Question

I’ve been given a big query to figure out. But there are so many subqueries that it’s nearly impossible.The number of subqueries is about 15-20.What do you suggest I do?

Answer 1

What I usually do when confronted with such monsters is:

1. Replace old style joins with new style ones
2. Modularize every non-trivial subquery into “virtual” tables using the with statement, for example:

with
 subquery1 as (select /*big query*/ ),
 subquery2 as (select /*big query*/ )
select *
from ...
     join subquery1
where foo in (select foo from subquery2)

At that point some patterns emerge and more often than not the query can be rewritten in a sensible way.