Selecting distinct values from two tables

Question

I have two rather large databases (+1 million rows each). Both tables have the same structure.

How can I check if each value in a column is unique across both tables?

Is there a
SELECT COUNT(DISTINCTcol) FROM tbl
type of query that will consider BOTH tables?

Thanks!

Solution 1

You can UNION two full sets in a subquery and then select DISTINCT col from that.

Something like:

SELECT DISTINCT col FROM (SELECT * FROM tbl1 UNION ALL SELECT * FROM tbl2)

Solution 2

You can use

UNION ALL

statement. It doesn't remove duplicate rows so you can see if there are any duplicates.