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Most people do not understand the knowledge of this article "mysql how to increase the unique index", so the editor summarizes the following contents, detailed content, clear steps, and has a certain reference value. I hope you can get something after reading this article. Let's take a look at this "mysql how to increase the unique index" article.

In mysql, you can use the "create unique index index name on table name (field name)" statement to increase the unique index; the unique index is similar to the ordinary index, the only difference is that the value to add the unique index must be unique, allowing null values, if the combined index, then the combination of column values must be unique.

The operating environment of this tutorial: windows10 system, mysql8.0.22 version, Dell G3 computer.

How does mysql add a unique index create unique index index name on table name (field name)

There are about five types of Mysql indexes:

General index (INDEX): the most basic index, without any restrictions

Unique index (UNIQUE): similar to a normal index, except that the value of the index column must be unique, but null values are allowed.

Primary key index (PRIMARY): it is a special unique index and no null values are allowed.

Full-text index (FULLTEXT): can be used for MyISAM tables, mysql5.6 can also be used for innodb tables, used in an article, to retrieve text information, for larger data, the generation of full-text index is time-consuming and space-consuming.

Union (combination) index: in order to improve the efficiency of mysql, a combination index can be established, following the principle of "leftmost prefix".

Here let's take a look at the unique index.

We looked at the primary key index before. it is a special unique index. the difference between the two is that the primary key index cannot have null values, but the unique index can have null values.

One: basic use

1: the unique index can be a single column or multiple columns. Let's take a look at the creation statement:

# create a single column unique index alter table sc add unique (name); # create a multi-column unique index alter table sc add unique key `name_ score` (`name`, `score`)

2: delete the unique index statement:

Alter table sc drop index name

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Second, the function of unique index

1: the greatest use is to ensure that the data written to the database is a unique value.

The only value in a single column is basically the primary key.

Commonly used is a multi-column unique index, such as: the current item, a user can only buy one item. We set the user id and product id columns to a unique index. Then you can avoid repeated purchases by a user.

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