Querying All Records of All Users who have any instance of purchase behavior in their history I am trying to write a query that shows me all user ids that in any instance of usage have a particular type of "purchase credit" associated with their account.  I have one table, "users" that shows user information, ie, user ids, names, birthdate, etc. and then I have another table that shows all users all instances of credit purchases.  I want to pull all credit purchases (not just the type I am looking up) from all users who in any instance of purchase have this one kind of credit purchase in their record.  Any help would be much appreciated!
It's common practice to write this sort of query using an exists construct, something like:

select *
from users join credit_purchases using (user_id)
where purchase_type = 'A'
and exists (
    select 'x'
    from credit_purchases
    where purchase_type = 'B'
    and user_id = users.user_id
);


Be careful that this type of query only works properly if you're looking for different records in the sub-query; if it is at all possible to "hit" the same record, you need to exclude that in the subquery's where clause, by doing something like:

select *
from users join credit_purchases cp using (user_id)
where purchase_type = 'A'
and exists (
    select 'x'
    from credit_purchases
    where purchase_type = 'A'
    and user_id = users.user_id
    and credit_purchase_id &lt;&gt; cp.credit_purchase_id
);


I'm assuming your tables have surrogate keys with the obvious names.