在多个表中选择一个不同的值（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;

Answer 2

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

Answer 3

SELECT DISTINCT TAB1.colb,TAB2.colb,TAB3.colb FROM TAB1, TAB2, TAB3

Answer 4

SELECT Distinct cola from tab1 union

SELECT Distinct cola from tab2 union

SELECT Distinct cola from tab3

Answer 5

In sybase, you can use the syntax that @cherouvim posted, you can imagine the subquery as a table.

You may also use temp tables

select colb into #t1 from TAB1 
insert into #t1(colb) values(select colb from TAB2) 
insert into #t1(colb) values(select colb from TAB3) 
select distinct colb from #t1