Where Predicate in “JOIN ON” Clause

Today it happened that my junior Engineer came to my desk and asks me a good question, his statement was like this

“What difference it makes if we write filter condition with ON CLAUSE along with join”

His mean to say; All those condition which we write in “WHERE” clause, if we write all those condition along with join, will it make any difference in the output of query?

Let’s stimulate the same, by doing a small practical.

Here we are creating two table, table1 as parent table and table2 as child with common field for joining “id” in parent table and “p_id” as child field as foreign table.

create table table1

(id int primary key,

name varchar(10))

Command(s) completed successfully.

create table table2

(id int primary key,

address varchar(15),

p_id int references table1(id))

Command(s) completed successfully.

Let’s insert some records in table1

insert into table1

select 1,'Lopa' union all

select 2, 'Abhipsha' union all

select 3,'Rubi' union all

select 4,'Soumya' union all

select 5,'Diksha'

Now lets insert some child records in table2

insert into table2

select 1,'Vegas', 1 union all

select 2,'New Jersey', 2 union all

select 3,'San Diego',3 union all

select 4,'San Francisco',3 union all

select 5,'Los Angeles',5

select * from table1

id          name

----------- ----------

1           Lopa

2           Abhipsha

3           Rubi

4           Soumya

5           Diksha

select * from table2

id          address         p_id

----------- --------------- -----------

1           Vegas           1

2           New Jersey      2

3           San Diego       3

4           San Francisco   3

5           Los Angeles     5

Lets write inner join query joining table1 with table2 on id of table1 with p_id of table2

select * from table1

inner join table2

on table1.id = table2.p_id

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

1           Lopa       1           Vegas           1

2           Abhipsha   2           New Jersey      2

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

5           Diksha     5           Los Angeles     5

Now here we will be adding a condition along with the join.  The condition is the p_id should be 3. And the condition is written with WhERE clause.

select * from table1

inner join table2

on table1.id=table2.p_id

where table2.p_id=3

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

Now we will write the same condition p_id equals 3 but this time the condition is written along with the JOIN clause.

select * from table1

inner join table2

on table1.id = table2.p_id and table2.p_id=3

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

Here we saw writing condition either with JOIN clause or WHERE did not make any difference with INNER JOIN.

Lets try the same with OUTER join.

select * from table1

left outer join table2

on table1.id = table2.p_id

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

1           Lopa       1           Vegas           1

2           Abhipsha   2           New Jersey      2

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

4           Soumya     NULL        NULL            NULL

5           Diksha     5           Los Angeles     5

Now we will put some filter criteria.

select * from table1

left outer join table2

on table1.id=table2.p_id

where table2.p_id=3

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

Here we saw after left outer join we have added a criteria of p_id =3… it filter the result set to p_id  equals 3.

Now we will try to write the same query but remove the where predicate and write the table2.p_id=3 condition along with join clause and check the result.

select * from table1

left outer join table2

on table1.id = table2.p_id and table2.p_id=3

id          name       id          address         p_id

----------- ---------- ----------- --------------- -----------

1           Lopa       NULL        NULL            NULL

2           Abhipsha   NULL        NULL            NULL

3           Rubi       3           San Diego       3

3           Rubi       4           San Francisco   3

4           Soumya     NULL        NULL            NULL

5           Diksha     NULL        NULL            NULL

Result are not same in case of Outer Join.

Conclusion: According to Logical Query Processing, the ON clause executed first then the where clause, Now here with the above Outer Join example, the table2 was filtered with p_id =3 condition then the result was joined with table1. So be aware if you are writing condition with ON CLAUSE with OUTER JOIN.

Blend your code with TEA, toast up your logic and enjoy your life with programming forever! J

 

Post Reference: Vikram Aristocratic Elfin Share