Casting fraction number to decimal throws Arithmetic overflow error

Question

I have a table called AssignmentMarks that stores Student's assignment marks for different subjects. This table has a column called Marks varchar(10) which stores the marks. The marks can be like 1, 2, and also it can be +, - or -+ which represents each sign a specific marks.

Note: + is 1 mark, - is -1 and -+ is 0.5

While getting a student marks from the table I am facing the Arithmetic Overflow error which I don't know what is the cause of this error.

The query is as follow:

SELECT SUM(
 CAST(
 CASE 
 WHEN a.Marks IS NULL OR a.Marks = '' THEN 0
 WHEN a.Marks = '+' THEN 1
 WHEN a.Marks = '-' THEN -1
 WHEN a.Marks = '-+' THEN 0.5
 ELSE a.Marks
 END AS DECIMAL(5,2)
 )
)
FROM AssignmentMarks AS a WHERE a.StudentID=10
AND a.SubjectID=1

After Executing the above query I get the following error.

Msg 8115, Level 16, State 8, Line 7

Arithmetic overflow error converting varchar to data type numeric.

Any idea what is the main cause of this error?

Sample Data: SQL Fiddle

• S3S: What does this return? select * from AssignmentMarks a where try_cast(a.Marks as decimal(5,2)) is null and a.Marks not in ('+','-+','-') and a.Marks is not null and a.Marks != ''
• Asrar Ahmad Ehsan: @scsimon returns nothing

Answer 1

From error message,

Arithmetic overflow error converting varchar to data type numeric.

My first place where I could change is :

 ELSE a.Marks

and change it to

ELSE try_convert(int, a.Marks) 

or convert to decimal , if you want

Later edit:
In case expression you have a few THEN with different data types: int, numeric, varchar. Due to data type precedence, when combines different data types, the lower data type is converted to the type with higher precedence See here information about it.

you can see this , by this example:

SELECT SQL_VARIANT_PROPERTY(col,'BaseType') AS 'Base Type', 
 SQL_VARIANT_PROPERTY(col,'Precision') AS 'Precision', 
 SQL_VARIANT_PROPERTY(col,'Scale') AS 'Scale'
FROM
 (select 0 as col) as a 
SELECT SQL_VARIANT_PROPERTY(col,'BaseType') AS 'Base Type', 
 SQL_VARIANT_PROPERTY(col,'Precision') AS 'Precision', 
 SQL_VARIANT_PROPERTY(col,'Scale') AS 'Scale'
FROM
 (select 1 as col) as a
SELECT SQL_VARIANT_PROPERTY(col,'BaseType') AS 'Base Type', 
 SQL_VARIANT_PROPERTY(col,'Precision') AS 'Precision', 
 SQL_VARIANT_PROPERTY(col,'Scale') AS 'Scale'
FROM
 (select -1 as col ) as a
SELECT SQL_VARIANT_PROPERTY(col,'BaseType') AS 'Base Type', 
 SQL_VARIANT_PROPERTY(col,'Precision') AS 'Precision', 
 SQL_VARIANT_PROPERTY(col,'Scale') AS 'Scale'
FROM
 (select 0.5 as col) as a 
SELECT SQL_VARIANT_PROPERTY(col,'BaseType') AS 'Base Type', 
 SQL_VARIANT_PROPERTY(col,'Precision') AS 'Precision', 
 SQL_VARIANT_PROPERTY(col,'Scale') AS 'Scale'
FROM
 (select '-+' as col) as a 
Base Type Precision Scale
int 10 0
Base Type Precision Scale
int 10 0
Base Type Precision Scale
int 10 0
Base Type Precision Scale
numeric 1 1
Base Type Precision Scale
varchar 0 0

• Asrar Ahmad Ehsan: Thanks it worked, One thing that I didn't understand is that if I change the 0.5 to 10.5 or higher then it doesn't give me the error. Why?
• Asrar Ahmad Ehsan: in case expression WHEN a.Marks = '-+' THEN 0.5 changing to WHEN a.Marks = '-+' THEN 10.5
• Asrar Ahmad Ehsan: so why casting 0.5 decimal gives error and casting 10.5 to decimal does not
• Sabin B: the error it wasn't from casting 0.5 to decimal. It is related to Data type precedence , see here learn.microsoft.com/en-us/sql/t-sql/data-types/…