SQL加入国家城市社区显示记录甚至没有匹配

Question

我一直试图在MySQL上搞定。

它们存储在3个不同的表（国家，城市，社区）中，可以通过country.country_link = city.country_link和city.city_link = community.city_link

连接在一起

有时需要添加国家/地区过滤器，如下所示

我试过这个，但仍无济于事：

SELECT
tbl_country.country_link AS country_link,
    tbl_city.city_link as city_link,
    tbl_community.community_link
FROM
tbl_country
LEFT JOIN
tbl_city ON tbl_country.country_link = tbl_city.country_link
LEFT JOIN
tbl_community ON tbl_city.city_link = tbl_community.city_link
WHERE
tbl_city.country_code = 'AE'
UNION
SELECT
tbl_country.country_link AS country_link,
    tbl_city.city_link as city_link,
    tbl_community.community_link
FROM
tbl_country
LEFT JOIN
tbl_city ON tbl_country.country_link = tbl_city.country_link
LEFT JOIN
tbl_community ON tbl_city.city_link = tbl_community.city_link
WHERE
tbl_city.city_link IS NULL AND
tbl_community.community_link IS NULL AND
    tbl_city.country_code = 'AE'
UNION
SELECT
tbl_country.country_link AS country_link,
    tbl_city.city_link as city_link,
    tbl_community.community_link
FROM
tbl_country
LEFT JOIN
tbl_city ON tbl_country.country_link = tbl_city.country_link
LEFT JOIN
tbl_community ON tbl_city.city_link = tbl_community.city_link
WHERE
tbl_community.community_link IS NULL AND
tbl_community.community_link IS NULL AND
    tbl_city.country_code = 'AE'

Answer 1

根据上面的评论，我在你的sql脚本中进行了更改。观察现在在连接级别条件下移动的WHERE子句条件。

SELECT tbl_country.country_link AS country_link,
       tbl_city.city_link as city_link,
       tbl_community.community_link
FROM tbl_country
LEFT JOIN tbl_city ON tbl_country.country_link = tbl_city.country_link AND tbl_city.country_code = 'AE'
LEFT JOIN tbl_community ON tbl_city.city_link = tbl_community.city_link

UNION
SELECT tbl_country.country_link AS country_link,
       tbl_city.city_link as city_link,
       tbl_community.community_link
FROM tbl_country
LEFT JOIN tbl_city ON tbl_country.country_link = tbl_city.country_link
LEFT JOIN tbl_community ON tbl_city.city_link = tbl_community.city_link AND
          tbl_community.community_link IS NULL AND tbl_city.country_code = 'AE'

UNION
SELECT tbl_country.country_link AS country_link,
       tbl_city.city_link as city_link,
       tbl_community.community_link
FROM tbl_country
LEFT JOIN tbl_city ON tbl_country.country_link = tbl_city.country_link
LEFT JOIN tbl_community ON tbl_city.city_link = tbl_community.city_link AND
                           tbl_community.community_link IS NULL AND 
                           tbl_community.community_link IS NULL AND
                           tbl_city.country_code = 'AE'