oracle - pl/sql How to use for loop with cursor for update operation -

i'm new in pl/sql , on book found example update cursors

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    declare       cursor c_grade(i_student_id in enrollment.student_id%type,         i_section_id in enrollment.section_id%type       )         select final_grade           enrollment         student_id_id = i_student_id         , section_id = i_section_id         update;         cursor c_enrollment         select e.student_id, e.section_id         enrollment enr, section sec         sec.course_no = 135 ,         enr.section_id = sec_section_id;     begin       r_enroll in c_enrollment loop         r_grade in c_grade(r_enroll.student_id. r_enroll.section_id) loop           update enrollment            set final_grade = 90           student_id = r_enroll.student_id           , section_id = r_enroll.section_id;               end loop;       end loop;     end; 

my question why need use update cursor in example? benefit compared this:

      r_enroll in c_enrollment loop            update enrollment            set final_grade = 90           student_id = r_enroll.student_id           , section_id = r_enroll.section_id;              end loop; 

as mentioned in comment, for update clause used lock rows within select statement such no other user able change these rows (update, delete, ... etc.) until lock release.

the lock typically released automatically once issue commit or rollback statements. particularly useful when there high number of dml statements running on same set of data different users. example, need update row highest id @ moment, before update row, row updated higher id different user.

for example, using

for r_grade in c_grade(r_enroll.student_id. r_enroll.section_id) loop 

will guarantee updating version locked, , nobody else has updated before you.

you can find elaborated description example here