Handlers

• These are declared code segments which deal with non-normal conditions like errors.
• In the example, he creates two tables, t2 and t3

  • create table t2 (
       s1 int, 
       primary key (s1)
    ) engine=innodb;
                 

  • create table t3 (
       s1 int, key(s1), 
       foreign key (s1) 
          references t2 (s1)
    ) engine=innodb;
         

• Inserting a 5 into t3 causes an error

  ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails
       

• He builds an error table:

  create table error_log (error_message char(80))
       

• And the following procedure:

  CREATE PROCEDURE InsertWithLog (parameter1 INT)
  BEGIN
     DECLARE EXIT HANDLER for 1452
        INSERT INTO error_log VALUES
           ( CONCAT ('Time: ', current_date,
  	    '. Foreign key failure for Value = ', parameter1));
     INSERT INTO t3 VALUES (parameter1);
  END;
  
       

• This s works for a CONDITION (warning)