MySQL join 与where的执行顺序

-- 写法ok 先过滤后join
select * from(select * from u1 where u1.id=3)t join u2  on t.id=u2.id;
id  name id(1) sex
3	c	3	male

-- 写法ok 先join后过滤
select * from u1 join u2 on u1.id=u2.id where u1.id=3;
id  name id(1) sex
3	c	3	male

-- 写法ok
select * from u1 join u2 on u1.id=u2.id and u1.id=3;
id  name id(1) sex
3	c	3	male

-- 此种写法不可以 报错
select * from u1 where u1.id=3 join u2 on u1.id=u2.id;
-- You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server version for the right syntax to use near 'join u2 on u1.id=u2.id' at line 1

select * from u1 left join u2 on u1.id=u2.id where u1.id=3;
id  name id(1) sex
3	c	3	male

select * from u1 left join u2 on u1.id=u2.id and u1.id=3;
id  name id(1) sex
3	c	3	male
1	a	null null	
2	b	null null

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