MySQL LEFT JOIN 3表

Question

我有3张桌子:

Persons (PersonID, Name, SS)
Fears (FearID, Fear)
Person_Fear (ID, PersonID, FearID)

现在我想列出每个人与他们有任何恐惧(可能是多重恐惧,但也可能没有).即使一个人没有与他们有关联的恐惧,也必须显示人员表.

我想我需要做LEFT JOIN,但我的代码似乎不起作用:

SELECT persons.name, 
       persons.ss, 
       fears.fear 
FROM   persons 
       LEFT JOIN fears 
              ON person_fear.personid = person_fear.fearid 

我在这做错了什么？

Answer 1

您试图加入Person_Fear.PersonID到Person_Fear.FearID-这并不是真正意义.你可能想要这样的东西:

SELECT Persons.Name, Persons.SS, Fears.Fear FROM Persons
LEFT JOIN Person_Fear
    INNER JOIN Fears
    ON Person_Fear.FearID = Fears.FearID
ON Person_Fear.PersonID = Persons.PersonID

这通过中间表连接Persons到.因为之间的连接和是一个,你会得到所有记录.FearsPerson_FearPersonsPerson_FearLEFT JOINPersons

或者:

SELECT Persons.Name, Persons.SS, Fears.Fear FROM Persons
LEFT JOIN Person_Fear ON Person_Fear.PersonID = Persons.PersonID
LEFT JOIN Fears ON Person_Fear.FearID = Fears.FearID

Answer 2

试试这个

    SELECT p.Name, p.SS, f.Fear 
    FROM Persons p 
    LEFT JOIN Person_Fear fp 
    ON p.PersonID = fp.PersonID
    LEFT JOIN Fear f
    ON f.FearID = fp.FearID

Answer 3

试试这个绝对有效.

SELECT p.PersonID AS person_id,
   p.Name, p.SS, 
   f.FearID AS fear_id,
   f.Fear 
   FROM person_fear AS pf 
      LEFT JOIN persons AS p ON pf.PersonID = p.PersonID 
      LEFT JOIN fears AS f ON pf.PersonID = f.FearID 
   WHERE f.FearID = pf.FearID AND p.PersonID = pf.PersonID

Answer 4

Select 
    p.Name,
    p.SS,
    f.fear
From
    Persons p
left join
        Person_Fear pf
    inner join
        Fears f
    on
        pf.fearID = f.fearID
 on
    p.personID = pf.PersonID

Answer 5

Select Persons.Name, Persons.SS, Fears.Fear
From Persons
LEFT JOIN Persons_Fear
ON Persons.PersonID = Person_Fear.PersonID
LEFT JOIN Fears
ON Person_Fear.FearID = Fears.FearID;