sql - Combine two SELECT queries in PostgreSQL -

i combine 2 select queries union.
how can use result first select in second select?

(select carto_id_key table1     tag_id = 16) union  (select * table2     carto_id_key = <the carto_id result above> ) 

use cte reuse result subquery in more 1 select.
need postgresql 8.4+ that:

with x (select carto_id_key table1 tag_id = 16)  select carto_id_key   x  union select t2.some_other_id_key   x join   table2 t2 on t2.carto_id_key = x.carto_id_key 

you want union all instead of union. doesn't exclude duplicates , faster way.