SQL JOIN with COALESCE 的条件是重复行

Question

我正在尝试将两个 table 连接在一起。第一个 table 包含我不想复制的数据记录。第二个 table 我加入第一个 table 以通过不同的 [profileId] 和 [role] 查找 [value]。第二个 table 中的 [profileId]、[role] 列对组合具有唯一约束，但 [role] 有时可以为 NULL，在这种情况下，我将该值视为该配置文件的默认值。

如何在不复制行且不使用多个左连接的情况下将这些 table 连接在一起？我的实际查询比示例更复杂。

请参阅下面的示例。

DECLARE @temp TABLE ([profileId] int, [role] int)
DECLARE @temp2 TABLE ([profileId] int, [role] int, [value] nvarchar(50))

INSERT INTO @temp ([profileId], [role]) VALUES (1, 1)
INSERT INTO @temp ([profileId], [role]) VALUES (1, 2)
INSERT INTO @temp ([profileId], [role]) VALUES (2, 1)
INSERT INTO @temp ([profileId], [role]) VALUES (2, 2)
INSERT INTO @temp2 ([profileId], [role], [value]) VALUES (1, 1, 'MATCH')
INSERT INTO @temp2 ([profileId], [role], [value]) VALUES (1, NULL, 'DEFAULT1')
INSERT INTO @temp2 ([profileId], [role], [value]) VALUES (2, NULL, 'DEFAULT2')

SELECT
    T1.[profileId],
    T1.[role],
    T2.value
FROM
    @temp T1
    JOIN @temp2 T2 ON T1.profileId = T2.profileId AND COALESCE(T2.[role], T1.[role]) = T1.[role]

这给了我（我明白为什么）

================================
| profileId | role  |  value   |
================================
|     1     |   1   |  MATCH   |
--------------------------------
|     1     |   1   | DEFAULT1 |
--------------------------------
|     1     |   2   | DEFAULT1 |
--------------------------------
|     2     |   1   | DEFAULT2 |
--------------------------------
|     2     |   2   | DEFAULT2 |
================================

虽然我想要

================================
| profileId | role  |  value   |
================================
|     1     |   1   |  MATCH   |
--------------------------------
|     1     |   2   | DEFAULT1 |
--------------------------------
|     2     |   1   | DEFAULT2 |
--------------------------------
|     2     |   2   | DEFAULT2 |
================================

Answer 1

如果您知道 DEFAULT 值，请使用 left join。

SQL Fiddle Demo

SELECT
    T1.[role],
    COALESCE(T2.value, 'DEFAULT') as value
FROM
    temp T1
LEFT JOIN temp2 T2 
       ON T1.[role] = T2.[role];

否则

SELECT
    T1.[role],
    COALESCE(T2.value, (SELECT value 
                        FROM temp2 
                        WHERE role is NULL and temp2.profileID = T1.profileID)) as value
FROM
    temp T1
LEFT JOIN temp2 T2 
       ON T1.[role] = T2.[role]
      AND T1.[profileID] = T2.[profileID]
       ;

Answer 2

这个 SQL 工作正常：

SELECT
    T1.[role],
    Value = coalesce(max(nullif(T2.value,'DEFAULT')),'DEFAULT')
FROM
    @temp T1
    JOIN @temp2 T2 ON COALESCE(T2.[role], T1.[role]) = T1.[role]
group by
    T1.[role]
;

Answer 3

您可以使用 APPLY 和 TOP:

SELECT
    t.profileId,
    t.role,
    x.value
FROM @temp t
OUTER APPLY(
    SELECT TOP 1 value
    FROM @temp2
    WHERE
        profileId = t.profileId
        AND (role = t.role OR role IS NULL)
    ORDER BY 
        CASE WHEN role IS NOT NULL THEN 0 ELSE 1 END
)x

SQL JOIN with COALESCE 的条件是重复行

SQL JOIN with COALESCE in condition is duplicating rows

sql-server

join

duplicates

left-join

coalesce