why this sql used union all returned no data

StackOverflow https://stackoverflow.com/questions/23112798

  •  04-07-2023
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Question

the result of sql 1st:

select p1.t1, p2.t2, p2.t3 from 
(select 'A' as t1,'' as t2,'' as t3 from dual ) p1 left join
(select '' as t1, '' as t2, '' as t3 from dual where 1=2) p2 on p1.t1 = p2.t1

is

A   NULL    NULL

the result of sql 2nd:

select p1.t1, p2.t2, p2.t3 from 
(select 'B' as t1,'' as t2,'' as t3 from dual ) p1 left join
(select '' as t1, '' as t2, '' as t3 from dual where 1=2) p2 on p1.t1 = p2.t1

is 

B   NULL    NULL

if I union all these two part of sql, my expected sql are:

A    NULL    NULL
B    NULL    NULL

but I get no result .

the sql is like below:

select p1.t1, p2.t2, p2.t3 from 
(select 'A' as t1,'' as t2,'' as t3 from dual ) p1 left join
(select '' as t1, '' as t2, '' as t3 from dual where 1=2) p2 on p1.t1 = p2.t1
union all
select p1.t1, p2.t2, p2.t3 from 
(select 'B' as t1,'' as t2,'' as t3 from dual ) p1 left join
(select '' as t1, '' as t2, '' as t3 from dual where 1=2) p2 on p1.t1 = p2.t1

I tested this sql on oracle. I want to know why the result is not like my expected. by the way, if use union instead of union all, the result is just like expected.

Was it helpful?

Solution

You haven't said which version you're encountering this on, but Gordon Linoff reproduced on SQL Fiddle which is running 11.2.0.2, and as others have said it's not seen in 10g and 11.2.0.3, so it seems reasonable to think you might be on 11.2.0.2 as well.

In which case, this looks identical to bug 12336962. If you have access to Oracle Support you (or your DBA) can look that up, but I can't reproduce what it says here, even though it's a published bug. Running your query and the example from the bug report produce the same results, and in both cases changing from union all to union produces correct results. You might want to raise a service request to get that confirmed though.

That bug is fixed in the 11.2.0.3 patch set - I'm not sure I'm really supposed to even share that, but it's already published here - so patching up might be your best bet, and Oracle might suggest that if you do raise an SR. If in doubt, ask Oracle directly.

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