mysql - SELECT 语句中表达式的执行顺序

Question

我想知道SELECT语句中表达式的执行顺序是否总是从左到右。

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;

是否保证上述查询始终会生成以下输出？

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

Answer 1

根据MySQL manual :

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

所以你的问题的答案是否定的，执行顺序无法保证。