SQL Beginner Tutorial Learning (iv)

1. Union,union All

SELECT E_name from Employees_china
UNION
SELECT E_name from Employees_usa

By default, the UNION operator chooses a different value. If duplicate values are allowed, use UNION all.

2. Create a table

CREATE TABLE Persons
(
id_p int not NULL,
LastName varchar (255) is not NULL,
FirstName varchar (255),
Address varchar (255),
City varchar (255),
CONSTRAINT uc_PersonID UNIQUE (Id_P,LastName)
)

If you need to name a unique constraint and define a UNIQUE constraint for multiple columns

3. Create a UNIQUE constraint in the "id_p" column when the table has been created

ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (Id_P,LastName)

If you want to revoke a UNIQUE constraint

ALTER TABLE Persons

DROP CONSTRAINT uc_PersonID

4.CREATE TABLE Persons
(
id_p int not NULL PRIMARY KEY ,
LastName varchar (255) is not NULL,
FirstName varchar (255),
Address varchar (255),
City varchar (255)
)

5.SQL FOREIGN KEY Constraint

CREATE TABLE Orders
(
id_o int not NULL PRIMARY KEY,
OrderNo int not NULL,
Id_P int FOREIGN KEY REFERENCES Persons(Id_P)
)

, the id_p column in Orders points to the id_p column in the Persons table.

The "id_p" column in the "Persons" table is the PRIMARY KEY in the Persons table.

The "id_p" column in the Orders table is the FOREIGN KEY in the Orders table.

The FOREIGN KEY constraint is used to prevent actions that disrupt the connection between tables.

Http://www.w3s.com.cn/sql/sql_foreignkey.asp

SQL Beginner Tutorial Learning (iv)