Nested Left Outer Join : Data Set

Question

Hi All
I am bit confused about data set used by Nested Left outer join.
Can anyone help me.
Here is sample data:
Tables (Name, 3 Column each, total rows and matched rows if any):
     Table 1          
     A     B     C
     Total 20 Rows          
     Table 2          
     A     D     E
     Total 50 Rows and 10 Matching on      2.A = 1.A     
     Table 3          
     D     M     N
     Total 15 Rows and 15 Matching on 3.D = 2.D     
     Table 4          
     M     X     Y
     Total 20 Rows and 10 Matching on 4.M = 3.M     
Sql
select *
From Table 1
Left Outer Join on Table 2 on
               2.A = 1.A
-- Data set 1 will contain 20 Rows (10 matching and 10 non matching)
Left Outer Join on Table 3 on
               3.D = 2.D
-- What will be data set? 20 Rows of Data set 1 or 15 Matching Rows?
Left Outer Join on Table 4 on
               4.M = 3.M
-- What will be data set? X Rows of Data set 2 or 10 Matching Rows?
Please have a look and clear my understanding.

Answer 1

SeshuGiri wrote:
I have two tables defined (below). Emp table has data and there is no data in Emp_Type table yet! Right now it is empty.
I want to write a query that returns data from both the tables even though there is no data in Emp_type table. I am using left outer join but it returning nothing. Anyone can help?
select *
from emp e
left outer join emp_Type t
on e.empid = t.empid
WHERE t.type_id = 1
and t.end_date is null;
The join is including all rows from emp, just like you want.
The WHERE clause is discarding all of those rows. Since all the columns from emp_type (alias t) are NULL, the condition "t.type_id = 1" in the WHERE clause is never true.
Perhaps you meant to include all those conditions in the join conditions, like this:
select *
  from emp e
  left outer join emp_Type t
    on e.empid = t.empid
   and t.type_id = 1
   and t.end_date is null;Edited by: Frank Kulash on Jan 30, 2012 3:56 PM