Question

当我在具有嵌入式'％'字符的查询中使用LIKE子句时，查询不会返回预期的行。但是如果我使用常量字符串，它会按预期工作。当我使用 StartsWith （LIKE 'x%'）而非包含（LIKE '%x%'）时，这只会导致问题。

我做错了什么！??！

Repro代码（最终查询是意外的）：

DECLARE @p__linq__StartsWith nvarchar 
SET @p__linq__StartsWith = N'm%'
DECLARE @p__linq__Contains nvarchar
SET @p__linq__Contains = N'%m%'

-- OK: Returns "Me" and "Not Me"
SELECT * FROM
    (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE N'%m%'

-- OK: Returns "Me"
SELECT * FROM
    (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE N'm%'


-- OK: Returns "Me" and "Not Me"
SELECT * FROM
    (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE @p__linq__Contains

-- Unexpected: Returns nothing
-- And why doesn't the last query do the same thing???
SELECT * FROM 
    (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE @p__linq__StartsWith

环境：实体框架5针对SQL Server 2012 SP1生成的SQL。检查SQL 2000和2008R2 SP2上发生的相同行为。

Answer 1

在代码末尾添加：

print @p__linq__StartsWith

结果解释了意外的结果。现在试试这段代码：

DECLARE @p__linq__StartsWith nvarchar(5)
SET @p__linq__StartsWith = N'm%'
DECLARE @p__linq__Contains nvarchar(5)
SET @p__linq__Contains = N'%m%'

-- OK: Returns "Me" and "Not Me"
SELECT * FROM (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE N'%m%'

-- OK: Returns "Me"
SELECT * FROM (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE N'm%'

-- OK: Returns "Me" and "Not Me"
SELECT * FROM (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE @p__linq__Contains

-- Unexpected: Returns nothing
-- And why doesn't the last query do the same thing???
SELECT * FROM  (SELECT N'Me' AS F UNION ALL SELECT N'Not Me' AS F) x
WHERE f LIKE @p__linq__StartsWith

print @p__linq__StartsWith

您只需指定nvarchar的长度即可解决此问题。

我在BOL http://technet.microsoft.com/en-us/library/ms179859.aspx找到了解释。看看第一句话。

为什么我的LIKE语句在使用参数时不会过滤

1 个答案: