SQL LEFT JOIN: difference between WHERE and condition inside AND [duplicate] - sql

Question

This question already has answers here:
SQL JOIN - WHERE clause vs. ON clause
(22 answers)
Closed 3 years ago.
What's the difference between
select t.*,
a.age
from t
left join a
on t.ID = a.ID and a.column > 10
and
select t.*,
a.age
from t
left join a
on t.ID = a.ID
where a.column > 10
?
Specifically, what's the difference when I put the condition on the table I am joining to the main table inside AND versus inside WHERE condition?

Answer 1

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.

Answer 2

So what's the difference between them?
An explanation through examples:
CREATE TABLE Students
(
StudentId INT PRIMARY KEY,
Name VARCHAR(100)
);
✓
CREATE TABLE Scores
(
ScoreId INT PRIMARY KEY,
ExamId INT NOT NULL,
StudentId INT NOT NULL,
Score DECIMAL(4,1) NOT NULL DEFAULT 0,
FOREIGN KEY (StudentId)
REFERENCES Students(StudentId)
);
✓
INSERT INTO Students
(StudentId, Name) VALUES
(11,'Joe Shmoe'),
(12,'Jane Doe'),
(47,'Norma Nelson');
✓
INSERT INTO Scores
(ScoreId, ExamId, StudentId, Score) VALUES
(1, 101, 11, 65.2),
(2, 101, 12, 72.6),
(3, 102, 11, 69.6);
✓
--
-- Using an INNER JOIN
--
-- Only Students that have scores
-- So only when there's a match between the 2 tables
--
SELECT stu.Name, sco.Score
FROM Students AS stu
INNER JOIN Scores AS sco
ON sco.StudentId = stu.StudentId
ORDER BY stu.Name
Name | Score
:-------- | :----
Jane Doe | 72.6
Joe Shmoe | 65.2
Joe Shmoe | 69.6
--
-- Using an LEFT JOIN
--
-- All Students, even those without scores
-- Those that couldn't be matched will show NULL's
-- for the fields from the joined table
--
SELECT stu.Name, sco.Score, sco.ScoreId
FROM Students AS stu
LEFT JOIN Scores AS sco
ON sco.StudentId = stu.StudentId
ORDER BY stu.Name
Name | Score | ScoreId
:----------- | :---- | :------
Jane Doe | 72.6 | 2
Joe Shmoe | 65.2 | 1
Joe Shmoe | 69.6 | 3
Norma Nelson | null | null
--
-- Using an LEFT JOIN
-- But with an extra criteria in the ON clause
--
-- All Students again.
-- That have scores >= 66
-- But also the unmatched without scores
--
SELECT stu.Name, sco.Score, sco.ScoreId
FROM Students AS stu
LEFT JOIN Scores AS sco
ON sco.StudentId = stu.StudentId
AND sco.Score >= 66.0
ORDER BY stu.Name
Name | Score | ScoreId
:----------- | :---- | :------
Jane Doe | 72.6 | 2
Joe Shmoe | 69.6 | 3
Norma Nelson | null | null
--
-- Using an LEFT JOIN
-- But with an extra criteria in the WHERE clause
--
-- Only students with scores >= 66
-- The WHERE filters out the unmatched.
--
SELECT stu.Name, sco.Score
FROM Students AS stu
LEFT JOIN Scores AS sco
ON sco.StudentId = stu.StudentId
WHERE sco.Score >= 66.0
ORDER BY stu.Name
Name | Score
:-------- | :----
Jane Doe | 72.6
Joe Shmoe | 69.6
--
-- Using an INNER JOIN
-- And with an extra criteria in the WHERE clause
--
-- Only Students that have scores >= 66
--
SELECT stu.Name, sco.Score
FROM Students AS stu
INNER JOIN Scores AS sco
ON sco.StudentId = stu.StudentId
WHERE sco.Score >= 66
ORDER BY stu.Name
Name | Score
:-------- | :----
Jane Doe | 72.6
Joe Shmoe | 69.6
db<>fiddle here
Did you notice how the criteria in the WHERE clause can make a LEFT JOIN behave like an INNER JOIN?