MySQL 在选择查询中使用生成的列

Question

I have a MySQL query that runs a brief operation (totalling the counts in a select statement) and I want to use the result to do a math operation, but I'm getting an error.

Table:

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

Query:

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

The error is because there is no real "total" column in the table. How can I make the query work?

Answer 1

You can save total as a variable , then use that in the division calculation.

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

NOTE: GROUP is a reserved word in MySQL. You should enclose it (and all other field/table names) in backticks (`).

Answer 2

You can also do this without introducing a variable:

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

Produces:

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

Answer 3

Your problem is that the inner query needs to generate 1 result per row, not 1 for every group. You want to add a where clause in the inner query saying something like

where inner_table.group = outer_table.group

so that only one result is returned.

Answer 4

This question already has an answer for MySql but by mistake if anyone lands here for MSSql as i did, it is as simple as below for MSSql

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

Answer 5

group is a reserved word in mysql, as is table , you should use it like:

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

For more information: MySQL Reserved Words

You'll see there that you can actually use count but I would put all table and column names in quotes anyway.