Q:

How to add flag variable based on source

I want to union two tables and add a flag column(1,0) indicating where they came from. Any help on how to perform this SQL? 

A:

Just add a static column to your SELECT
SELECT
    a
    ,b
    ,0 AS flag
FROM
    t1
UNION ALL
SELECT
    a
    ,b
    ,1 AS flag
FROM
    t2

