SQL Tech Insider Two DDL

• To Create a database :

If db_id (' DBTest ') is null
Create DATABASE DBTest

• Create a table

Use Eb_fy_data_test---The database context in which the use switch resides

If object_id (' usertest ', ' u ') is not null

Drop TABLE Usertest

CREATE TABLE Usertest {}

• Data integrity

1. PRIMARY KEY constraints

ALTER TABLE Usertest

Add Constraint Pk_usertest

Primary Key (ID)

When you create a primary key, SQL Server automatically creates a unique index behind the scenes, guaranteeing uniqueness.

Syntax for adding a unique index (a physical mechanism):

ALTER TABLE Usertest

Add Constraint Pk_usertest

Unique (USERCNO)

2. FOREIGN KEY constraints

ALTER TABLE Usertest

Add Constraint Pk_usertest

Foreign Key (OrderID)

References OrderInfo (OrderID)

You can also add a foreign key constraint to other fields in this table, but you can allow null values at this time

3. Check constraints

ALTER TABLE Usertest

Add Constraint Pk_usertest

Check (age>18)

4. Default constraint is default value

SQL Tech Insider Two DDL