Question

让我们说我有两张桌子：

table 1 columns: A,B,C
table 2 columns: D,E,F

JOIN Conditions
table1.A = table2.D, table1.B=table2.E, table1.C = table2.F

任何列都可以具有空值。我希望将null值视为任何值的匹配项。（即，如果A = D且B = E，但C为空.F有两个值，A = D，B = E.两行应包括在结果表中）

Table1:                                Table2:
A:         B:       C:                D:           E:          F:
a          b         c                 a           b           c
e          <null>    f                 e           a           f
o          <null>    q                 e           b           f             
h          i         j                 h           i           k
l          <null>    m                 l           a           m
o          <null>    q                 o           <null>      q


results(show table 2 values...):
D:   E:     F:
a    b      c
e    a      f
e    b      f
o   <null>  q
l    a      m

Answer 1

我会使用类似于JNevill的方法，但我会发现这样做是为了让我更有意义：

ON (Table1.A=Table2.D OR Table1.A IS NULL OR Table2.D IS NULL)
AND (Table1.B=Table2.E OR Table1.B IS NULL OR Table2.E IS NULL)
AND (Table1.C=Table2.F OR Table1.C IS NULL OR Table2.F IS NULL)

Answer 2

你可以写出这个条件。我觉得可能有一些方法可以做到这一点更多偷偷摸摸/更简洁，但这应该让你在球场：

ON
(table1.A = table2.D AND table1.B = table2.E AND (table1.C = table2.F OR table1.C IS NULL))
OR (table1.A = table2.D AND (table1.B = table2.E OR table1.B IS NULL) AND table1.C = table2.F)
OR ((table1.A = table2.D OR table1.A IS NULL) AND table1.B = table2.E AND table1.C = table2.F)

Answer 3

我认为这可以满足您的需求。 SQL实际上会相当有效地优化它。

DECLARE @t1 AS TABLE
  (
       [A]   NVARCHAR(25)
       , [B] NVARCHAR(25)
       , [C] NVARCHAR(25)
  );
DECLARE @t2 AS TABLE
  (
       [D]   NVARCHAR(25)
       , [E] NVARCHAR(25)
       , [F] NVARCHAR(25)
  );

INSERT INTO @t1
            ([A]
             , [B]
             , [C])
VALUES      (N'one',N'one',N'one'),
            (N'two',N'two',NULL),
            (N'three',N'three',N'three');

INSERT INTO @t2
            ([D]
             , [E]
             , [F])
VALUES      (N'one',N'one',N'one'),
            (N'two',N'two',NULL),
            (N'three',N'three',NULL);

SELECT *
FROM   @t1 AS [t1]
       JOIN @t2 AS [t2]
         ON [t1].[A] = [t2].[D]
            AND [t1].[B] = [t2].[E]
            AND [t1].[C] = [t2].[F];

SELECT *
FROM   @t1 AS [t1]
       JOIN @t2 AS [t2]
         ON
       (
           [t1].[A] = [t2].[D]
            OR [t1].[A] IS NULL
            OR [t2].[D] IS NULL
        )
       AND
       (
           [t1].[B] = [t2].[E]
            OR [t1].[B] IS NULL
            OR [t2].[E] IS NULL
        )
       AND
       (
           [t1].[C] = [t2].[F]
            OR [t1].[C] IS NULL
            OR [t2].[F] IS NULL
        )
;

我想加入两个表，我希望null值等同于ALL

3 个答案: