SQL表连接中ON和WHERE子句之间的区别

select e.last_name,e.hire_date
from employees e join employees m
on (m.last_name = 'Davies')
and (e.hire_date > m.hire_date);

select e.last_name,e.hire_date
from employees e join employees m
on (m.last_name = 'Davies')
where (e.hire_date > m.hire_date);

select e.last_name,e.hire_date
from employees e join employees m
on (e.hire_date > m.hire_date)
where (m.last_name = 'Davies');

这三个陈述具有相同的结果.除了不能单独使用,不使用on的事实,是否有任何特殊的理由在表连接中使用哪里？

解决方法

通常,如果要使用内部联接,则应该看到相同的结果,但是一旦开始使用LEFT联接,结果将会更改.

看看下面的例子

SQL Fiddle DEMO

And have a look at the following article (very explanatory)

编辑@ShannonSeverance

架构和测试数据

CREATE TABLE Table1 (
  ID INT,Val VARCHAR(20)
 );

INSERT INTO Table1 VALUES (1,'a');
INSERT INTO Table1 VALUES (2,'a');

CREATE TABLE Table2 (
  ID INT,Val VARCHAR(20)
 );

INSERT INTO Table2 VALUES (1,'a');

和测试

SELECT t1.ID,t1.Val,t2.ID ID2,t2.Val Val2
FROM Table1 t1 INNER JOIN
Table2 t2 ON t1.ID = t2.ID AND t1.Val = t2.Val;

SELECT  t1.ID,t2.Val Val2
FROM Table1 t1,Table2 t2 
WHERE t1.ID = t2.ID
 AND t1.Val = t2.Val;

SELECT  t1.ID,t2.Val Val2
FROM Table1 t1 LEFT JOIN
Table2 t2 ON t1.ID = t2.ID  AND t1.Val = t2.Val;

SELECT  t1.ID,t2.Val Val2
FROM Table1 t1 LEFT JOIN
Table2 t2 ON t1.ID = t2.ID  
WHERE t1.Val = t2.Val;

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