Question

I’m new to relational sql. I’m trying to figure out a query to return the names of customers who have more than one type of account.

customers:

+------------+--------------+ 
| cid        | Name         | 
+------------+--------------+ 
| 1          | Bob          |
| 2          | John         | 
| 3          | Jane         | 
+------------+--------------+ 

accounts:

+------------+--------------+ 
| aid        | type         | 
+------------+--------------+ 
| 1          | Checking     |
| 2          | Saving       | 
| 3          | CD           | 
+------------+--------------+ 

transactions:

+------------+--------------+--------------+ 
| tid        | cid          | aid          | 
+------------+--------------+--------------+ 
| 1          | 1            | 1            | 
| 2          | 2            | 1            | 
| 3          | 1            | 2            | 
| 4          | 2            | 3            | 
| 5          | 3            | 1            | 
+------------+--------------+--------------+

With these tables, the query should return Bob and John. I’m having some trouble with how to write such a query. More specifically, how do I keep count of how many accounts a customer has and how do I compare if the accounts are different without adding a new column to the table?

Answer 1

Okay, this seems to work in SQL Fiddle with my test data structure. Try it out with your real data structure and see if it gives you what you’re looking for.

SELECT name FROM customers c WHERE EXISTS(
  SELECT DISTINCT aid FROM transactions
  WHERE cid = c.cid
  HAVING COUNT(DISTINCT aid)>1
)