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;

是否保证上面的查询总是会产生下面的输出？

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

Answer 1

是的，无论如何都没有执行顺序的配置，所以你总是得到这样的输出。

Answer 2

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，不能保证执行顺序。

SELECT语句中表达式的执行顺序

Execution order of expressions in SELECT statement

mysql

sql

select

order-of-execution