对于这种内部联接，它们在逻辑上是等效的.但是，您可能会遇到join子句中的条件与where子句中的条件有所不同的情况.

For inner joins like this they are logically equivalent. However, you can run in to situations where a condition in the join clause means something different than a condition in the where clause.

作为一个简单的例子，想象一下您像这样进行左连接；

As a simple illustration, imagine you do a left join like so;

select x.id
from x
       left join y
         on x.id = y.id
;

在这里，我们要从x中获取所有行，而不管y中是否有匹配的id.现在说我们的加入条件越来越大-我们不仅在基于id的y中寻找匹配项，而且还在基于id_type的情况中寻找匹配项.

Here we're taking all the rows from x, regardless of whether there is a matching id in y. Now let's say our join condition grows - we're not just looking for matches in y based on the id but also on id_type.

select x.id
from x
       left join y
         on x.id = y.id
         and y.id_type = 'some type'
;

同样，这将给出x中的所有行，而不管y中是否存在匹配的(id，id_type).

Again this gives all the rows in x regardless of whether there is a matching (id, id_type) in y.

这是非常不同的:

select x.id
from x
       left join y
         on x.id = y.id
where y.id_type = 'some type'
;

在这种情况下，我们选择x的所有行，并尝试与y的行匹配.现在，对于y中不匹配的行，y.id_type将为null.因此，y.id_type ='some type'不被满足，因此那些没有匹配项的行将被丢弃，从而有效地将其转换为内部联接.

In this situation, we're picking all the rows of x and trying to match to rows from y. Now for rows for which there is no match in y, y.id_type will be null. Because of that, y.id_type = 'some type' isn't satisfied, so those rows where there is no match are discarded, which effectively turned this in to an inner join.

长话短说:对于内部联接而言，条件在哪里都无所谓，对于外部联接而言，则可以.

Long story short: for inner joins it doesn't matter where the conditions go but for outer joins it can.