Difference between on .. and and on .. where in SQL Left Join?
create table A(id int);
create table B(id int);
INSERT INTO A VALUES(1);
INSERT INTO A VALUES(2);
INSERT INTO A VALUES(3);
INSERT INTO B VALUES(1);
INSERT INTO B VALUES(2);
INSERT INTO B VALUES(3);
SELECT * FROM A;
SELECT * FROM B;
id
-----------
1
2
3
id
-----------
1
2
3
Filter on the JOIN to prevent rows from being added during the JOIN process.
select a.*,b.*
from A a left join B b
on a.id =b.id and a.id=2;
id id
----------- -----------
1 NULL
2 2
3 NULL
WHERE will filter after the JOIN has occurred.
select a.*,b.*
from A a left join B b
on a.id =b.id
where a.id=2;
id id
----------- -----------
2 2
SQL LEFT JOIN: difference between WHERE and condition inside AND
with a left join
there is a difference
with condition on left join
rows with column > 10
will be there filled with nulls
with where
condition rows will be filtered out
with a inner join
there is no difference
example:
declare @t table (id int, dummy varchar(20))
declare @a table (id int, age int, col int)
insert into @t
select * from (
values
(1, 'pippo' ),
(2, 'pluto' ),
(3, 'paperino' ),
(4, 'ciccio' ),
(5, 'caio' ),
(5, 'sempronio')
) x (c1,c2)
insert into @a
select * from (
values
(1, 38, 2 ),
(2, 26, 5 ),
(3, 41, 12),
(4, 15, 11),
(5, 39, 7 )
) x (c1,c2,c3)
select t.*, a.age
from @t t
left join @a a on t.ID = a.ID and a.col > 10
Outputs:
id dummy age
1 pippo NULL
2 pluto NULL
3 paperino 41
4 ciccio 15
5 caio NULL
5 sempronio NULL
While
select t.*, a.age
from @t t
left join @a a on t.ID = a.ID
where a.col > 10
Outputs:
id dummy age
3 paperino 41
4 ciccio 15
So with LEFT JOIN
you will get ALWAYS all the rows from 1st table
If the join condition is true, you will get columns from joined table filled with their values, if the condition is false their columns will be NULL
With WHERE
condition you will get only the rows that match the condition.
What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?
Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.
Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.
Find original one at: Difference between JOIN and OUTER JOIN in MySQL.
What's the difference between where clause and on clause when table left join?
The where
clause applies to the whole resultset; the on clause
only applies to the join in question.
In the example supplied, all of the additional conditions related to fields on the inner side of the join - so in this example, the two queries are effectively identical.
However, if you had included a condition on a value in the table in the outer side of the join, it would have made a significant difference.
You can get more from this link: http://ask.sqlservercentral.com/questions/80067/sql-data-filter-condition-in-join-vs-where-clause
For example:
select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 and t2.f4=1
select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 where t2.f4=1
- do different things - the former will left join to t2 records where f4 is 1, while the latter has effectively been turned back into an inner join to t2.
Difference between Where and Join on Id
This is very good practice -- in fact, you cannot (easily) get this logic in a WHERE
clause.
A LEFT JOIN
returns all rows in the first table -- even when there are no matches in the second.
So, this returns all rows in the preceding tables -- and any rows from Client
where ClientId = 1
. If there is no match on that ClientId
, then the columns will be NULL
, but the rows are not filtered.
What is the difference between On or Where in LEFT JOIN?
Rewriting the 2 queries without the LEFT JOIN / IS NULL
so it's obvious that they can return different data sets in certain cases:
---Query 1
SELECT COUNT(*)
FROM a
WHERE a.status = 2
AND NOT EXISTS
( SELECT *
FROM b
WHERE a.id = b.id
)
and
---Query 2
SELECT COUNT(*)
FROM a
WHERE NOT ( a.status = 2
AND EXISTS
( SELECT *
FROM b
WHERE a.id = b.id
)
)
--- or:
---Query 2
SELECT COUNT(*)
FROM a
WHERE a.status <> 2
OR NOT EXISTS
( SELECT *
FROM b
WHERE a.id = b.id
)
Difference between and and where in joins
Firstly there is a semantic difference. When you have a join, you are saying that the relationship between the two tables is defined by that condition. So in your first example you are saying that the tables are related by cd.Company = table2.Name AND table2.Id IN (2728)
. When you use the WHERE clause, you are saying that the relationship is defined by cd.Company = table2.Name
and that you only want the rows where the condition table2.Id IN (2728)
applies. Even though these give the same answer, it means very different things to a programmer reading your code.
In this case, the WHERE clause is almost certainly what you mean so you should use it.
Secondly there is actually difference in the result in the case that you use a LEFT JOIN instead of an INNER JOIN. If you include the second condition as part of the join, you will still get a result row if the condition fails - you will get values from the left table and nulls for the right table. If you include the condition as part of the WHERE clause and that condition fails, you won't get the row at all.
Here is an example to demonstrate this.
Query 1 (WHERE):
SELECT DISTINCT field1
FROM table1 cd
LEFT JOIN table2
ON cd.Company = table2.Name
WHERE table2.Id IN (2728);
Result:
field1
200
Query 2 (AND):
SELECT DISTINCT field1
FROM table1 cd
LEFT JOIN table2
ON cd.Company = table2.Name
AND table2.Id IN (2728);
Result:
field1
100
200
Test data used:
CREATE TABLE table1 (Company NVARCHAR(100) NOT NULL, Field1 INT NOT NULL);
INSERT INTO table1 (Company, Field1) VALUES
('FooSoft', 100),
('BarSoft', 200);
CREATE TABLE table2 (Id INT NOT NULL, Name NVARCHAR(100) NOT NULL);
INSERT INTO table2 (Id, Name) VALUES
(2727, 'FooSoft'),
(2728, 'BarSoft');
In SQL / MySQL, what is the difference between ON and WHERE in a join statement?
WHERE
is a part of the SELECT
query as a whole, ON
is a part of each individual join.
ON
can only refer to the fields of previously used tables.
When there is no actual match against a record in the left table, LEFT JOIN
returns one record from the right table with all fields set to NULLS
. WHERE
clause then evaluates and filter this.
In your query, only the records from gifts
without match in 'sentgifts' are returned.
Here's the example
gifts
1 Teddy bear
2 Flowers
sentgifts
1 Alice
1 Bob
---
SELECT *
FROM gifts g
LEFT JOIN
sentgifts sg
ON g.giftID = sg.giftID
---
1 Teddy bear 1 Alice
1 Teddy bear 1 Bob
2 Flowers NULL NULL -- no match in sentgifts
---
SELECT *
FROM gifts g
LEFT JOIN
sentgifts sg
ON g.giftID = sg.giftID
WHERE sg.giftID IS NULL
---
2 Flowers NULL NULL -- no match in sentgifts
As you can see, no actual match can leave a NULL
in sentgifts.id
, so only the gifts that had not ever been sent are returned.
what is the difference between left join and left outer join?
The OUTER
keyword is optional across most popular SQL distributions, which means there is absolutely no difference between a LEFT JOIN
and a LEFT OUTER JOIN
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