Mysql用户定义的变量和COUNT

Question

mysql变量存在问题并且计数
代码如下：

SELECT @var := 2, @var := @var+2,
count(birey_dil.birey_id) as count,
@var := @var+count(birey_dil.birey_id), 
@var+1
from birey_dil

以下是结果集：

+-----------+----------------+-------+----------------------------------------+--------+
| @var := 2 | @var := @var+2 | count | @var := @var+count(birey_dil.birey_id) | @var+1 |
+-----------+----------------+-------+----------------------------------------+--------+
|         2 |              4 |     8 |                                     12 |      5 |
+-----------+----------------+-------+----------------------------------------+--------+

我将var设置为2
然后2 + 2 = 4
然后我计算行数，结果是8

  

@var：= @ var + count（birey_dil.birey_id）

返回4 + 8 12 但 @ var + 1为5
为什么mysql会这样呢？ @ var + 1现在应该是13并且有办法解决这个问题吗？

Answer 1

试试这个：
  SELECT @var+1 FROM
(
SELECT @var := 2, @var := @var+2,
count(birey_dil.birey_id) as count,
@var := @var+count(birey_dil.birey_id),
from birey_dil
)x 似乎mysql首先评估所有@var [+ constant];这是有道理的，因为它不需要数据库访问，然后才计算count(birey_dil.birey_id)。最后它是@var := @var+count。