SQL 服务器 function 无法按预期工作

Question

I am working on a project for class & course management. I want to create a function that automatically creates a course code based on the course name and then adds a number based on whether there already exist a course name with the same first 4 characters.

Here is what my function looks like:

CREATE FUNCTION [dbo].[fxGenerateCourseCode]
    (@course_name VARCHAR(50))
RETURNS VARCHAR(8)
AS
BEGIN
    DECLARE @course_code VARCHAR(8), @count_course INT

    SET @count_course = (SELECT COUNT(1) FROM dbo.COURSE_TB 
                         WHERE SUBSTRING(course_name, 1, 4) = SUBSTRING(@course_name, 1, 4)) + 1 

    SELECT @course_code = UPPER(SUBSTRING(@course_name, 1, 4)) + ' - ' + '00' + CAST(@count_course AS VARCHAR)

    RETURN @course_code
END

The problem is, whenever I execute this function, it is not returning the right count values and it doesn't include the 00 either.

For example, executing this:

SELECT dbo.fxGenerateCourseCode('french')

returns FREN-0 rather than FREN-001 (which is what I expect).

However, when I execute the code manually, it gives me the result that I expect:

DECLARE @course_code VARCHAR(8), @count_course INT, @course_name VARCHAR(50)

SET @course_name = 'french'

SET @count_course = (SELECT COUNT(1) + 1 FROM dbo.PARAMETRES_COURS 
                     WHERE SUBSTRING(nom_cours, 1, 4) = SUBSTRING(@course_name, 1, 4))

SELECT course_code = UPPER(SUBSTRING(@course_name, 1, 4)) + ' - ' + '00' + CAST(@count_course AS VARCHAR)

Result: FREN-001 .

I have tried changing the way I write the function with not much result. Can someone help me know where the problem here is coming from? Am I doing something wrong in the function?

Thank you

Answer 1

I suspect that you are putting the value into a column with a length of 6. That said, the problem would appear to be the spaces around the hyphen. Also, always include a length for varchar , because the defaut length varies by context and it might not do what you want:

You can simplify your code and make it more accurate:

declare @course_code varchar(8), @count_course int;

select @count_course = count(1) + 1
from dbo.COURSE_TB
where left(course_name, 4) = left(@course_name, 4);

select @course_code = concat(left(upper(@course_name, 4)), '-', '00', @count_course);

Note that this still doesn't do what you want, which is padding the number. This version only works for 9 courses. You seem to want:

select @course_code = concat(left(upper(@course_name, 4)), '-',
                             right(concat('00', @count_course), 3)
                            );

Note the use of the concat() function, so you don't have to worry about lengths of strings when doing a conversion.