What is a UNIQUE constraint

A UNIQUE constraint in SQL is a rule or restriction applied to one or more columns in a database table, ensuring that all values in those columns are unique across all rows in the table. In other words, a UNIQUE constraint prevents duplicate values from being entered into the specified column(s) of the table. Here are some key points about UNIQUE constraints:- 

1. Uniqueness:-  The UNIQUE constraint ensures that every value in the specified column(s) is unique within the table. It prevents the insertion of duplicate values into those columns.

2. Column-Level or Table-Level:-  UNIQUE constraints can be applied at the column level or at the table level. When applied at the column level, the constraint ensures uniqueness for individual columns. When applied at the table level, the constraint ensures uniqueness for combinations of columns.

3. Null Values:-  In most SQL database systems, a UNIQUE constraint allows NULL values. However, because NULL values are considered distinct from one another, multiple NULL values are allowed. If the constraint is defined on multiple columns, each combination of NULL values must be unique.

4. Indexing:-  UNIQUE constraints are often indexed by the database management system (DBMS) to improve query performance when searching for unique values in the column(s) covered by the constraint.

5. Enforcement:-  UNIQUE constraints are automatically enforced by the database system. If an attempt is made to insert or update a row with a value that violates the UNIQUE constraint, the DBMS will raise an error and reject the operation.

6. Definition:-  UNIQUE constraints are typically defined when creating or altering a table using SQL. The syntax for defining a UNIQUE constraint varies slightly between different database systems, but generally involves using the UNIQUE keyword followed by the column(s) to which the constraint applies.

For example, in a Products table, you might have a UNIQUE constraint on the ProductCode column to ensure that each product code is unique across all products in the table, preventing duplicate product codes from being entered.