Unique Constraint

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Details

• You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it’s possible to insert rows that appear to be duplicates if one of the values is NULL. If you need to strictly enforce uniqueness, use the NOT NULL constraint in addition to the UNIQUE constraint. You can also achieve the same behavior through the table’s Primary Key.
• Columns with the UNIQUE constraint automatically have an index created with the name _<columns_key. To avoid having two identical indexes, you should not create indexes that exactly match the UNIQUE constraint’s columns and order. The UNIQUE constraint depends on the automatically created index, so dropping the index also drops the UNIQUE constraint. Conversely, dropping the UNIQUE constraint also drops the automatically created index.
• When using the UNIQUE constraint on multiple columns, the collective values of the columns must be unique. This does not mean that each value in each column must be unique, as if you had applied the UNIQUE constraint to each column individually.
• You can define the UNIQUE constraint when you create a table, or you can add it to an existing table through ADD CONSTRAINT.

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Syntax

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Table level

> CREATE TABLE logon (
    login_id  INT PRIMARY KEY,
    customer_id   INT,
    logon_date    TIMESTAMP,
    UNIQUE (customer_id, logon_date)
  );

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Column level

> CREATE TABLE warehouses (
    warehouse_id    INT        PRIMARY KEY NOT NULL,
    warehouse_name  STRING(35) UNIQUE,
    location_id     INT
  );

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Usage example

> CREATE TABLE IF NOT EXISTS logon (
    login_id INT PRIMARY KEY,
    customer_id   INT NOT NULL,
    sales_id INT,
    UNIQUE (customer_id, sales_id)
  );

> INSERT INTO logon (login_id, customer_id, sales_id) VALUES (1, 2, 1);

> INSERT INTO logon (login_id, customer_id, sales_id) VALUES (2, 2, 1);

duplicate key value (customer_id,sales_id)=(2,1) violates unique constraint "logon_customer_id_sales_id_key"

> INSERT INTO logon (login_id, customer_id, sales_id) VALUES (3, 2, NULL);

> INSERT INTO logon (login_id, customer_id, sales_id) VALUES (4, 2, NULL);

> SELECT customer_id, sales_id FROM logon;

+-------------+----------+
| customer_id | sales_id |
+-------------+----------+
|           2 |        1 |
|           2 | NULL     |
|           2 | NULL     |
+-------------+----------+

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See also

• Constraints
• DROP CONSTRAINT
• CHECK constraint
• DEFAULT value constraint
• Foreign key constraint
• NOT NULL constraint
• PRIMARY key constraint
• SHOW CONSTRAINTS