What is the use of sql unique constraint?

The role of the unique constraint in SQL is to ensure that each record has a unique identifier so that there are no two identical record values ​​in the column; the primary key of the table is a unique constraint, but the primary key only There can be one, so if the data in other columns is not allowed to be repeated, a unique constraint can be established.

#The role of the unique constraint in SQL is to ensure that each record has a unique identifier so that there are no two identical record values ​​in the column.

Unique constraint

The unique constraint in SQL prevents the same two record values ​​​​in a specific column, that is to say, it is used to ensure that each record Each has a unique identifier so that there are no duplicate values ​​in the column. The primary key of the table is a unique constraint, but there can only be one primary key, so if the data in other columns is not allowed to be repeated, a unique constraint can be established.

Create unique constraints

For example, the following SQL creates a new table named CUSTOMERS and adds five columns. Here, the AGE column is set to be unique, so there cannot be two records with the same age:

CREATE TABLE CUSTOMERS(
       ID   INT              NOT NULL,
       NAME VARCHAR (20)     NOT NULL,
       AGE  INT              NOT NULL UNIQUE,
       ADDRESS  CHAR (25) ,
       SALARY   DECIMAL (18, 2),       
       PRIMARY KEY (ID));

If the CUSTOMERS table has been created, then you want to add a unique constraint to the AGE column, similar to the following statement:

ALTER TABLE CUSTOMERS;
MODIFY AGE INT NOT NULL UNIQUE;

You can also use the following syntax, which supports named constraints on multiple columns:

ALTER TABLE CUSTOMERS;
ADD CONSTRAINT myUniqueConstraint UNIQUE(AGE, SALARY);

Delete a unique constraint

To delete a UNIQUE constraint, Please use the following SQL:

ALTER TABLE CUSTOMERS;
DROP CONSTRAINT myUniqueConstraint;

If you are using MySQL, you can use the following syntax:

ALTER TABLE CUSTOMERS
DROP INDEX myUniqueConstraint;

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