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Getting Started with SQL and Relational Databases

Unique Indexes

Discover how to use unique indexes in MySQL to ensure no duplicate values exist in critical columns. Learn to create and verify unique indexes, preventing data entry errors and enhancing relational database reliability.

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Show existing customers

First, let’s see the current customers table again.

MySQL
select * from customers;

Now, let’s execute the following command. This will try to create a new customer, John Boo, but with the identity value equal to the identity of an existing customer (that of John Woo).

MySQL
insert into customers(name, identity) values ('John Boo','JW0001');

We’re doing a business error on purpose here. We’ve inserted the new customer with the wrong identity. However, the query has been successfully executed, and the new customer has ...