How to create double query SQL? -

i know how receive 1 result, and, result, give new query. have now:

   select a.id_buc               buc,         a.id_promotor_altair     ejecutivo      cl_cliente           a.id_buc=(select b.id_contrato       contrato,                            b.id_buc            buc,                            b.fec_apertura      fecha                          el_contrato       b                         b.id_contrato in (80121221158)); 

but query throws me errors.

there 2 problems in query:

1. you should use in rather = in case there 2 or more results inner query.
2. the inner query should return single column, values of column server filter field retrieve.

so, query should this:

select a.id_buc          buc,     a.id_promotor_altair ejecutivo cl_cliente a.id_buc in (select b.id_buc                 el_contrato b                 b.id_contrato in (80121221158)); 

but query bad , perform horribly. better if rewritten in single query using join , can retrieve data of both tables, needed:

select a.id_buc             buc,        a.id_promotor_altair ejecutivo,        b.id_contrato        contrato,        b.id_buc             buc,        b.fec_apertura       fecha cl_cliente      inner join el_contrato b on a.id_buc = b.id_buc b.id_contrato in (80121221158);