Question

Given the following tables

      orders
      +----+---------+---------+
      | id | user_id | details |
      +----+---------+---------+  
      | 1  |    2    | blue    | 
      +----+---------+---------+
      | 2  |    1    | red     | 
      +----+---------+---------+
      | 3  |    2    | yellow  | 
      +----+---------+---------+
      | 4  |    2    | cyan    | 
      +----+---------+---------+

      users
      +---------+---------+---------+
      | user_id |    ph   |   name  | 
      +---------+---------+---------+
      |   1     |   123   |   fred  | 
      +---------+---------+---------+
      |   2     |   456   |   Stan  | 
      +---------+---------+---------+
      |   3     |   189   |   Levi  | 
      +---------+---------+---------+

I know how to select only one occurance of each user in the first table using distinct

     SELECT DISTINCT user_id FROM orders

How could I pull just the phone number from users?

I could probably go for a loop and pick out each number like…

     SELECT ph from users WHERE user_id = user_id

Can’t help thinking there might be a one liner query I could use.

result would be

     123
     456

Answer 1

Selecting Distinctively from the Orders table might be a heavy operation. Depending on how massive that table is going to become.

Maybe this one will be faster:

Select u.ph 
  from users u 
  where exists (select id from orders where user_id = u.user_id);