Question

i try to create a trigger to prevent the insertion of duplicate entries in SQL Developer (Oracle 11g XPRESS) but it ‘s not compiled correctly. Can you help me why because I can’t see any obvious error in syntax.

CREATE OR REPLACE TRIGGER trig1

BEFORE INSERT ON table1

BEGIN 
  DECLARE CURSOR C1
  IS
    SELECT value1,value2 FROM inserted;

  DECLARE value11 number;
  DECLARE value22 number;
  OPEN C1;
  FETCH NEXT FROM C1 INTO @value11, @value22;

  WHILE FETCH_STATUS = 0
  LOOP
    IF NOT EXISTS (SELECT * FROM table1 WHERE value1 = @value11 AND value2 = @value22)
    THEN
        INSERT INTO table1 (value1,value2)
        VALUES
        (@value11, @value22);
    ELSE
        ROLLBACK TRANSACTION
        --DELETE FROM table1 WHERE value1 = @value11 AND value2 = @value22
        PRINT 'Cannot add duplicate entry.'
    END IF;
    FETCH NEXT FROM C1 INTO @value11, @value22;
  END LOOP; 
  CLOSE C1;
END;

Answer 1

Most of the problems with your trigger are down to you using the wrong syntax; this looks like MySQL not PL/SQL. I would recommend reading the documentation and looking at some examples before continuing.

Having said all that; you’re going about this all the wrong way. In order to prevent the insertion of duplicates you have to create a unique constraint on your table. It is the only way to guarantee that you prevent them; trying to work around it in code is bound to fail at some point.

You can create a unique constraint inline, or if your table already exists you could create a unique index:

CREATE UNIQUE INDEX index_name
    ON table_name (column1, column2, ... column_n);

or use an ALTER TABLE statement:

ALTER TABLE table_name
  add CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);

If the set of columns you’re testing against are the primary key you can add a primary key constraint instead.

In addition to enforcing integrity no matter what your users decide enabling a unique constraint enables you to simply insert data and catch the errors. There’s no need to query the table prior to insertion, which should speed up your application.