SELECT语句中表达式的执行顺序 [英] Execution order of expressions in SELECT statement

问题描述

我想知道SELECT语句中表达式的执行顺序是否总是从从左到右.

I want to know if the execution order of expressions in SELECT statement always takes place from left to right.

SET @a := 0;

SELECT 
@a AS first,
@a := @a + 1 AS second,
@a := @a + 1 AS third,
@a := @a + 1 AS fourth,
@a := @a + 1 AS fifth,
@a := @a + 1 AS sixth;

是否保证上述查询将始终生成以下输出?

Is it guaranteed that the above query will always generate the following output?

first second third fourth fifth sixth 
  0     1      2     3     4      5

推荐答案

根据 MySQL manual :

但是，涉及用户的表达式的求值顺序 变量未定义

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

所以您的问题的答案是否"，不能保证执行顺序.

So the answer to your question, is no, the execution order is not guaranteed.

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