MySQL查询分配错误

Question

当我关注查询时

Select
case
        when (@nume=Start_Date)  then  null        
        when (@nume!=Start_Date) then @num
End,
case
        when (@nume=Start_Date and @nume:=End_Date)  then  null 
        when (@nume!=Start_Date) then @nume
End,
case
        when (@nume!=Start_Date and @nume:=End_Date and @num:=Start_Date)  then  @nume 
End
from P,(Select @num:=(Select Start_Date from P order by Start_Date desc Limit 1),@nume:=(Select End_Date from P order by Start_Date desc Limit 1 )) as A
;

在下一轮中，它将整数 1 值分配给 @nume，而不是它应该分配给 Date。

Answer 1

“如其所愿”。嗯，documentation明确指出:

As a general rule, other than in SET statements, you should never assign a value to a user variable and read the value within the same statement. For example, to increment a variable, this is okay:

SET @a = @a + 1;

For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate @a first and then do an assignment second:

SELECT @a, @a:=@a+1, ...;

However, the order of evaluation for expressions involving user variables is undefined.

您对您的代码应该做什么的解释与文档直接矛盾。我建议您问另一个问题，描述您要解决的问题，并包括样本数据和期望的结果。