Select a distinct value in multiple tables (sql)

Question

HI,

I have a database with 3 tables TAB1, TAB2, TAB3 which have exactly the same columns, for example :

TAB1
cola, colb, colc, cold
TABB
cola, colb, colc, cold
...

Now I would like to search all distinct "colb" values, this is the query :

SELECT DISTINCT colb FROM TAB1

Works perfectly but now I would search all distinct "colb" values in my 3 tables "TAB1", "TAB2", "TAB3" :

SELECT DISTINCT colb FROM TAB1, TAB2, TAB3

And now SQL return me an error: "Column 'colb' in field list is ambiguous" After some search, I understood that was because 'colb' column exist in my 3 tables.

So how search in my 3 tables a distinct value from the same column ? I cannot use the LEFT JOIN because I wan to search in my all 3 tables and not in one of them.

Do you have an idea ? Thanks

Answer 1

This single query with union will take care of distinct values for you.

select colb from tab1 union
  select colb from tab2 union
  select colb from tab3;