How to add unique index in mysql

Three ways to add: 1. Use the "CREATE INDEX" statement to add, the syntax "CREATE UNIQUE INDEX index name ON table name (column name list);"; 2. Add when creating a table, the syntax "CREATE TABLE Table name (... UNIQUE KEY (column name list));"; 3. Added when modifying the table, the syntax is "ALTER TABLE table name ADD CONSTRAINT index name UNIQUE KEY (column name list);".

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

mysql unique index (UNIQUE)

To enforce unique values ​​for one or more columns, PRIMARY KEY constraints are usually used. However, each table can only have one primary key. Therefore, you cannot use primary key constraints if you want to include multiple columns or a set of columns with unique values.

Fortunately, MySQL provides another index called a UNIQUE index that allows you to force values ​​to be unique to one or more columns. Unlike PRIMARY KEY indexes, UNIQUE can have multiple indexes per table.

To create a UNIQUE index, use the following <span style="background-color: rgb(255, 255, 255); color: rgb(68, 68, 68); font-family: " helvetica neue yahei arial sans-serif>CREATE INDEX</span> statement:

CREATE UNIQUE INDEX 索引名
ON 表名(列名1,列名2,...);

Another way to enforce uniqueness of values ​​in one or more columns One way is to use UNIQUE constraints.

When creating a UNIQUE constraint, MySQL will UNIQUE create the index behind the scenes.

The following statements illustrate how to create a unique constraint when creating a table.

CREATE TABLE 表名(
...
   UNIQUE KEY(列名1,列名2,...) 
);

You can also use the ALTER TABLE statement to add a unique index (UNIQUE) to an existing table

ALTER TABLE 表名
ADD CONSTRAINT 索引名 UNIQUE KEY(列名1,列名2,...);

MySQL UNIQUE index example

Suppose you want to manage contacts in the application. You also want the email for each contact in the contacts table to be unique.

To enforce this rule, create a unique constraint in the CREATE TABLE statement as follows:

CREATE TABLE IF NOT EXISTS contacts (
    id INT AUTO_INCREMENT PRIMARY KEY,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL,
    phone VARCHAR(15) NOT NULL,
    email VARCHAR(100) NOT NULL,
    UNIQUE KEY unique_email (email)
);

If you use the SHOW INDEXES statement, you will See that MySQL UNIQUE creates an index for the email column.

SHOW INDEXES FROM contacts;

Let us insert a row in the contacts table.

INSERT INTO contacts(first_name,last_name,phone,email)
VALUES('John','Doe','(408)-999-9765','john.doe@mysqltutorial.org');

Now if you try to insert the row whose email is john.doe@mysqltutorial.org you will get an error message.

INSERT INTO contacts(first_name,last_name,phone,email)
VALUES('Johny','Doe','(408)-999-4321','john.doe@mysqltutorial.org');

ERROR 1062 (23000): Duplicate entry 'john.doe@mysqltutorial.org' for key 'unique_email'

Assume that the combination you want first_name, last_name and phone is also unique among contacts. In this case, you can create indexes for these columns using CREATE INDEX statement UNIQUE as follows:

CREATE UNIQUE INDEX idx_name_phone
ON contacts(first_name,last_name,phone);

Add the following row to contacts because combining the tables will cause an error first_name, last_name and phone already exist.

INSERT INTO contacts(first_name,last_name,phone,email)
VALUES('john','doe','(408)-999-9765','john.d@mysqltutorial.org');

ERROR 1062 (23000): Duplicate entry 'john-doe-(408)-999-9765' for key 'idx_name_phone'

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