Conditional Duplicating MySQL tables by using trigger - mysql

Question

I want to write a trigger in MySQL which copy table1’s column1 to table2’s column1
But table2’s column1 must to be unique, table1’s column1 is duplicable.
Any one suggest me how to conditionally sync up between this 2 tables using triggers.

Answer 1

You could improve your question by stating what kind of trigger you want (insert,update,delete) and what you want to happen if a duplicate is found. Assuming you want an insert trigger and table2.id has unique key you could use insert ignore to ignore the error and throw away the the attempted insert from table1 , you could use insert..on duplicate key to update table2. If table2.id does not have a unique key you could code to find if the key would be duplicated and then apply the insert option you prefer. I have assumed that you don't simply want to throw an error if a duplicate is encountered. Here's some sample code and data for you.
DROP TABLE IF EXISTS TABLE1,TABLE2;
CREATE TABLE TABLE1(ID INT, VALUE INT);
CREATE TABLE TABLE2(ID INT PRIMARY KEY, VALUE INT);
DROP TRIGGER IF EXISTS T;
DELIMITER $$
CREATE TRIGGER T AFTER INSERT ON TABLE1
FOR EACH ROW
BEGIN
INSERT IGNORE INTO TABLE2 VALUES (NEW.ID,NEW.VALUE);
END $$
INSERT INTO TABLE1 VALUES(1,1);
SELECT * FROM TABLE2;
INSERT INTO TABLE1 VALUES(1,2);
SELECT * FROM TABLE1;
SELECT * FROM TABLE2;
DROP TRIGGER IF EXISTS T;
DELIMITER $$
CREATE TRIGGER T AFTER INSERT ON TABLE1
FOR EACH ROW
BEGIN
INSERT INTO TABLE2 VALUES (NEW.ID,NEW.VALUE)
ON DUPLICATE KEY UPDATE VALUE = NEW.VALUE;
END $$
DELIMITER ;
SELECT * FROM TABLE2;
INSERT INTO TABLE1 VALUES(1,3);
SELECT * FROM TABLE1;
SELECT * FROM TABLE2;
DROP TRIGGER IF EXISTS T;
DELIMITER $$
CREATE TRIGGER T AFTER INSERT ON TABLE1
FOR EACH ROW
BEGIN
DECLARE FOUND INT DEFAULT 0;
SELECT 1 INTO FOUND FROM DUAL WHERE EXISTS(SELECT ID FROM TABLE2 WHERE ID = NEW.ID);
IF FOUND = 0 THEN
INSERT INTO TABLE2 VALUES (NEW.ID,NEW.VALUE);
ELSE
INSERT INTO TABLE2 VALUES (NEW.ID,NEW.VALUE)
ON DUPLICATE KEY UPDATE VALUE = NEW.VALUE;
END IF;
END $$
DELIMITER ;
SELECT * FROM TABLE2;
INSERT INTO TABLE1 VALUES(1,4),(2,1);
SELECT * FROM TABLE1;
SELECT * FROM TABLE2;