Question

idI有一个像这样的表

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

我正试图找回一系列结果，向我展示男性（1）和女性（2）的种族群体

因此，此示例中的结果行将是：

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

到目前为止，我接近我想要的东西：

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，那么count(id)只需要计算该种族行的广告。

Answer 1

请改为尝试：

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;

上述内容几乎适用于任何SQL产品，而不仅仅适用于MySQL。但是，如果您愿意，也可以使用此版本，该版本使用MySQL隐式将布尔值转换为整数（true - ＆gt; 1，false - ＆gt; 0 ）：

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

Answer 2

SELECT 
  ethnicity, 
  CASE WHEN gender = 1 THEN 1 ELSE 0 END AS Male,
  CASE WHEN gender = 2 THEN 1 ELSE 0 END AS Female
FROM students

MySQL多连接/组查询

2 个答案: