Primary Key vs. Unique Index: When Should I Use Which?

The primary key and unique index: in -depth discussion of its differences

In the database design, is the data integrity constraint a basic decision: Should the primary key or the only index? Although the two concepts are similar, choosing one in actual projects may have a significant impact.

Unique index: Maintenance data unique

The only index compulsory restrictions cannot have the same value in the index column. This attribute ensures that the data remain unique in this column. Consider the following example:

In this scene, the only index on the "name" column ensures that there are not two lines of the same name.

Primary key: Multiple -way constraints

CREATE TABLE my_table (
  id INT NOT NULL,
  name VARCHAR(255) UNIQUE
);

The primary key not only guarantees the uniqueness, but also guarantees that the column is not NULL. This means that each row will have a unique and non -empty value in the primary column. In addition, the table can only have one primary key, and the primary key will automatically establish an index. For example, in the following example:

The "ID" column acts as the primary key, forcing the uniqueness and non -empty value.

<缺> advantages and disadvantages

CREATE TABLE my_table (
  id INT PRIMARY KEY NOT NULL,
  name VARCHAR(255)
);

The advantages of the only index:

The uniqueness of the data is enforced, and there is no need to force non -empty value.

Allow definition of multiple unique indexes.
• Do not automatically create a gathering index.
  The advantages of the main key:
  • Make sure the uniqueness and non -empty value of data integrity.
  Automatically creating gathering indexes may improve performance.
• Select the correct method
  • The best choice of primary key and unique index depends on the specific requirements of applications and database design. If the maintenance of the uniqueness and the non -empty value is crucial, the primary key is the first choice. On the contrary, if only the uniqueness is enforced, the only index provides greater flexibility and customization.
  Copy precautions in MS SQL Server

In MS SQL Server, the primary key plays a vital role in copying. Each table used in copies requires a unique identifier, which is usually implemented as the main key. This unique identifier ensures that the data can be consistent and updated between the databases that can be copied. Therefore, when designing a database you want to copy, the primary key is usually recommended.

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