Left join returns fewer rows than expected? - plsql

Question

Following is my query:
Select Count(*)
from Table1 s
left join Table2 d
ON s.subjectid = d.subjectid
and s.PROJECTID = d.projectid
and s.SITEName = d.SITENAME
left join Table3 dev
on s.subjectid = dev.subjectid
and s.projectid = dev.projectid
and s.siteid = dev.siteid
Where s.isprod =1
and d.isprod =1
and dev.isprod = 1
and s.projectid =107
-- Output 301 ROWS
This query returns 301 rows. However, if I don't use Table3 then the join returns 2203 rows, as shown in the query below:
Select Count(*)
from Table1 s
left join Table2 d
ON s.subjectid = d.subjectid
and s.PROJECTID = d.projectid
and s.SITEName = d.SITENAME
Where s.isprod =1
and d.isprod =1
and s.projectid =107
-- OutPut 2203 ROWS
By my understanding of left join, all the rows from the left table should remain even if they don't match with the right table. However in this case, the number of rows is reduced from 2203 in query 2 to 301 in query 1. How is that possible?
Please suggest what could be going wrong here. For more clarification:
Table1 with the same where conditions as above has 1300 rows
Table2 with the same where conditions as above has 2203 rows
Table3 with the same where conditions as above has 129 rows

Answer 1

When you have conditions in your where clause that put non-null constraints on the records from the table you have outer joined, you effectively destroy the effect of the outer join, and make it act as an inner join
The solution is to move such constraints into the on clause of the outer join:
Select Count(*)
from Table1 s
left join Table2 d
ON s.subjectid = d.subjectid
and s.PROJECTID = d.projectid
and s.SITEName = d.SITENAME
and d.isprod =1
left join Table3 dev
on s.subjectid = dev.subjectid
and s.projectid = dev.projectid
and s.siteid = dev.siteid
and dev.isprod = 1
Where s.isprod =1
and s.projectid =107

Answer 2

The first SQL has additional "where" clause of "and dev.isprod = 1". Most likely this is reducing the number of rows returned.