Primary Key vs. Unique Index: When to Use Which?

Primary key and unique index: Choose the correct method

In database design, the choice of primary key and unique index may be a problem, especially when considering practical applications. This article discusses the advantages and disadvantages of each method to help you make wise decisions for the project.

What is the unique index?

The only index on the column ensures that none of the two lines in the table have the same value in the column. This constraint is particularly useful for preventing repeated data input and maintenance data integrity. The primary key and the unique index: different similarities

Similarity: The primary key and the unique index are exerted the unique constraint on the specified columns.

• Different:

It can be an empty field:
• The primary key cannot contain air value, and the only index can allow the empty value.

  The number of keys: A table can only have one primary key, but there can be multiple unique indexes.

  Gathering Index:
  • If there is no definition of gathering indexes, the primary key will automatically become a gathering index, thereby improving the query performance. The only index is not a gathering index.
  • The advantages of the main key
  • Ensure data integrity: The non -empty constraints of the primary key can prevent data loss and ensure the only labeling.
  Gathering index:
The primary key provides efficient data access by gathers relevant data on the disk.

The disadvantages of the main key

• Strict non -empty constraints: Non -empty constraints may sometimes be restricted, especially when processing can be empty data.
Limited flexibility:
• Each table can only define a primary key, which may not always meet complex data needs.
The advantages of the only index

Restrictions:

The only index allows vacancy to provide flexibility in the data representation.

• Multiple indexes: You can create multiple unique indexes on the table to provide additional flexibility for data organizations.
• Select the correct method

Use the main key:

For the unique and non -empty key data, such as customer ID or order number. Using unique index:

For data that does not require non -empty data, such as address or phone number, or data that requires multiple unique constraints.

• Copy on the MS SQL Server
• In Microsoft SQL Server, the primary key is used to identify the main copy during replication to ensure the consistency of data crossing multiple database instances. The only index does not work in copying. In the end, the choice of primary key and unique index depends on your specific data requirements and project goals. Consider the advantages and disadvantages of each method and choose the most suitable solution for your application.

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