In this post I will described how to alter table and add new FOREIGN KEY constraints on table.Its very easy 3 steps work. After that you can add predefined trigger like on Delete, On updated, Not set etc.
Which is very useful if you want remove/update parent data with all relationship data which is stored in child table.

Step 1: first we add Column in child table.

ALTER TABLE Role ADD COLUMN Parent int(11) UNSIGNED DEFAULT NULL;

Step 2: first we add Index on foreign Key, because MySQL says foreign Key must Index.

ALTER TABLE Table1 ADD INDEX ParentTabl(ParentID)

Step 3: Now alter table and add constraints.

ALTER TABLE `ChildTbl` ADD CONSTRAINT `Parent` FOREIGN KEY(`Parent`) REFERENCES `ChildTbl`(`Id`)

Step 4: Now define trigger if you want add with constraints.
ALTER TABLE ` ChildTbl ` ADD CONSTRAINT `Parent` FOREIGN KEY(`Parent`) REFERENCES ` ChildTbl `(`Id`) ON DELETE CASCADE;

Note: If you are finding ‘Errno 150’ thats mean your PK column(s) in the parent table and the FK column(s) in child table is not same data type.