This trigger will raise an ORA-4091 mutating table error. : Trigger Error « Trigger « Oracle PL / SQL

1. Oracle PL / SQL
2. Trigger
3. Trigger Error

This trigger will raise an ORA-4091 mutating table error.

 

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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