Select query : from clause is having multple tables, but i want where clause to have condition on only two tables - sql

Question

For eg:
SELECT aa.name, aa.id
FROM ba_acct_memo aa, acct_mast bb
WHERE (aa.cod_acct_no=bb.cod_acct_no AND aa.flg_mnt_status='U'
and bb.flg_mnt_status='A')
gives 1 row(for eg)
but
SELECT aa.name, aa.id
FROM ba_acct_memo aa, acct_mast bb, college cc
WHERE (aa.cod_acct_no=bb.cod_acct_no AND aa.flg_mnt_status='U'
and bb.flg_mnt_status='A')
gives same result as in first query but giving it, as many number of rows which are there in college table.
But it should work just as like first query.
Can someone explain the weird behaviour of my query?

Answer 1

Following Steps will help you to understand (Inner) Join.
--Create Table 1 :
Create Table #Table1
(
Roll_No INT ,
Student_Name Varchar(50)
)
Go
--Create Table 2 :
Create Table #Table2
(
Roll_No INT,
Student_Address Varchar(200)
)
Go
--Create Table 3 :
Create Table #Table3
(
Roll_No INT,
Student_Contact_No Varchar(10)
)
Go
-- Insert Values into #Table1:
Insert into #Table1 Values ('1','John')
Insert into #Table1 Values ('2','Alex')
Insert into #Table1 Values ('3','Mike')
Insert into #Table1 Values ('4','Steve')
Insert into #Table1 Values ('5','Jack')
Insert into #Table1 Values ('6','Vicky')
Insert into #Table1 Values ('7','Sid')
Insert into #Table1 Values ('8','Tom')
-- Insert Values into #Table2:
Insert into #Table2 Values ('1','1st Street')
Insert into #Table2 Values ('2','2rd Street')
Insert into #Table2 Values ('3','3rd Street')
Insert into #Table2 Values ('4','4th Street')
Insert into #Table2 Values ('5','5th Street')
Insert into #Table2 Values ('6','6th Street')
Insert into #Table2 Values ('7','7th Street')
Insert into #Table2 Values ('8','8th Street')
-- Insert Values into #Table3:
Insert into #Table3 Values ('1','123-123')
Insert into #Table3 Values ('2','123-111')
Insert into #Table3 Values ('3','123-122')
Insert into #Table3 Values ('4','123-125')
Insert into #Table3 Values ('5','123-126')
Insert into #Table3 Values ('6','123-116')
Insert into #Table3 Values ('7','123-145')
Insert into #Table3 Values ('8','123-132')
--View Data :
Select * from #Table1
Select * from #Table2
Select * from #Table3
-- First Query : (With Out Third Table) :
Select A.Roll_No,A.Student_Name,B.Student_Address from #Table1 A, #Table2 B
Where a.Roll_No = B.Roll_No
and a.Roll_No = 1
--It will Retrun One Record. If you add third table in this query with out join with any other tables ( Table 1 and Table 2 )
-- Secord Query :
Select A.Roll_No,A.Student_Name,B.Student_Address from #Table1 A, #Table2 B , #Table3 C
Where a.Roll_No = B.Roll_No
and a.Roll_No = 1
--It will return 8 Same Records. (Because Table 3 having 8 Records)
--You can understand when execute why it is return 8 records.
-- Third Query :
Select * from #Table1 A, #Table2 B , #Table3 C
Where a.Roll_No = B.Roll_No
and a.Roll_No = 1
--Delete some records from #Table 3 :
Delete #Table3 Where Roll_No > 5
--Execute again Third Query :
-- Third Query :
Select * from #Table1 A, #Table2 B , #Table3 C
Where a.Roll_No = B.Roll_No
and a.Roll_No = 1
--Clean Up:
Drop table #Table1
Drop table #Table2
Drop table #Table3