Db2 sql query

query 1
select a from tab1 where a='xaa'

output:
1
2
3

query2
select b from tab2 where a=1
select b from tab2 where a=2
select b from tab2 where a=3

i want to build one query
and want the output in the format

b b b..

Please note that query1 will have 3 rows as output

query 1
select TOP 3 a from tab1 where a='xaa'

query2

select
case when a=1 then b else null end as b1
case when a=2 then b else null end as b2
case when a=3 then b else null end as b3
from tab2

output:
b1 b2 b3

TOP is not a db2 command. To limit the number of rows received use fetch first n rows only.

As far as the OP goes

you can either use a join on the two tables
i.e.
select tb.b from tab2 as tb inner join tab1 as ta on tb.a=ta.a where ta.a = 'xaa'