There are joins and unions. Joins work horizontally, so to speak,and unions work vertically. i.e.
select * from table1 t1, table2 t2 on (t1.id=t2.id)
will produce output something like this:
t1.id | t1.field1 | t2.id | t2.field2
0 | something | 0 | somethingelse
1 | anotherthing | 1 | anotherthingelse
While
select from table1 union select from table2
will produce something more like:
field1 | field 2
data | more data (<- row from table1)
otherdata | evenmoredata (<- row from table2)
Do yourself a favor and pick up an introductory SQL / Database theory book and spend the weekend reading it. It'll save you lots of time and headaches later on. Anything written by Celko is a good choice.