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sqluniondistinctdistinct-values

Getting a distinct value across 2 union sql server tables

I'm trying to get all distinct values across 2 tables using a union.

The idea is to get a count of all unique values in the columnA column without repeats so that I can get a summation of all columns that contain a unique columnA.

This is what I tried (sql server express 2008)

select 
    count(Distinct ColumnA) 
from 
( 
    select Distinct ColumnA as ColumnA from tableX where x = y
    union
    select Distinct ColumnA as ColumnA from tableY where y=z
)
Solution 
  • SELECT COUNT(distinct tmp.ColumnA) FROM ( (SELECT ColumnA FROM TableX WHERE x=y) 
UNION (SELECT ColumnA FROM TableY WHERE y=z) ) as tmp

    The extra distincts on TableX and TableY aren't necessary; they'll get stripped in the tmp.ColumnA clause. Declaring a temporary table should eliminate the ambiguity that might've prevented your query from executing.