Question

I am using SQL Server 2008.

Whats is the difference between :

With conditions in the Where clause

select customer.Id,
       order.Id
  from Customer customer
      left outer join Order order on order.customerId = customer.Id
  where 
      order.deleted =0

With conditions on the ON clause

select customer.Id,
       order.Id
  from Customer customer
      left outer join Order order 
           on order.customerId = customer.Id 
           and order.deleted =0

And how do the two compare with

select customer.Id,
       order.Id
  from Customer customer
      inner join Order order on order.customerId = customer.Id
  where 
      order.deleted =0

Answer 1

I think you should learn about sql joins. Here is a nice link to start.

Basically,

• Case 1: You are filtering order.deleted =0 which could be null as it
  is the right table of a left join.If right table does not have a mapping row, columns from that table will be null. It should be as ISNULL(order.deleted,0)=0 Or order.delete IS NULL or order.delete=0

• Case 2: ON is the condition where the two tables are joining and
  condition order.deleted =0 is checked (I think) before the join

• Case 3: Simple JOIN or INNER JOIN where exact rows are mapped from both sides