SQL UNIQUE Constraint. The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has …

Mar 7, 2025 · In this article, we will explore the SQL UNIQUE Constraint in detail, including its syntax, usage, and examples. What is the SQL UNIQUE Constraint? The SQL UNIQUE constraint ensures that all values in a column or a set of columns are different from one another.

We can create one or more Unique Keys on one or more columns in an SQL table. Syntax. Following is the syntax to create unique key constraints on multiple columns in a table −. CREATE TABLE table_name( column1 datatype UNIQUE KEY, column2 datatype UNIQUE KEY, ..... ..... columnN datatype ); Example

SQL UNIQUE Constraint Syntax. The syntax of the SQL UNIQUE constraint is: CREATE TABLE table_name ( column_name data_type UNIQUE ); Here, table_name is the name of the table to be created; column_name is the name of the column where the constraint is to be implemented; data_type is the data type of the column such as INT, VARCHAR, etc.

When a Unique index is added to an existing column in a table, the database engine first checks for the uniqueness of the specified column data. If a duplicate entry is found, then the engine returns an error and does not add the constraint. A Unique key in a table can be referenced by a Foreign Key from another table.

Mar 27, 2023 · Guide to Unique Key in SQL. Here we discuss the need and the Working of Unique Key in SQL with respective syntax, parameters, and examples.

Mar 21, 2025 · Using a UNIQUE Key helps prevent duplicate entries and maintains data consistency. It’s especially useful when certain columns require distinct values but aren’t the primary identifiers. To define a UNIQUE Key during table creation, use the following syntax: SQL Server / Oracle / MS Access Example (Single Column): Emp_ID INT NOT NULL UNIQUE, .

A Unique Key is a constraint in SQL that prevents duplicate values in one or more columns. It's like a bouncer at your data party, making sure no uninvited (duplicate) guests sneak in. Why Do We Need Unique Keys?

The UNIQUE key constraint is used for ensuring that all values in a column or a set of columns are unique. This chapter covered the basic syntax for defining UNIQUE keys, inserting data while maintaining unique values, and using composite UNIQUE keys.

Mar 3, 2024 · When it comes to ensuring data integrity in your databases, knowing how to properly implement the UNIQUE constraint is key. This constraint prevents duplicate entries in a column or a set of columns, which is paramount for maintaining the uniqueness of each record.