Using SQL variables in sum () rows causes unpredictable results

Query 1)

Select * From test;

No1	NO2
1	1
2	2
3	3
4	4
5	5

Query 2)

select @wokao:= (no1 + no2), @wokao from test group by no1;

No1	NO2
2	2
4	4
6	6
8	8
10	10

Query 3)

select @wokao:= (no1 + no2), sum(@wokao) from test group by no1;

No1	NO2
2	Null
4	2
6	4
8	6
10	8

The result of the third SQL query is strange, in theory it should get the same results as Query2, and why the results are different.

I asked this question in StackOverflow.

The answer is big. In the third SQL query, the sum (@wokao) is executed first, followed by @wokao:= (No1 + NO2), which results in the above result.

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Using SQL variables in sum () rows causes unpredictable results