SQL三连接表中的某些行可能不存在

Question

我有1个表包含主要行，我希望所有这些行。然后有两个表可能没有匹配的行，但如果这些表没有匹配的行，我仍然需要来自主表的行。

该查询有效，但是如果在所有3个表中没有一行，则不会返回任何结果。我使用MySQL与PHP。

SELECT PS_Info.* , PS_Guides.Guide, PS_User.UserID FROM PS_Info 
JOIN PS_Guides ON PS_Info.ID = PS_Guides.InfoID 
JOIN PS_User ON PS_Info.ID = PS_User.InfoID 
WHERE PS_Info.GameID = :ID AND PS_User.UserID = :UserID 

我想显示在PS_Info中的列表。然后，如果用户将它们设置为隐藏（PS_User = UserID，InfoID），我想隐藏这些项目中的一部分。如果PS_User中没有任何内容，那么他​​们没有设置这个项目被隐藏，所以我想显示它。 PS_Guides包含有关列表中每个项目的额外信息。并非所有的项目都包含额外的信息，所以如果这个表格中没有行，那么我就不会显示它。

Answer 1

SELECT PS_Info.* , PS_Guides.Guide, PS_User.UserID FROM PS_Info 
left JOIN PS_Guides ON PS_Info.ID = PS_Guides.InfoID //use left join 
left JOIN PS_User ON PS_Info.ID = PS_User.InfoID //use left join 
WHERE PS_Info.GameID = :ID AND PS_User.UserID = :UserID 

Answer 2

您正在寻找'left join'。

SELECT PS_Info.* , PS_Guides.Guide, PS_User.UserID FROM PS_Info 
LEFT JOIN PS_Guides ON PS_Info.ID = PS_Guides.InfoID 
LEFT JOIN PS_User ON PS_Info.ID = PS_User.InfoID 
WHERE PS_Info.GameID = :ID AND PS_User.UserID = :UserID