操作数类型冲突：日期与用CONVERT（）解决的int不兼容...为什么？

Question

My question is about WHY? WHAT is SQL doing to cause this error message in the situation below.

There are a few of these queries on stack but usually because of a missing DateAdd(). Here I am simply trying to put the date passed in as part of an audit note string against the record. I am not specifically asking the two variables to play together and give me a result.

Below is the line of issue. At the bottom the fuller query for context.

@Days INT, @Date DATE

'Extension to claim of ' + @Days + ' from ' + @Date + '.' ,

SELECT TOP 1
                ID,
                WorkFlowStatusCode ,
                'Number of days: ' + @Days + ' from ' + @Date + '.' ,
                @UserId ,
                @UserId
        FROM    dbo.Table
        WHERE   ID= @VariableA
        ORDER BY ID DESC;

What I have found is that wrapping the @Date in a Convert() to 103 solves the issue. See Below.

'Extension to claim of ' + @Days + ' from ' + CONVERT(NVARCHAR(10),@Date,103) + '.' ,

I am simply trying to understand why I would need to do that in this situation from SQL/Database point of view. I can't find that answer online to help my understanding.

All comments greatly appreciated.

Lee

Answer 1

Presumably @Day is an integer. Remember that + can mean both addition and string concatenation, so SQL can get confused. Try this:

'Number of days: ' + convert(varchar(255), @Days) + ' from ' + convert(varchar(10), @Date, 103) + '.' 

Notes:

• When using varchar() or nvarchar() , always include a length. SQL Server has a default length that varies by context. Not including the length is an easy way to create a very-hard-to-debug error.
• Your code suggests varchar() rather than nvarchar() because the rest of the string are not national character sets.