sql-server - 一个表中的多个列引用回一个表获取它们的值

Question

我有一个表table_One，其中有多个columns(Column1, Column2, Column3, Column4.....) 两个引用 ( Contains PK values for Table_two) 到另一个表Table_Two。有没有什么有效的方法来加入这两个表，而不是将 Table_one 加入到 table_Two 多次.br/>

两个表和所需结果集的结构如下。Table_One

Answer 1

您是否尝试过为 table2 表起别名并将其两次加入 Table_One，如下所示？

SELECT
    t1.PrimaryKey,
    c1.ColumnA AS Column1,
    c2.ColumnA AS Column2
FROM Table_One t1
JOIN Table_two c1 ON t1.Column1 = c1.ID
JOIN Table_two c2 ON t1.Column2 = c2.ID;

Answer 2

Select Table_One.PrimaryKey,
       T2_Column1.ColumnA As Column1,
       T2_Column2.ColumnA As Column2
From   Table_One
       Inner Join Table_Two As T2_Column1
         On Table_One.Column1 = T2_Column1.ID
       Inner Join Table_Two As T2_Column2
         On Table_One.Column2 = T2_Column2.Id

基本上，您加入表 2 两次。当你这样做时，你必须至少给其中一个起别名，这样 SQL 才不会混淆。作为实践问题，通常最好将两者都取别名，这样当您在 6 个月后再次阅读此代码时，会更容易理解。

Answer 3

以下解决方案（SQLFiddle）仅从第二个表中读取行一次：

SET STATISTICS IO ON;
...
PRINT 'Test #1'
SELECT  *
FROM
(
    SELECT  ca.PrimaryKey, ca.[Type], y.ColumnA
    FROM    @Table1 x
    UNPIVOT( Value FOR [Type] IN ([Column1], [Column2]) ) ca
    INNER MERGE /*HASH*/ JOIN @Table2 y ON ca.Value = y.ID
) src
PIVOT( MAX(src.ColumnA) FOR src.[Type] IN ([Column1], [Column2]) ) pvt
PRINT 'End of Test #1'

结果：

Test #1
PrimaryKey Column1   Column2
---------- --------- -------
1          ALPHA     CHARLIE
2          BETA      DELTA
3          CHARLIE   ALPHA
4          DELTA     CHARLIE
5          ALPHA     DELTA
6          CHARLIE   ALPHA
7          ALPHA     DELTA
8          DELTA     CHARLIE

Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#65B6F546'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#61E66462'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

End of Test #1