Inconsistent results observed with union and distinct - Source

Revision: 120642
User: Harinath Maddela
Date: 2022-11-09 17:13

I have UNION of 2 queries(query1 and query2). When i run 2 queries separately they yield 4 records each. 
UNION ALL is expected to yield 8 records(all  8 records being distinct) instead it yields just 5 records.

Following is the combined query which has 2 queries joined by union all :

select distinct if (period_type = '1', 'Month', if(period_type = '2', 'Week', period_type)) as value, if (period_type = '1', 'Month', if(period_type = '2', 'Week', period_type)) as description, ('Y') as planningonly from gl_periods
union all
select distinct if (period_type = '1', 'Fiscal_Month', if(period_type = '2', 'Fiscal_Week', if(period_type = 'Quarter', 'Fiscal_Quarter', if(period_type = 'Year', 'Fiscal_Year', null)))) as value, if (period_type = '1', 'Fiscal_Month', if(period_type = '2', 'Fiscal_Week', if(period_type = 'Quarter', 'Fiscal_Quarter', if(period_type = 'Year', 'Fiscal_Year', null)))) as description, ('Y') as planningonly from gl_periods

Not sure what is the issue in this. Can someone explain this behaviour.

Thanks in advance,
Harinath

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Inconsistent results observed with union and distinct - Source

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