mysql - SELECT 中的临时变量操作可见性

Question

MySQL 中如何定义临时变量操作？似乎值的任何变化都会在赋值表达式之后立即“可见”。如果我运行查询

SELECT 
  @x as x,
  @x + (@x:=@x+1) + @x as magic,
  @x as new_x
FROM 
  (SELECT @x:=0) x,
  Values1to100 numbers

返回

+---+-------+-------+
| x | magic | new_x |
+===+=======+=======+
| 0 |     2 |     1 |
+---+-------+-------+
| 1 |     5 |     2 |
+---+-------+-------+
| 2 |     8 |     3 |
+---+-------+-------+
| ...               |

现在我想知道这是 MySQL(或一般的 SQL)的明确定义的行为还是不是(即结果取决于实现)？

Answer 1

MySQL documentation说

For 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.