Database primary key, foreign key, relationship, cascading save, update, delete

Data | database-building a database
Create DATABASE test;
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--Establish table customers
CREATE TABLE Customers (
ID int identity (1,1) not NULL,
Name varchar (15),
Age int,
Primary KEY (ID)
);
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--Establish the table orders
CREATE TABLE Orders (
ID int identity (1,1) not NULL,
Order_number varchar (15),
Price,
customer_id int,
Primary KEY (ID)
);
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--Inserts data into the table orders to detect the Money data type, and the result is: 69.0000
INSERT into orders (price) values (69);
SELECT * from Orders;

--adding foreign keys and relationship constraints
ALTER TABLE orders add constraint Fk_customer foreign key (customer_id) references customers (ID);
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-Cascade Delete, update
ALTER TABLE orders add constraint Fk_customer foreign key (customer_id) references customers (ID)
On the DELETE cascade on UPDATE cascade;
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--Cascading deletion
ALTER TABLE orders add constraint Fk_customer foreign key (customer_id) references customers (ID)
ON DELETE cascade;
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--
INSERT into customers values (' John ', 25);
INSERT into customers values (' Dick ', 25);

--If a cascade insert is set, the customer_id (foreign key) inserted into the child table orders
--This ID must be present in the parent table customers (primary key)
INSERT into orders values (1,50,5);

--If a cascade deletion is set, the id=5 record in the parent table customers is deleted.
--also deletes customer_id=5 records in the child table orders,
Delete from customers where id=5


--Query
SELECT * from Customers;
SELECT * from Orders;