任何列不为空时对多列的唯一约束

Question

我正在尝试在允许多个 (null, null) 但不允许多个 ("a", null) 的 2 列上添加唯一约束。我写了以下 SQL 语句，但出现错误

Incorrect syntax near the keyword 'with'

SQL 语句：

CREATE UNIQUE NONCLUSTERED INDEX [UQ] 
ON [dbo].[MyTable] ([Column_A], [Column_B])
WHERE [Column_A] IS NOT NULL OR [Column_B] IS NOT NULL
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY];

然而在

 WHERE [Column_A] IS NOT NULL OR [Column_B] IS NOT NULL

当我用 AND 替换 OR 时，不再有语法错误。但我想要的是 OR 逻辑，而不是 AND 因为使用 AND 允许多个 ("a", null) 条目。

那么为什么使用OR会出现不正确的语法错误呢？谢谢。

Answer 1

有点笨拙，但如果重点是强制唯一性（NULL/NULL 除外）并且您很高兴拥有两个索引，则可以通过将 NOT NULL 过滤器放在单独的索引上来实现，例如,

CREATE UNIQUE NONCLUSTERED INDEX
[UQ_A] ON [dbo].[MyTable]
(
    [Column_A],
    [Column_B]
)
WHERE [Column_A] IS NOT NULL
GO

CREATE UNIQUE NONCLUSTERED INDEX
[UQ_B] ON [dbo].[MyTable]
(
    [Column_A],
    [Column_B]
)
WHERE [Column_B] IS NOT NULL
GO

虽然唯一性检查不是必需的，但您可能希望更改第二个索引中字段的顺序以获得索引的额外优势（取决于这些字段在您的应用程序中的使用方式 - 它可能最好在第一个索引中交换它们），例如，

CREATE UNIQUE NONCLUSTERED INDEX
[UQ_B] ON [dbo].[MyTable]
(
    [Column_B],
    [Column_A] 
)
WHERE [Column_B] IS NOT NULL
GO

Answer 2

根据 Martin Smith 的评论，我认为我可以通过以下方式实现它：

CREATE VIEW [dbo].[TableAView]
WITH SCHEMABINDING AS
SELECT
[ColumnA] = [ColumnA],
[ColumnB] = [ColumnB]
FROM [dbo].[TableA]
WHERE [ColumnA] IS NOT NULL OR [ColumnB] IS NOT NULL;
GO

CREATE UNIQUE CLUSTERED INDEX
[UQ] ON [dbo].[TableAView]
(
    [ColumnA],
    [ColumnB]
)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
ON [PRIMARY];
GO

任何列不为空时对多列的唯一约束

Unique constraint on multiple columns when any columns are not null

sql-server

unique-constraint