MySQL在select查询中使用生成列

Question

我有一个MySQL查询运行一个简短的操作(总计一个select语句中的计数),我想使用结果进行数学运算,但我收到一个错误.

表:

id  |   group   |   count   |
-----------------------------
1       1           3
2       1           2

查询:

select id, count,
  (select sum(count) from table group by group) as total,
  count/total as percent 
from table

该错误是因为表中没有真正的"总"列.如何使查询工作？

Answer 1

您可以保存total为变量,然后在除法计算中使用它.

SELECT 
  `id`, `count`, 
   @total:=(SELECT sum(`count`) FROM `table` GROUP BY `group`) AS `total`, 
  `count`/@total AS `percent` 
FROM `table`

注意:GROUP是MySQL中的保留字.你应该将它(和所有其他字段/表名)括在反引号(`)中.

Answer 2

您也可以在不引入变量的情况下执行此操作:

select id, 
   count, 
   (select sum(count) from `table` group by `group`) as total, 
   (select count/total) as percent 
from `table`;

生产:

+------+-------+-------+---------+
| id   | count | total | percent |
+------+-------+-------+---------+
|    1 |     3 |     5 |  0.6000 |
|    2 |     2 |     5 |  0.4000 |
+------+-------+-------+---------+
2 rows in set (0.05 sec)