1.在没有设置默认值的情况下:

SELECT userinfo.id, user_name, role, adm_regionid, region_name , create_time
FROM userinfo
LEFT JOIN region ON userinfo.adm_regionid = region.id
结果:

 

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设置显示默认值:

SELECT userinfo.id, user_name, role, adm_regionid, IFNULL(region_name,0) as region_name, create_time
FROM userinfo
LEFT JOIN region ON userinfo.adm_regionid = region.id
即:如果region_name一列的值为空,则显示为0,结果为:

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