Question

我有以下查询：

select *
from House h
join Users u
on h.ID_COORD = u.ID
join PA_USERS createdBy
on h.USR_N_ID_CREATION = createdBy.ID
join Contacts c
on h.ConID= c.ID
left join Price p
on h.PriD = p.ID
join Car c
on h.CID = c.ID
left join Person p
on c.PID = p.ID
left join Assistance a
on c.AID = a.ID
left join Travel t
on p.SER_N_ID = t.SER_N_ID
left join Travel at
on a.SER_N_ID = at.SER_N_ID
where h.LEA_N_ID = 9132
and t.lang = 'en'
and p.lang = 'en'
and at.lang = 'en'

但是由于at.lang ='en'，查询没有返回任何数据。我在Assistance和Person上都需要左连接，并且在Travel表上的两个表都需要左连接。关于如何执行此操作的任何想法。

Answer 1

在WHERE中放置子句而不处理LEFT JOIN中的可空性将导致联接隐式变为INNER JOIN。以下面的简单示例为例：

SELECT *
FROM TableA A
     LEFT JOIN TableB B ON A.PID = B.FID
WHERE B.[name] = 'John';

这会导致LEFT JOIN隐式成为INNER JOIN，因为条款B.[name] = 'John'必须对所有行为真（无论{ {1}}已返回）。

通常，解决方案是将需求放在TableB子句中：

ON

因此，您的查询将变为：

SELECT *
FROM TableA A
     LEFT JOIN TableB B ON A.PID = B.FID
                       AND B.Name = 'John';

请注意，由于您已经重用了别名，因此无论如何也无法解决您的查询。例如SELECT * FROM House h JOIN Users u ON h.ID_COORD = u.ID JOIN PA_USERS createdBy ON h.USR_N_ID_CREATION = createdBy.ID JOIN Contacts c ON h.ConID = c.ID LEFT JOIN Price p ON h.PriD = p.ID JOIN Car c ON h.CID = c.ID LEFT JOIN Person p ON c.PID = p.ID AND p.lang = 'en' LEFT JOIN Assistance a ON c.AID = a.ID LEFT JOIN Travel t ON p.SER_N_ID = t.SER_N_ID AND t.lang = 'en' LEFT JOIN Travel at ON a.SER_N_ID = at.SER_N_ID AND at.lang = 'en' WHERE h.LEA_N_ID = 9132;和Person p。我没有为您解决这些错误，因为我怀疑我们这里没有完整的查询（由于别名重用，您的问题中的那个查询永远不会运行）。

Answer 2

外部联接的意思是：当没有匹配记录时，添加一个伪记录，将所有列设置为空。

因此，如果没有匹配的旅行记录，t.lang将为空；不会是“ en”。

将条件移至ON子句以使其起作用：

left join Travel t on p.SER_N_ID = t.SER_N_ID and t.lang = 'en'

等

Answer 3

使用LEFT JOIN时，ON条件不等同于where条件，因为前者仍将为您提供行，而后者则不会。 >

在您的位置删除此内容

and at.lang = 'en'

并将其添加到LEFT JOIN的打开位置：

left join Travel at
on a.SER_N_ID = at.SER_N_ID
and at.lang = 'en'

Answer 4

将AND at.lang = 'en'从WHERE子句移到LEFT JOIN子句。放入WHERE子句会将LEFT JOIN更改为INNER JOIN

SELECT  *
FROM
        House       h
JOIN
        Users       u ON h.ID_COORD                     = u.ID
JOIN
        PA_USERS    createdBy ON h.USR_N_ID_CREATION    = createdBy.ID
JOIN
        Contacts    c ON h.ConID                        = c.ID
LEFT JOIN
        Price       p ON h.PriD                         = p.ID
JOIN
        Car         c ON h.CID                          = c.ID
LEFT JOIN
        Person      p ON c.PID                          = p.ID
LEFT JOIN
        Assistance  a ON c.AID                          = a.ID
LEFT JOIN
        Travel      t ON p.SER_N_ID                     = t.SER_N_ID
LEFT JOIN
        Travel      at  ON a.SER_N_ID                   = at.SER_N_ID
                        AND at.lang         = 'en'
WHERE
    h.LEA_N_ID  = 9132
AND t.lang          = 'en'
AND p.lang          = 'en'
;

几个左联接在查询中

4 个答案: