understanding foreign key integrity constraint

Author: techysanoj

foreign_key_integrity_constrain.sql

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+use temp_db;
+
+show tables;
+
+
+-- here it is most important constraint because using this we can have relationship constraint
+-- like 1 - 1, 1 - M, M-1, M-M
+-- it will work when there are multiple tables 
+-- lets say there are two tables so there must be a common column between them
+-- one is parent table and another is child table and in parent table that column must be primary key 
+-- and in child table it will work as foreign key, also child table is dependent on parent table.
+/*
+RULES:
+1. Data inserted into the child table should have same data of the common column of parent table.
+ 2. Cannot delete the record from the parent table having the associated data in child table.
+ if you have to delete this then you need to delete the child table entry first.
+ You can do this using ON DELETE CASCADE;
+*/
+
+-- parent table
+create table emp_parent(id int, name varchar(15), exp int, primary key(id));
+
+insert into emp_parent values(1, 'SANOJ', 1);
+insert into emp_parent values(2, 'KUMAR', 2);
+insert into emp_parent values(3, 'DEEPAK', 3);
+
+-- child table
+create table salary_child(id int, salary int, foreign key(id) references emp_parent(id));
+
+insert into salary_child values(1, 10000); -- no error
+insert into salary_child values(9, 1000000); -- will give error because 9 is not present in parent table
+insert into salary_child values(2, 200000); -- no error
+
+
+-- now deleting the value in parent table
+delete from emp_parent where id = 1; -- will give error because it has value associated in child table
+
+-- but using the ON DELETE CASCADE we can do this (also deleting one by one can also help);
+-- but we need to change the table strucutre and then use ON DELETE CASCADE;
+
+create table salary_new(id int, salary int, foreign key(id) references emp_parent(id) on delete cascade); -- like this
+insert into salary_new values(1, 10000); -- no error
+insert into salary_new values(9, 1000000); -- will give error because 9 is not present in parent table
+insert into salary_new values(2, 200000); -- no error
+
+delete from emp_parent where id = 1; -- here it will not work because it has dependencies on two tables now so we need to remove that first
+
+-- now deleted from salary_child but not from the salary_new
+delete from salary_child where id = 1;
+
+-- this will work
+delete from emp_parent where id = 1;
+
+
+select * from emp_parent;
+
+select * from salary_child;
+
+select * from salary_new;