SELECT
a.foo
b.bar
c.foobar
FROM tableOne AS a
INNER JOIN tableTwo AS b ON a.pk = b.fk
LEFT JOIN tableThree AS c ON b.pk = c.fk
WHERE a.foo = 'something'
AND c.foobar = 'somethingelse'

将where子句放在where子句后似乎将左连接变为内连接.我看到的行为是如果tableThree中没有“somethingelse”,那么将返回0行.

如果我将c.foobar =’somethingelse’移动到join子句中,则存储的连接将像左连接那样运行.

SELECT
    a.foo
    b.bar
    c.foobar
    FROM tableOne AS a
    INNER JOIN tableTwo AS b ON a.pk = b.fk
    LEFT JOIN tableThree AS c ON b.pk = c.fk
    AND c.foobar = 'somethingelse'
    WHERE a.foo = 'something'

有人可以指出一些文件,说明为什么会这样吗？非常感谢你