Anonymous One Anonymous One - 3 months ago 18
MySQL Question

Execution order of expressions in SELECT statement

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;


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

Answer

According to the MySQL manual:

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

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