How to fix ERROR 1449: The user specified as a definer does not exist in MySQL

SELECT DEFINER, ROUTINE_NAME, ROUTINE_TYPE FROM information_schema.ROUTINES 
WHERE ROUTINE_SCHEMA = "your_database";

SELECT DEFINER, TABLE_NAME FROM information_schema.VIEWS 
WHERE TABLE_SCHEMA = "your_database";

SELECT * FROM mysql.user WHERE User = "missing_user" AND Host = "localhost";

CREATE USER IF NOT EXISTS "app_user"@"localhost" IDENTIFIED BY "secure_password";
GRANT ALL PRIVILEGES ON your_database.* TO "app_user"@"localhost";
FLUSH PRIVILEGES;

UPDATE mysql.proc SET DEFINER = "root@localhost" 
WHERE DEFINER = "missing_user@localhost" 
AND db = "your_database";

FLUSH PRIVILEGES;

SELECT DEFINER, ROUTINE_NAME FROM information_schema.ROUTINES 
WHERE ROUTINE_SCHEMA = "your_database";

DROP PROCEDURE IF EXISTS your_procedure;

CREATE PROCEDURE your_procedure()
DEFINER = "root"@"localhost"
SQL SECURITY DEFINER
BEGIN
  -- Your procedure code here
END;

ALTER VIEW your_view SQL SECURITY INVOKER AS
SELECT * FROM your_table;

mysqldump --no-definer --single-transaction your_database > backup.sql

sed -i "/DEFINER=/s/DEFINER=`[^*]*`\*/DEFINER=CURRENT_USER*/g" backup.sql
mysql your_database < backup.sql

-- For a procedure
CALL your_procedure();

-- For a view
SELECT * FROM your_view LIMIT 5;

-- For a trigger (will execute on the next INSERT/UPDATE/DELETE)
INSERT INTO your_table VALUES (...);