sql 左鏈接查詢條件and與where

 用一條SQL查詢一張表中不一樣級別的字段(字段名稱相同)。

需求：用一條SQL同時查出城市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

版本二查詢出來的結果

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'

查詢出的結果同版本二同樣

別人寫的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'

查詢出的結果同版本一 同樣

別人寫的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

仔細觀察上述SQL會發現

我寫的版本三和別人寫的版本一惟一的區別在於on後面的條件鏈接關鍵詞，我用的是ON，別人用的是where。這裏延伸出一個問題:

left join on後面跟and和where的區別

1.on後面and 條件表示先過濾以後，再鏈接

2.on後面where條件表示先關聯以後，再過濾 至關內連了。。