Question

在左加入时，我对这些事情提出了一些问题。

我有这些表格：

dbo.ARoom_type

id | roomtype_name |  isactive
---+---------------+-----------
1  | Presidential  |    1
2  |   De Luxe     |    1

dbo.AAmenities

id | amenities_name | isactive
---+----------------+----------
1  |   Internet     |   1
2  |   Cable TV     |   1
3  |   Bath Tub     |   1
4  |   Air-con      |   1
5  |   Telephone    |   1

dbo.AAmenities_details：

id | roomtype_id | amenities_id
---+-------------+--------------
1  |      1      |      1
2  |      1      |      2
3  |      1      |      4
4  |      2      |      1

我试过了这个查询

SELECT
    B.id as 'details_id',
    C.roomtype_name,
    A.amenities_name
FROM 
    AAmenities AS A
LEFT JOIN 
    AAmenities_details AS B ON B.amenities_id = A.id
LEFT JOIN 
    ARoom_type as C ON C.id = B.roomtype_id

这就是我得到的

结果：

details_id | roomtype_name  | amenities_name 
-----------+----------------+----------------
    1      |   Presidential |   Internet
    2      |   De Luxe      |   Internet
    3      |   Presidential |   Cable TV
    4      |   NULL         |   Bath Tub
    5      |   Presidential |   Air-con
    6      |   NULL         |   Telephone

我可以获得所有的便利设施，并获得所有的房型名称，这是我想要的但我希望这是房型，所以如果总统只有总统和我试过这个查询

  @id INT
  AS
  BEGIN TRY

  SELECT
      B.id AS 'details_id',
      C.roomtype_name,
      A.amenities_name
  FROM
      AAmenities AS A
  LEFT JOIN 
      AAmenities_details AS B ON B.amenities_id = A.id
  LEFT JOIN 
      ARoom_type AS C ON C.id = B.roomtype_id
  WHERE 
      C.id = @id

我只能得到这些结果

  id  |  roomtype_name  | amenities_name
  ----+-----------------+----------------
   1  |    Presidential |   Internet
   2  |    Presidential |   Cable TV
   4  |    Presidential |   Air-con

这是结果，当我想要的是它会是这样的：

  details_id | roomtype_name  | amenities_name 
  -----------+----------------+-----------------
      1      |   Presidential |   Internet
      3      |   Presidential |   Cable TV
      4      |   NULL         |   Bath Tub
      5      |   Presidential |   Air-con
      6      |   NULL         |   Telephone

什么是正确的查询，当我想要id并仍然得到所有的amenities_name，即使我的roomtype_name的值为NULL，如何LEFT JOIN答案？非常感谢你

Answer 1

您需要AAmenities_details的所有数据，然后尝试

SELECT
   B.id as 'details_id',
   C.roomtype_name,
   A.amenities_name
 FROM 
 AAmenities_details as B
 LEFT JOIN AAmenities  as A
   on B.amenities_id = A.id
 LEFT JOIN ARoom_type as C
   on C.id = B.roomtype_id
 WHERE B.roomtype_id = @id

但我认为每个AAmenities_details都必须有AAmenities 因此，您可以在INNER JOIN和AAmenities_details

之间使用AAmenities

SELECT
   B.id as 'details_id',
   C.roomtype_name,
   A.amenities_name
 FROM 
 AAmenities_details as B
 INNER JOIN AAmenities  as A
   on B.amenities_id = A.id
 LEFT JOIN ARoom_type as C
   on C.id = B.roomtype_id
 WHERE B.roomtype_id = @id

在存储过程中LEFT JOIN的正确方法

1 个答案: