MYSQL unique constraint

First, the unique constraint (UNIQUE)

Unique constraints are used to ensure that the data in a column (or group of columns) is unique.Similar to primary key, but with the following differences:

• Tables can contain multiple unique constraints, but only one primary key is allowed per table.
• Unique constraint columns can contain NULL values.
• Unique constraint columns can be modified or updated.
• The value of the unique constraint column can be reused.
• Unique constraints cannot be used to define foreign keys.

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Second, create a unique constraint

1. Define a unique constraint for a single column in a table

CREATE TABLE Employees(? ? social_num INTEGER UNIQUE);-- Add the keyword UNIQUE to the social_num column definition of the table to make it a unique constraint.If an insert or update in the social_num column results in a duplicate value, MySQL will issue an error message and reject the change

2, define the unique constraint of the table (single column)

CREATE TABLE Employees(employee_name CHAR(50) NOT NULL,social_num INTEGER,phone INTEGER,UNIQUE (social_num));-- If an insert or update in the social_num column results in a duplicate value, MySQL will issue an error message and reject the change

3. Define the unique constraint of the table (multi-column union)

CREATE TABLE Employees(employee_name CHAR(50) NOT NULL,social_num INTEGER,phone INTEGER,UNIQUE (employee_name, social_num));-- The two fields are required to be unique when they are combined, that is, employee_name + social_num is unique

4. Use CONSTRAINT syntax to define a unique constraint (the constraint name can be specified)

(1) Defined when creating a table

-- Defined when creating a tableCREATE TABLE Employees(employee_name CHAR(50) NOT NULL,social_num INTEGER,phone INTEGER UNIQUE,employee_address CHAR(255),CONSTRAINT name_address UNIQUE (employee_name, employee_address));-- insert dataINSERT INTO Employees(employee_name, employee_address) VALUES('a', 'NJ');

• The first UNIQUE constraint is applied to the phone column, indicating that each employee must have a different phone number
• The second UNIQUE constraint name is name_address, which means that the employee employee_name can be repeated, and the employee_address can be repeated, but employee_name+employee_address must be unique and not repeated.Example: a + NJ and a + SH
  a + NJ and b + NJ
  a + NJ and b + SH
  a + NJ and a + NJ error "1062 - Duplicate entry"

(2) Add a unique constraint to the created table

-- Add a unique constraint to the created table and specify the constraint name as uni_nameALTER TABLE EmployeesADD CONSTRAINT uni_name UNIQUE (employee_name);-- Add a unique constraint to the created table without specifying a constraint nameALTER TABLE EmployeesADD UNIQUE (employee_name);

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Third, delete the unique constraint

When adding a unique constraint, MySQL will create a corresponding BTREE index for the database, so delete the index to delete the constraint.Query the index with the following command:

SHOW INDEX FROM table name;

-- Format 1:DROP INDEX constraint name ON table name;-- E.g:DROP INDEX name_address ON Employees;-- Format 2:ALTER TABLE table nameDROP INDEX constraint name;-- E.g:ALTER TABLE EmployeesDROP INDEX name_address;

Reference learning link:

mysql joint unique constraintAdd unique constraints and joint unique constraints in _MySQL - Baidu Library

MySQL Unique Constraint - MySQL Tutorial?

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