Mysql计算行数和空行数

Question

大家好， 我有一张这样的桌子：

+----+------+
| id | grade| 
+----+------+
|  1 |    1 |
|  2 |    1 |
|  3 |    2 |
|  4 | NULL |
|  5 | NULL |
|  6 | NULL |
+----+------+

在哪里

1 坏

2 好

3 非常好

我试图得到类似的结果：

+--------------+------+
| grade        | count| 
+--------------+------+
| "Bad"        |    2 |
| "Good"       |    1 |
| "Very Good"  |    0 |
| "Not Ranked" |    3 |
+--------------+------+

我正在尝试计数但没有成功

Answer 1

您可以使用CASE 更改grade 的值，而不是您想要的字符串值，

SELECT  CASE WHEN grade = 1 THEN 'Bad'
            WHEN grade = 2 THEN 'Good'
            WHEN grade = 3 THEN 'Very Good'
            ELSE 'Not Ranked'
        END as grade,
        COUNT(IFNULL(grade, 0)) as `count`
FROM    TableName
GROUP   BY CASE WHEN grade = 1 THEN 'Bad'
            WHEN grade = 2 THEN 'Good'
            WHEN grade = 3 THEN 'Very Good'
            ELSE 'Not Ranked'
        END

由于您想显示所有值，因此您需要创建一个返回所有值的子查询并使用LEFT JOIN 连接到您的表。

SELECT  a.grade,
        COUNT(b.id)
FROM    
        (
            SELECT 1 id, 'Bad' grade UNION ALL
            SELECT 2 id, 'Good' grade UNION ALL
            SELECT 3 id, 'Very Good' grade UNION ALL
            SELECT 999 id, 'Not Ranked' grade 
        ) a
        LEFT JOIN TableName b ON a.id = IFNULL(b.grade, 999)
GROUP   BY a.grade

这是Demo。

Answer 2

如果您可以使用数字，并稍后将它们与字符串匹配，那么这可能是一个解决方案：

SELECT grade, count(*) as id FROM tbl GROUP BY grade

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