MySQL联合删除零

Question

I would like to merge two tables,but it removes zero when i used union all in stroeprocedure in mysql.

Two tables as the following:

TBL1:

ID VAL1 VAL2
2    3    3
3    3    0

TBL2 

ID VAL1 VAL2
3   0   3
4   0   4

Select ID,VAL1,VAL2 FROM TBL1
UNION ALL
Select ID,VAL2,VAL2 FROM TBL2;

ERROR RESULT:
   ID VAL1 VAL2
    2    3   3
    3    3    
    3        3
    4        4

RIGHT RESULT:
   ID VAL1 VAL2
    2    3   3
    3    3   0
    3    0   3
    4    0   4

How to keep Zero value when we use union all ?

Regards,

Answer 1

try this

     Select ID,IF(VAL1 =0, 0,val1) val1,VAL2 FROM TBL1
    UNION ALL
   Select ID,IF(VAL1 =0, 0,val1) val1,VAL2 FROM TBL2;

DEMO HERE

if same thing is with you VAL2 then use also IF(VAL2 =0, 0,val2) val2 for val2

your query looks right look demo http://sqlfiddle.com/#!2/9e0845/2