Unique Key in SQL

In SQL, a unique key is a field or set of fields in a table that must contain unique values across all rows in the table. It is used to ensure data integrity and to prevent duplicate values from being entered into the table.

Here is an example of how to define a unique key in SQL:

CopyCREATE TABLE customers ( customer_id INT PRIMARY KEY, name VARCHAR(255), email VARCHAR(255), UNIQUE KEY (email) ); 

In this example, the customers table has a unique key on the email field, which means that the email values must be unique across all rows in the table.

You can also define a composite unique key, which is a unique key made up of two or more fields:

CopyCREATE TABLE sales ( product_id INT, store_id INT, sale_date DATE, UNIQUE KEY (product_id, store_id, sale_date) ); 

In this example, the sales table has a composite unique key that is made up of the product_id, store_id, and sale_date fields. This means that the combination of these three fields must be unique across all rows in the table.

Unique keys are often used in conjunction with primary keys to ensure data integrity. They can also be used to create indexes, which can improve the performance of SELECT, INSERT, UPDATE, and DELETE statements.

Choosing Between Primary Keys and Unique Keys

There are a few differences between primary keys and unique keys in SQL:

• Null values: Primary keys cannot contain null values, while unique keys can.

• Multiple unique keys: You can define multiple unique keys on a table, while you can only define one primary key.

• Referential integrity: Primary keys can be used to establish relationships between tables through foreign keys, while unique keys cannot.

• Indexes: Both primary keys and unique keys can be used to create indexes, which can improve the performance of SELECT, INSERT, UPDATE, and DELETE statements.

Dropping Unique Keys

You can drop a unique key using the DROP INDEX statement:

CopyDROP INDEX index_name ON table_name; 

In this example, the unique key with the name index_name is dropped from the table_name table.

Keep in mind that dropping a unique key can result in data integrity issues, so it should be done with caution.

Unique Key Constraints

In SQL, you can define unique key constraints to enforce the uniqueness of a field or set of fields in a table.

Here is an example of how to define a unique key constraint in SQL:

CopyCREATE TABLE customers ( customer_id INT PRIMARY KEY, name VARCHAR(255), email VARCHAR(255), CONSTRAINT uq_email UNIQUE (email) ); 

In this example, the customers table has a unique key constraint called uq_email on the email field. This means that the email values must be unique across all rows in the table.

You can also define a composite unique key constraint, which is a unique key constraint that is made up of two or more fields:

CopyCREATE TABLE sales ( product_id INT, store_id INT, sale_date DATE, CONSTRAINT uq_product_store UNIQUE (product_id, store_id) ); 

In this example, the sales table has a composite unique key constraint called uq_product_store that is made up of the product_id and store_id fields. This means that the combination of these two fields must be unique across all rows in the table.

Unique Key Indexes

In SQL, you can create an index on a unique key to improve the performance of SELECT, INSERT, UPDATE, and DELETE statements. An index is a data structure that allows the database engine to quickly locate and retrieve specific rows based on the values of the indexed fields.

Here is an example of how to create an index on a unique key in SQL:

CopyCREATE UNIQUE INDEX index_name ON table_name (field_name); 

In this example, an index called index_name is created on the field_name field of the table_name table. The UNIQUE keyword specifies that the index is a unique index, which means that the values in the indexed field must be unique across all rows in the table.