Getting a distinct value across 2 union sql server tables

Question

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
)

Answer 1

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.

Answer 2

SELECT COUNT(*)
FROM
(
SELECT DISTINCT ColumnA From TableX WHERE x = y
UNION
SELECT DISTINCT ColumnA From TableY WHERE y = z
) t

Using a "UNION" will not return duplicates. If you used "UNION ALL" then duplicate ColumnA values from each table WOULD be return.

Answer 3

To get distinct values in Union query you can try this

Select distinct AUnion.Name,AUnion.Company from (SELECT Name,Company from table1 UNION SELECT Name,Company from table2)AUnion

Answer 4

SELECT DISTINCT Id, Name
FROM   TableA
UNION ALL
SELECT DISTINCT Id, Name
FROM   TableB
WHERE  TableB.Id NOT IN (SELECT Id FROM TableA)