SQL Server is a powerful relational database management system that allows developers to store and retrieve data efficiently. However, there are times when developers encounter error messages that can be confusing and difficult to understand. One such error is Error 1505, which occurs when attempting to create a primary key on a column that contains duplicate values.

Let’s take a look at an example to better understand this error:

-- Create Table
CREATE TABLE test (
    ID INT NOT NULL,
    Col1 INT,
    Col2 VARCHAR(100)
)

-- Populate Table
INSERT INTO test (ID, Col1, Col2)
SELECT 1, 1, 'First'
UNION ALL
SELECT 1, 2, 'Second' -- Duplicate ID col
UNION ALL
SELECT 3, 3, 'Third'
UNION ALL
SELECT 4, 4, 'Fourth'

-- Now create PK on ID Col
ALTER TABLE test
ADD CONSTRAINT [PK_test] PRIMARY KEY CLUSTERED ([ID] ASC)

When executing the above code, you will encounter Error 1505:

Msg 1505, Level 16, State 1, Line 2
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name 'dbo.test' and the index name 'PK_test'. The duplicate key value is (1).
Msg 1750, Level 16, State 0, Line 2
Could not create constraint. See previous errors. The statement has been terminated.

The error message indicates that a duplicate key was found for the object ‘dbo.test’ and the index ‘PK_test’. In this case, the duplicate key value is (1).

So, how can we fix this issue? The solution is to ensure that the primary key column does not contain duplicate values. In our example, the column ‘ID’ contains duplicate values of 1. To create a primary key on this column, we need to delete the duplicate row that exists.

If your business logic does not allow you to delete the duplicate row, it means that the column is not a good candidate for the primary key. In such cases, you can either select another column to serve as the primary key or use a composite primary key, where you use multiple columns.

It’s important to understand the requirements and constraints of your data when designing a database schema. By ensuring that primary keys are unique and non-nullable, you can maintain data integrity and avoid errors like Error 1505.

For more information on how to delete duplicate records from a table, you can refer to our blog post and video tutorial: Select and Delete Duplicate Records – SQL in Sixty Seconds #036 – Video.

By understanding the causes and solutions to common SQL Server errors, you can become a more proficient developer and effectively manage your database systems.