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If you need values in some of the columns to be unique, you can specify your index as UNIQUE.

When a unique index is applied to two or more columns, the combined values in these columns can't be duplicated in multiple rows. Note that because a NULL value is treated as a distinct value, you can have multiple NULL values in a column with a unique index.

If a table has a primary key defined, a corresponding unique index is created automatically.

Syntax

CREATE UNIQUE INDEX index_name ON table_name(column_list);

Example

1. Create a keyspace and a table as follows:

   ycqlsh> CREATE KEYSPACE yb_demo;
   ycqlsh> USE yb_demo;
   ycqlsh> CREATE TABLE employees(employee_no integer,name text,department text, PRIMARY KEY(employee_no))
           WITH transactions = {'enabled': 'true'};
   

2. Create a UNIQUE index for the name column in the employees table to allow only unique names in your table.

   CREATE UNIQUE INDEX index_employee_name ON employees(name);
   

3. Use the DESCRIBE INDEX command to verify the index creation.

   ycqlsh:yb_demo> DESCRIBE INDEX index_employee_name;
   

   CREATE UNIQUE INDEX index_employee_name ON yb_demo.employees (name) INCLUDE (employee_no)
       WITH transactions = {'enabled': 'true'};
   

4. Insert values into the table and verify that no duplicate names are created.

   ycqlsh:yb_demo> INSERT INTO employees(employee_no, name, department) VALUES (1, 'John', 'Sales');
   ycqlsh:yb_demo> INSERT INTO employees(employee_no, name, department) VALUES (2, 'Bob', 'Marketing');
   ycqlsh:yb_demo> INSERT INTO employees(employee_no, name, department) VALUES (3, 'Bob', 'Engineering');
   

   InvalidRequest: Error from server: code=2200 [Invalid query] message="Execution Error. Duplicate value disallowed by unique index index_employee_name
   INSERT INTO employees(employee_no, name, department) VALUES (3, 'Bob', 'Engineering');
          ^^^^
    (ql error -300)"
   

Learn more

CREATE INDEX