Exploring Default Constraints in SQL Server: A Comprehensive Guide（sqlserver默认约束）
Most relational databases in the world today use Structured Query Language (SQL). SQL Server is a popular version of SQL which includes default constraints as an important part of the database architecture. Default constraints are an essential part of database design and they add value to the data by maintaining the integrity of the data and enforcing certain behaviours. This article will provide an overview of default constraints in SQL Server, explain when and how to use them, and discuss best practices for their implementation.
Default constraints are rules or restrictions applied to a column in a table. The purpose of these constraints is to ensure that all data entering and stored in the database, follows the set rules or behaviours. This ensures that the data remains consistent and is valid for the task or purpose it was designed for.
The most common type of default constraint is the CHECK constraint. This constraint allows the DBA to limit the types of values which can be entered into the column by setting conditions. For example, a CHECK constraint could be used to limit the gender column in a table to only accept either ‘Male’ or ‘Female’. This can be implemented in SQL Server by writing the following command:
ALTER TABLE [MyTable]
ADD CONSTRAINT [MyTable_Gender_CHECK]CHECK (Gender IN ('Male', 'Female'))
Apart from the CHECK constraint, other types of default constraints available in SQL Server are PRIMARY KEY, UNIQUE, and FOREIGN KEY constraints.
The PRIMARY KEY constraint is used to specify a column or combination of columns in the table which must identify all related rows and should never be empty. This means that each table should only have one primary key and any two (or more) rows should never have the same primary key.
The UNIQUE constraint works in much the same way as the PRIMARY KEY constraint, except that a single column or combination of columns must be unique across the entire table. This means that each value within that column or combination of columns must only appear once in the table and cannot be repeated.
The FOREIGN KEY constaint defines a relationship between two or more columns in different tables. It is used to ensure that the value in a column in one table matches the value of a column in another table. An example of this could be if you have an ‘orders’ table and a ‘customers’ table. The Orders table could have a ‘Customer ID’ column which would contain the ID of the customer for that order. This column would have a FOREIGN KEY constraint linked to the ‘Customers’ table, so that the customer record from the customer table is always linked to the order.
In conclusion, default constraints are an essential part of SQL Server and database design. Default constraints allow for more efficient and rigorous data storage, by enforcing standards and providing further data integrity.As a DBA it is important to understand the different types of default constraints and the best ways in which they can be implemented.
