Here is another example where the error messages are very clear but often developers get confused with the message. I think the reason for the confusion is the word “Key” used in the error message. After I explained this to a developer who sent me the error he realize that it is about how we all interpret a same statement.
Following code will generate the error 1505.
-- 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'
SELECT 1, 2, 'Second' -- Duplicate ID col
SELECT 3, 3, 'Third'
SELECT 4, 4, 'Fourth'
After creating a table, I am inserting same a key in the first column multiple times.
-- Now create PK on ID Col
ALTER TABLE test
ADD CONSTRAINT [PK_test] PRIMARY KEY CLUSTERED
Now when I attempt to create a Primary Key on the column it gives us following error.
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.
In SQL Server Primary Key can’t have duplicate records as well can’t have a column which is nullable. In our case, you can notice that we are creating a primary key on the column ID which contains duplicate values of 1. The only way to create primary key on this column is to delete the duplicate row which exists. If your business logic does not allow to delete the duplicate row, this means that your column is not a good candidate for the Primary Key and you will have to either select another column or use a composite Primary Key (where you use multiple columns).
On Separate note, here is the blog post with video which explains how you can delete the duplicate row from the table: Select and Delete Duplicate Records – SQL in Sixty Seconds #036 – Video.
Reference: Pinal Dave (http://blog.SQLAuthority.com)