Question

假设我有一个包含各种类型的编码表，例如：

    ID  EncodingName
    ------------------
    1   UTF-8
    2   ISO-8859-1

另一个 EncodingMapping ，它使用这些ID来跟踪要转换为From和To的编码：

    ID  ItemId_FK  EncodingFromId_FK  EncodingToId_FK
    -------------------------------------------------
    1   45         2                  1  
    2   78         1                  2

我想创建一个SQL语句，当ItemId_FK = 45时（例如）创建以下结果：

   FromEncoding  ToEncoding
   -------------------------
   ISO-8859-1    UTF-8

看起来它很简单，但我不能通过以这种格式返回单行来获得JOIN工作。

到目前为止（这是错的）：

   SELECT EncodingName As FromEncoding, EncodingName As ToEncoding
   FROM Encoding
   LEFT JOIN EncodingMapping As em
   ON Encoding.ID = em.EncodingFromId_FK OR Encoding.ID = em.EncodingToId_FK
   WHERE ItemId_FK = 45

Answer 1

关闭，但不是雪茄：

SELECT 
  FromEnc.EncodingName AS FromEncoding, 
  ToEnc.EncodingName AS ToEncoding
FROM EncodingMapping
INNER JOIN Encoding AS FromEnc
  ON FromEnc.ID=EncodingMapping.EncodingFromId_FK
INNER JOIN Encoding AS ToEnc
  ON ToEnc.ID=EncodingMapping.EncodingToId_FK
WHERE EncodingMapping.ItemId_FK = 45

Answer 2

你只需要加入同一个表：

SELECT from.EncodingName AS FromEncoding,
  to.EncodingName AS ToEncoding
FROM EncodingMapping AS m
LEFT JOIN Encoding AS from ON m.EncodingFromId_FK=from.ID
LEFT JOIN Encoding AS to ON m.EncodingToId_FK=to.ID
WHERE ItemId_FK=45

注意，在简单ID列上包含_FK扩展名似乎有点多余。

Answer 3

你应该有两个连接。一个用于“FromEncoding”，另一个用于“ToEncoding”。

像这样：

Select  FromEncoding.Title
,   ToEncoding.Title
    From    EncodingOrder
    Inner   Join
        Encoding        As  FromEncoding
    On  FromEncoding.Id     =   EncodingOrder.From_Encoding_Id
    Inner   Join
        Encoding        As  ToEncoding
    On  ToEncoding.Id       =   EncodingOrder.To_Encoding_Id

干杯

MySQL JOIN用另一个表中的值替换ID

3 个答案: