SQL LEFT JOIN：WHERE 和 AND 内部条件之间的区别 [重复]

Question

有什么区别

select t.*,
a.age
from t
left join a
on t.ID = a.ID and a.column > 10

和

select t.*,
a.age
from t
left join a
on t.ID = a.ID
where a.column > 10

？ 具体来说，当我将条件放在 AND 内与 WHERE 内加入主表的表上时有什么区别？

Answer 1

那么它们之间有什么区别呢？

举例说明：

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

姓名 |分数 :-------- | :---- 简·多伊 | 72.6 乔什莫 | 65.2 乔什莫 | 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

姓名 |分数 |分数 ID :----------- | :---- | :------ 简·多伊 | 72.6 | 2 乔什莫 | 65.2 | 1 乔什莫 | 69.6 | 3 诺玛纳尔逊 | 空 | 空

--
-- 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

姓名 |分数 |分数 ID :----------- | :---- | :------ 简·多伊 | 72.6 | 2 乔什莫 | 69.6 | 3 诺玛纳尔逊 | 空 | 空

--
-- 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

姓名 |分数 :-------- | :---- 简·多伊 | 72.6 乔什莫 | 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

姓名 |分数 :-------- | :---- 简·多伊 | 72.6 乔什莫 | 69.6

db小提琴here

您是否注意到 WHERE 子句中的条件如何使 LEFT JOIN 表现得像 INNER JOIN？

Answer 2

跟left join有区别

left join 行的条件为column > 10 将在那里填充空值

带有where条件的行会被过滤掉

跟inner join没有区别

示例：

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

输出：

id  dummy       age
1   pippo       NULL
2   pluto       NULL
3   paperino    41
4   ciccio      15
5   caio        NULL
5   sempronio   NULL

虽然

select t.*, a.age
from @t t
left join @a a on t.ID = a.ID
where a.col > 10

输出：

id  dummy       age
3   paperino    41
4   ciccio      15

因此，使用LEFT JOIN，您将始终从第一个表中获得所有行

如果连接条件为真，您将从连接表中获取填充其值的列，如果条件为假，则它们的列将为NULL

使用WHERE 条件，您将只获得符合条件的行。