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MySQL 字段统计操作
source link: https://blog.moonlightwatch.com/%E4%BB%A3%E7%A0%81%E7%AC%94%E8%AE%B0/2021/07/06/MySQL-%E5%AD%97%E6%AE%B5%E7%BB%9F%E8%AE%A1%E6%93%8D%E4%BD%9C.html
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MySQL 字段统计操作
代码笔记 | 📅 2021-07-06 | mysql sql
对单表内，字段数据进行统计
使用 count 方法
SELECT
count( `status` = 1 OR NULL ) AS queuing,
count( `status` = 2 OR `status` = 3 OR NULL ) AS running,
count( `status` = 5 OR NULL ) AS finished
FROM
some_table
WHERE
is_delete = 0;
执行结果如下：
+---------+---------+----------+
| queuing | running | finished |
+---------+---------+----------+
| 387 | 79 | 64 |
+---------+---------+----------+
COUNT() 函数返回匹配指定条件的行数。
COUNT() 函数的特性是：统计时不计入NULL数据。因此可以构造 SQL 如下：
SELECT
`status` = 6 OR NULL
FROM
some_table
WHERE
is_delete = 0;
此时，结果集中，status 字段为 6 的将返回 1，否则返回 NULL
+--------------------+
| `status`=6 or null |
+--------------------+
| 1 |
| 1 |
| NULL |
+--------------------+
应用 COUNT() 函数，即可得设定条件下的统计结果：
SELECT
count(`status` = 6 OR NULL)
FROM
some_table
WHERE
is_delete = 0;
+---------------------------+
| count(`status`=6 or null) |
+---------------------------+
| 2 |
+---------------------------+
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