行将是:

Ethnicity    Male    Female
    2         1        0
    3         1        0
    4         0        1

到目前为止，我已经接近我想要的了:

SELECT ethnicity, 
(SELECT 
        count(id)
    FROM
        table_name
    WHERE
        gender = '2' ) as female,
(SELECT 
        count(id)
    FROM
        table_name
    WHERE
        gender = '1') as male
FROM table_name
GROUP BY ethnicity

这给了我:

Ethnicity    Male    Female
2            2        1
3            2        1
4            2        1

但是如果有意义的话，需要 count(id) 仅作为该种族行的 adno 的计数。

Answer 1

试试这个:

SELECT 
  ethnicity, 
  SUM(CASE WHEN gender = 1 THEN 1 ELSE 0 END) AS female,
  SUM(CASE WHEN gender = 2 THEN 1 ELSE 0 END) AS Male
FROM students
GROUP BY ethnicity;

以上内容几乎适用于任何 SQL 产品，而不仅仅是 MySQL。不过，如果您愿意，也可以使用此版本，它采用 MySQL 将 bool 值隐式转换为整数 (true -> 1, false -> 0):

SELECT 
  ethnicity, 
  SUM(gender = 1) AS female,
  SUM(gender = 2) AS Male
FROM students
GROUP BY ethnicity;