The field is too small to accept the amount of data... (URGENT HELP NEEDED)

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SQL

DateTime

MS-Access

database

I am doing some university coursework, and i have created a query to extract some data from the database but i keep receiving the error 'the field is too small to accept the amount of data...' and i have no clue what i have done wrong.

Here is what i have written:

SELECT SMM.StudentId, Student_Module.YearStudied, AVG(SMM.FinalMark) AS AverageMarks
FROM SMM, Student_Module, Student
WHERE Student.StudentId=SMM.StudentId
AND SMM.StudentId=Student_Module.StudentId
AND SMM.ModuleCode=Student_Module.ModuleCode
AND Student_Module.ModuleCode=Module.ModuleCode
AND Student.GraduationDate="09/08/2013"
AND YearStudied>1
GROUP BY SMM.StudentId, YearStudied; 

UNION SELECT SMM.StudentId, "DegreeResult" AS YearStudied, AVG( SMM.FinalMark) AS AverageMarks
FROM SMM, Student_Module, Student
WHERE Student.StudentId=SMM.StudentId
AND SMM.StudentId=Student_Module.StudentId
AND SMM.ModuleCode=Student_Module.ModuleCode
AND Student_Module.ModuleCode=Module.ModuleCode
AND Student.GraduationDate="09/08/2013"
AND YearStudied>1
GROUP BY SMM.StudentId;

When i go to the 'StudentModule' and change the data type in 'YearStudied' to text the error changes to 'Data Type Mismatch'
Any help would be much appreciated (P.S I am a beginner on Access)

[Edit member="Tadit"]
Added pre tags.
[/Edit]