Rule Definition
In SQL the UNION operator combines the results of two SQL queries into a single table of all matching rows. The two queries must have matching fields and data types in order to join them. Any duplicate records are automatically removed unless UNION ALL is used. The problem is that MOST of the time it is unnecessary to remove duplicate as there are none and doing the search for duplicate rows can be very costly.

Remediation
Add the ALL after the union keyword (ensure that this does not change the returned data)/

Violation Code Sample

select C1a, C2a
from Table1
union  <- Failure here
select C1b, C2b
from Table2

=> The database server will read the values, sort them and eliminate duplicates. This is costly and likely to be useless.

Fixed Code Sample

select C1a, C2a
from Table1
union ALL <- Correction here
select C1b, C2b
from Table2