IDNTIFYING INDEX

Question

How can i determine the index of a row in a dataset if one column value is known?
 
here the code to fill the data set:
string str = "SELECT ROUND(AVG((EXAMRESULTS.assessM1 + EXAMRESULTS.assessM2 + EXAMRESULTS.assessM3)/3), 2), STUDENT.studentId FROM EXAMRESULTS, STUDENT where EXAMRESULTS.Term = '" + comboBox15.Text + "' AND EXAMRESULTS.class = '" + comboBox14.Text + "' AND EXAMRESULTS.StudentId = STUDENT.studentId GROUP BY STUDENT.studentId,  EXAMRESULTS.StudentId ORDER BY AVG((EXAMRESULTS.assessM1 + EXAMRESULTS.assessM2 + EXAMRESULTS.assessM3)/3) DESC";
               
The i want to know the row that will have a particular STUDENT.studentId (say the id   = 23)