I have oracle sql table of students, students are assigned to groups.
I have to set constraint so there will be no more than 10 students in a group (ideally no more than N students in a group).
Basically restrict table from having more than N rows with the same GROUP_ID value.
Here is table declaration.
CREATE TABLE "students"
( "STUDENT_ID" NUMBER(6,0),
"GROUP_ID" NUMBER(6,0),
"FULL_NAME" VARCHAR2(40),
CONSTRAINT "student_id_PK" PRIMARY KEY ("STUDENT_ID")
USING INDEX ENABLE
)
/
ALTER TABLE "students" ADD FOREIGN KEY ("GROUP_ID")
REFERENCES "groups" ("GROUP_ID") ENABLE
/
CREATE TABLE "groups"
( "GROUP_ID" NUMBER(6,0),
"NAME" VARCHAR2(40),
CONSTRAINT "group_id_PK" PRIMARY KEY ("GROUP_ID")
USING INDEX ENABLE
)
/
I've tried to use trigger that throws error when there is more than 10 students in a group. It worked with inserts but it was useless for updates of table, as i was constantly getting error ORA-04091: table students is mutating
Here is shortened version of the trigger:
SELECT count(*) INTO stud_in_g
FROM "students"
WHERE GROUP_ID = :NEW.GROUP_ID;
IF stud_in_g>10 THEN
raise_application_error(-20001, 'to many students');
END IF;
I've read that oracle forbids to read from table when it is updating. If that's so then how can i ensure that there can't be more than 10 students in a group?
CodePudding user response:
The "shortened version" of the trigger doesn't show what type of trigger it is - most probably a row level trigger. That won't work because of the mutating table trigger error. A statement level trigger does work:
create table test_table (
id number generated by default on null as identity
constraint test_table_id_pk primary key,
name varchar2(100 char),
the_group varchar2(100 char)
);
Table TEST_TABLE created.
BEGIN
insert into test_table (name, the_group ) values ('Chandler', 'A');
insert into test_table (name, the_group ) values ('Ross', 'A');
insert into test_table (name, the_group ) values ('Joey', 'A');
insert into test_table (name, the_group ) values ('Phoebe', 'B');
insert into test_table (name, the_group ) values ('Jennifer', 'B');
insert into test_table (name, the_group ) values ('Monica', 'B');
commit;
END;
/
PL/SQL procedure successfully completed.
CREATE OR REPLACE TRIGGER test_table_as
AFTER UPDATE OR INSERT ON test_table
DECLARE
l_max_members NUMBER := 3;
BEGIN
-- determine the transaction type
FOR r IN (SELECT
the_group,
COUNT(*) AS cnt
FROM
test_table
GROUP BY
the_group
HAVING
COUNT(*) > l_max_members)
LOOP
IF r.cnt > l_max_members THEN
raise_application_error(-20100,'Max '||l_max_members||' per group');
END IF;
END LOOP;
END;
/
Trigger TEST_TABLE_AS compiled
insert into test_table (name, the_group ) values ('Jill', 'B');
Error starting at line : 39 in command -
insert into test_table (name, the_group ) values ('Jill', 'B')
Error report -
ORA-20100: Max 3 per group
ORA-06512: at "KOEN.TEST_TABLE_AS", line 8
ORA-06512: at "KOEN.TEST_TABLE_AS", line 8
ORA-04088: error during execution of trigger 'KOEN.TEST_TABLE_AS'