sql 左链接查询条件and与where

 用一条SQL查询一张表中不一样级别的字段(字段名称相同)。

需求：用一条SQL同时查出城市id、城市name、省份id和省份name。

版本一以下

SELECT
    tlUp.LOCATION_ID as cityId,
    tlUp.LOCATION_NAME as cityName,
    tlDown.LOCATION_ID as provinceId,
    tlDown.LOCATION_NAME as provinceName
FROM
(SELECT
    tl.LOCATION_ID,
    tl.LOCATION_NAME,
    tl.SUPER_LOCATION_ID
FROM
    t_location tl) tlUp,t_location tlDown
WHERE
    tlUp.SUPER_LOCATION_ID = tlDown.LOCATION_ID

版本二

SELECT

    tlOne.LOCATION_ID as cityId,

    tlOne.LOCATION_NAME as cityName,

    tlTwo.LOCATION_ID as provinceId,

    tlTwo.LOCATION_NAME as provinceName

FROM

    t_location tlOne

LEFT JOIN 

    t_location tlTwo 

ON

    tlOne.SUPER_LOCATION_ID = tlTwo.LOCATION_ID

版本二查询出来的结果

SQL版本三

SELECT

    tlOne.LOCATION_ID as cityId,

    tlOne.LOCATION_NAME as cityName,

    tlTwo.LOCATION_ID as provinceId,

    tlTwo.LOCATION_NAME as provinceName

FROM

    t_location tlOne

LEFT JOIN 

    t_location tlTwo 

ON

    tlOne.SUPER_LOCATION_ID = tlTwo.LOCATION_ID

AND

    tlOne.LOCATION_LEVEL = '2'

查询出的结果同版本二同样

别人写的SQL版本一以下

SELECT

    tlOne.LOCATION_ID as cityId,

    tlOne.LOCATION_NAME as cityName,

    tlTwo.LOCATION_ID as provinceId,

    tlTwo.LOCATION_NAME as provinceName

FROM

    t_location tlOne

LEFT JOIN 

    t_location tlTwo 

ON

    tlOne.SUPER_LOCATION_ID = tlTwo.LOCATION_ID

WHERE

    tlOne.LOCATION_LEVEL = '2'

查询出的结果同版本一 同样

别人写的SQL版本二以下

SELECT

    tlUp.LOCATION_ID as cityId,

    tlUp.LOCATION_NAME as cityName,

    tlDown.LOCATION_ID as provinceId,

    tlDown.LOCATION_NAME as provinceName

FROM t_location tlUp,t_location tlDown

WHERE

    tlUp.SUPER_LOCATION_ID = tlDown.LOCATION_ID

仔细观察上述SQL会发现

我写的版本三和别人写的版本一惟一的区别在于on后面的条件链接关键词，我用的是ON，别人用的是where。这里延伸出一个问题:

left join on后面跟and和where的区别

1.on后面and 条件表示先过滤以后，再链接

2.on后面where条件表示先关联以后，再过滤 至关内连了。。