Question

使用NULL检查有条件地查询值的正确语法是什么？

DECLARE @TargetDialogHandle UNIQUEIDENTIFIER
DECLARE @EventMessage XML
DECLARE @EventMessageTypeName sysname

WAITFOR(
    RECEIVE @TargetDialogHandle = conversation_handle,
    @EventMessage = CONVERT(XML, message_body),
    @EventMessageTypeName = message_type_name FROM dbo.EventNotificationQueue ), TIMEOUT 1000

SELECT 
    @EventMessage.value('(/EVENT_INSTANCE/TSQLCommand/CommandText)[1]', 'nvarchar(max)') AS TSQLCommand, 
    CASE @EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)') 
       WHEN NULL @EventMessage.value('(/EVENT_INSTANCE/ObjectName)[1]', 'varchar(128)') 
       ELSE @EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)') 
    END AS TableName

这会引发错误：

第15级州立1行9的消息102
“ @EventMessage”附近的语法不正确。

以下声明：

DECLARE @TargetDialogHandle UNIQUEIDENTIFIER
DECLARE @EventMessage XML
DECLARE @EventMessageTypeName sysname

WAITFOR(
    RECEIVE @TargetDialogHandle = conversation_handle,
    @EventMessage = CONVERT(XML, message_body),
    @EventMessageTypeName = message_type_name FROM dbo.EventNotificationQueue ), TIMEOUT 1000

SELECT 
    @EventMessage.value('(/EVENT_INSTANCE/TSQLCommand/CommandText)[1]', 'nvarchar(max)') AS TSQLCommand, 
    IIF( @EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)') IS NULL, @EventMessage.value('(/EVENT_INSTANCE/ObjectName)[1]', 'varchar(128)'), @EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)')) AS TableName

返回

第156条消息，第15级，状态1，第9行
关键字“ IS”附近的语法不正确。

Answer 1

CASE表达式有两种样式：

CASE (column name or SQL variable)
   WHEN (value 1) 
      THEN (return value 1)
   WHEN (value 2) 
      THEN (return value 2)       
   ELSE (return else value)
END

这将检查CASE之后的列或变量名是否等于给定值之一，并在每种情况下返回某些值。

但是，如果您无法检查是否相等-使用NULL，您永远不会！ -您需要使用其他样式：

CASE 
   WHEN (expression 1) 
      THEN (return value 1)
   WHEN (expression 2) 
      THEN (return value 2)       
   ELSE (return else value)
END

因此，您在这里检查公式化表达式（可以使用 other 而不是相等检查（例如，检查> =，或使用BETWEEN进行范围检查等）。

对于您来说，由于要检查NULL，因此必须使用方法2：

SELECT 
    @EventMessage.value('(/EVENT_INSTANCE/TSQLCommand/CommandText)[1]', 'nvarchar(max)') AS TSQLCommand, 
    CASE 
       -- check if your value IS NULL
       WHEN @EventMessage.value(.....) IS NULL 
          THEN @EventMessage.value('(/EVENT_INSTANCE/ObjectName)[1]', 'varchar(128)') 
       ELSE @EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)') 
    END AS TableName

Answer 2

您不能只使用COALESCE吗？

DECLARE @TargetDialogHandle UNIQUEIDENTIFIER
DECLARE @EventMessage XML
DECLARE @EventMessageTypeName sysname

WAITFOR(
    RECEIVE @TargetDialogHandle = conversation_handle,
    @EventMessage = CONVERT(XML, message_body),
    @EventMessageTypeName = message_type_name FROM dbo.EventNotificationQueue ), TIMEOUT 1000

SELECT 
    @EventMessage.value('(/EVENT_INSTANCE/TSQLCommand/CommandText)[1]', 'nvarchar(max)') AS TSQLCommand, 
    COALESCE(@EventMessage.value('(/EVENT_INSTANCE/TargetObjectName)[1]', 'varchar(128)'), @EventMessage.value('(/EVENT_INSTANCE/ObjectName)[1]', 'varchar(128)')) AS TableName

条件空选择

2 个答案: