Casting fraction number to decimal throws Arithmetic overflow error 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
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