Скачать или смотреть Session8 - SQL Server - Column Constraints (NOT NULL, NULL, DEFAULT, and CHECK)

SQL Server NOT NULL, NULL, DEFAULT, and CHECK constraints are used to control the data that can be stored in a column.

NOT NULL constraints prevent a column from containing a NULL value. A NULL value is a placeholder for a value that is not yet known or does not exist.
NULL constraints allow a column to contain NULL values.

DEFAULT constraints specify a default value for a column. If a NULL value is inserted into a column with a DEFAULT constraint, the default value will be used instead.

CHECK constraints specify a condition that all values in a column must meet. If a value does not meet the condition, it will not be inserted into the column.

Constraints can be created when a table is created or they can be added to an existing table using the ALTER TABLE statement.

Here are some examples of SQL Server constraints:

NOT NULL constraint:
CREATE TABLE Customers (
CustomerID INT NOT NULL,
CustomerName VARCHAR(50) NOT NULL,
CustomerAddress VARCHAR(100)
);
This constraint prevents the CustomerID, CustomerName, and CustomerAddress columns from containing NULL values.

NULL constraint:
CREATE TABLE Products (
ProductID INT,
ProductName VARCHAR(50),
ProductDescription VARCHAR(100) NULL
);
This constraint allows the ProductDescription column to contain NULL values.

DEFAULT constraint:
CREATE TABLE Orders (
OrderID INT NOT NULL,
OrderDate DATETIME DEFAULT CURRENT_TIMESTAMP,
OrderTotal DECIMAL(10, 2)
);
This constraint specifies that the OrderDate column will be automatically populated with the current date and time when a new row is inserted into the Orders table.

CHECK constraint:
CREATE TABLE Employees (
EmployeeID INT NOT NULL,
EmployeeName VARCHAR(50) NOT NULL,
EmployeeAge INT CHECK (EmployeeAge GreaterThan 18)
);

This constraint specifies that the EmployeeAge column can only contain values that are greater than or equal to 18.

Constraints can be very helpful in ensuring the accuracy and reliability of your data. They can also help to prevent errors and make your database more efficient.

Here are some of the benefits of using SQL Server constraints:

Accuracy: Constraints can help to ensure that the data in your database is accurate by preventing the entry of invalid data.
Reliability: Constraints can help to ensure that the data in your database is reliable by preventing the loss or corruption of data.
Efficiency: Constraints can help to improve the efficiency of your database by preventing unnecessary queries and updates.
If you are using SQL Server, I encourage you to use constraints to help ensure the accuracy, reliability, and efficiency of your data.