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This article mainly introduces the purpose of creating an index of the data table, which has a certain reference value. Interested friends can refer to it. I hope you will gain a lot after reading this article. Let's take a look at it.

1. The purpose of SQL's index creation is as follows:

1. Uniqueness of data can be ensured by uniqueness index (unique).

2. Speed up the retrieval of data

3. Speed up the connection between tables

4. Reduce grouping and sorting time

5. Use the optimized hidden device to improve the performance of the system.

Second, the syntax for creating an SQL index:

CREATE [UNIQUE] [CLUSTERED | NONCLUSTERED] (index type) INDEX ON ([ASC | DESC] [, [ASC | DESC]...]).

Extended data:

Category description of the index:

1. Unique index:

A unique index is an index that does not allow any two rows to have the same index value. When there are duplicate key values in existing data, most databases do not allow the newly created unique index to be saved with the table. The database may also prevent the addition of new data that will create duplicate key values in the table.

2. Primary key index:

Database tables often have a combination of one or more columns whose values uniquely identify each row in the table. This column is called the primary key of the table. Defining a primary key for a table in a database diagram automatically creates a primary key index, which is a specific type of unique index. The index requires that each value in the primary key be unique. It also allows quick access to data when a primary key index is used in a query.

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