Insert into table requires specific condition from another table - sql

Question

I have two SQL tables with the following schema:
road_test (test_ID, examiner_ID, student_ID, vin, test_date)
lessons_count (student_ID, lessons_taken)
I am looking for some way to require a student to have at least 5 lessons_taken before they can insert into the road_test table.
Is there some sort of trigger or constraint that allows for this?

Answer 1

Don't store count of lessons; calculate it whenever needed.
Here's what I'd suggest:
SQL> -- the final table
SQL> create table road_test
2 (test_id number, student_id number, vin number);
Table created.
SQL> -- table that shows which lessons were taken by which student
SQL> create table lesson
2 (student_id number, lesson_id number);
Table created.
SQL>
Trigger which is supposed to control whether you're allowed to insert student's record into the road_test table: count number of lessons taken and raise an error if it is too low (I set it to 3 for simplicity):
SQL> create or replace trigger trg_bi_road
2 before insert on road_test
3 for each row
4 declare
5 l_cnt number;
6 begin
7 select count(*)
8 into l_cnt
9 from lesson
10 where student_id = :new.student_id;
11 if l_cnt < 3 then
12 raise_application_error(-20001,
13 'You have to take at least 3 lessons');
14 end if;
15 end;
16 /
Trigger created.
SQL>
Testing (as I said: restricted to 3 lessons, for simplicity):
SQL> -- initial record
SQL> insert into lesson(student_id, lesson_id) values (1, 100);
1 row created.
SQL> -- can I enter that student into the ROAD_TEST table? Nope
SQL> insert into road_test (test_id, student_id, vin) values (555, 1, 123456);
insert into road_test (test_id, student_id, vin) values (555, 1, 123456)
*
ERROR at line 1:
ORA-20001: You have to take at least 3 lessons
ORA-06512: at "SCOTT.TRG_BI_ROAD", line 9
ORA-04088: error during execution of trigger 'SCOTT.TRG_BI_ROAD'
SQL> -- Let's insert 2 more lessons for the same student
SQL> insert into lesson(student_id, lesson_id) values (1, 200);
1 row created.
SQL> insert into lesson(student_id, lesson_id) values (1, 300);
1 row created.
SQL> -- New attempt for the ROAD_TEST table:
SQL> insert into road_test (test_id, student_id, vin) values (555, 1, 123456);
1 row created.
SQL> select * From lesson;
STUDENT_ID LESSON_ID
---------- ----------
1 100
1 200
1 300
SQL> select * from road_test;
TEST_ID STUDENT_ID VIN
---------- ---------- ----------
555 1 123456
SQL>