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SQL>
SQL> CREATE TABLE lecturer (
2 id NUMBER(5) PRIMARY KEY,
3 first_name VARCHAR2(20),
4 last_name VARCHAR2(20),
5 major VARCHAR2(30),
6 current_credits NUMBER(3)
7 );
Table created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major,current_credits)
2 VALUES (10001, 'Scott', 'Lawson','Computer Science', 11);
1 row created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major, current_credits)
2 VALUES (10002, 'Mar', 'Wells','History', 4);
1 row created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major,current_credits)
2 VALUES (10003, 'Jone', 'Bliss','Computer Science', 8);
1 row created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major,current_credits)
2 VALUES (10004, 'Man', 'Kyte','Economics', 8);
1 row created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major,current_credits)
2 VALUES (10005, 'Pat', 'Poll','History', 4);
1 row created.
SQL>
SQL> INSERT INTO lecturer (id, first_name, last_name, major,current_credits)
2 VALUES (10006, 'Tim', 'Viper','History', 4);
1 row created.
SQL>
SQL>
SQL> CREATE OR REPLACE TRIGGER LimitMajors
2 BEFORE INSERT OR UPDATE OF major ON lecturer
3 FOR EACH ROW
4 DECLARE
5 studentMax CONSTANT NUMBER := 5;
6 studentCount NUMBER;
7 BEGIN
8 SELECT COUNT(*)
9 INTO studentCount
10 FROM lecturer
11 WHERE major = :new.major;
12
13 IF studentCount + 1 > studentMax THEN
14 RAISE_APPLICATION_ERROR(-20000,
15 'Too many lecturer in major ' || :new.major);
16 END IF;
17 END LimitMajors;
18 /
Trigger created.
SQL>
SQL> UPDATE lecturer
2 SET major = 'History'
3 WHERE ID = 10003;
UPDATE lecturer
*
ERROR at line 1:
ORA-04091: table JAVA2S.LECTURER is mutating, trigger/function may not see it
ORA-06512: at "JAVA2S.LIMITMAJORS", line 5
ORA-04088: error during execution of trigger 'JAVA2S.LIMITMAJORS'
SQL>
SQL> DROP TRIGGER LimitMajors;
Trigger dropped.
SQL>
SQL> drop table lecturer;
Table dropped.
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