sql 左连接查询条件and与where

摘要: sql 左连接查询条件and与where

用一条SQL查询一张表中不同级别的字段(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。

版本一如下

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二一样

别人写的SQL版本一如下

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 一样

别人写的SQL版本二如下

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唯一的区别在于on后面的条件连接关键词，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 条件表示先过滤之后，再连接
2.on后面where条件表示先关联之后，再过滤 相当内连了。。