三表连接的选择性滤波器

Question

我有三张桌子表1、表2和表3。

表1与表2有一对多的关系。表2与表3有一对多的关系。

假设表1已经

-------------
t1key
-------------
a
b

Table2 has 
-------------
t1key | t2key
--------------
a        c
a        d
b        x
b        y


Table 3
------------
t2key | t3key
-------------
c      e
c      f
c      g
d      h
d      i
d      j
x      m
x      n
x      o
y      p
y      q
y      r

我希望连接这三个表，以便只返回表3中t2key的第一个唯一匹配

join的结果应该是

a c  e
a d  h
b x  m
b y  p

目前，我的应用程序连接了所有三个表，返回所有可能的行。我需要用上述条件过滤掉这个。

SELECT * FROM Table1 AS T1  
LEFT OUTER JOIN Table2 T2 ON T1.t1Key = T2.t1Key 
LEFT OUTER JOIN Table2 T3 ON T2.t2Key = T3.t2Key

Answer 1

DECLARE @table1 table
(
    t1 char(1)
)
INSERT INTO @table1
SELECT 'a' UNION ALL
SELECT 'b' 


DECLARE @table2 table
(
    t1 char(1),
    t2 char(1)
)
INSERT INTO @table2
SELECT 'a', 'c' UNION ALL
SELECT 'a', 'd' UNION ALL
SELECT 'b', 'x' UNION ALL
SELECT 'b', 'y' 

DECLARE @table3 table
(
    t1 char(1),
    t2 char(1)
)
INSERT INTO @table3
SELECT 'c', 'e' UNION ALL
SELECT 'c', 'f' UNION ALL
SELECT 'c', 'g' UNION ALL

SELECT 'd', 'h' UNION ALL
SELECT 'd', 'i' UNION ALL
SELECT 'd', 'j' UNION ALL

SELECT 'x', 'm' UNION ALL
SELECT 'x', 'n' UNION ALL
SELECT 'x', 'o' UNION ALL

SELECT 'y', 'p' UNION ALL
SELECT 'y', 'q' UNION ALL
SELECT 'y', 'r' 



SELECT 

    t1.t1,
    t2.t2,
    min(t3.t2)
FROM @table2 t2
INNER JOIN @table3 t3
ON t3.t1 = t2.t2
INNER JOIN @table1 t1
ON t1.t1 = t2.t1
GROUP BY t1.t1,
    t2.t2

Answer 2

首先需要使用GROUP BY子句并使用MIN（）

SELECT t2.t1key, t2.t2key, MIN(t3.t3key) AS t3key
FROM Table1 AS T1  
LEFT JOIN Table2 T2 ON T1.t1Key = T2.t1Key 
LEFT JOIN Table3 T3 ON T2.t2Key = T3.t2Key
GROUP BY t2.t1Key, t2.t2key;

如果只想显示每个表中可用的数据，请使用内部连接，而不是左连接<例如，假设表2中有一行“b”、“q”
如果使用左连接，则结果如下所示

| T1KEY | T2KEY |  T3KEY |
--------------------------
|     a |     c |      e |
|     a |     d |      h |
|     b |     q | (null) | <-- It will show null value too
|     b |     x |      m |
|     b |     y |      p |

使用内部连接将不会显示该记录

Answer 3

这样地：

SELECT 
  t2.t1Key, 
  t2.t2key, 
  MIN(t3.t3key) t3key
FROM Table1 AS T1  
INNER JOIN Table2 T2 ON T1.t1Key = T2.t1Key 
INNER JOIN Table3 T3 ON T2.t2Key = T3.t2Key
GROUP BY t2.t1Key, 
         t2.t2key;

这将为您提供：

| T1KEY | T2KEY | T3KEY |
-------------------------
|     a |     c |     e |
|     a |     d |     h |
|     b |     x |     m |
|     b |     y |     p |