REPOST: UNION & MINUS - strange work together at different situation

Hi,

I have two tables: A and B, they both have the same fields. Table A contains some records. Table B has the same records and some additional records.

If I use such code:

 select * from A
 minus
 select * from B
union
 select * from B
 minus
 select * from A

I get correct result - some string (difference bitween B and A)

But if I use such code

 select * from B
 minus
 select * from A
union
 select * from A
 minus
 select * from B

I get no records as a result. What can be a problem here? Why if last part return null string UNION does not work ?

Thanks
Mike

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