SQL JOINS

              You have following tables 

Salesperson	Customer
ID Name Age Salary 1 Abe 61 140000 2 Bob 34 44000 5 Chris 34 40000 7 Dan 41 52000 8 Ken 57 115000 11 Joe 38 38000	ID Name City Industry Type 4 Samsonic pleasant J 6 Panasung oaktown J 7 Samony jackson B 9 Orange Jackson B

Orders

Number order_date cust_id salesperson_id Amount 10 8/2/96 4 2 540 20 1/30/99 4 8 1800 30 7/14/95 9 1 460 40 1/29/98 7 2 2400 50 2/3/98 6 7 600 60 3/2/98 6 7 720 70 5/6/98 9 7 150

Given the tables above, find the following:

a. The names of all salespeople that have an order with Samsonic. 

solution:

SELECT distinct name FROM `salesperson` WHERE id in(select salesperson_id from customer,orders where id=cust_id AND name='Samsonic');

if you write  above query like:

SELECT distinct name FROM `salesperson` WHERE id=(select salesperson_id from customer,orders where id=cust_id AND name='Samsonic');
It is invalid query because subquery returning multiple records so it can't be compared.