Violation of PRIMARY KEY constraint

 This is SQL error 2627 - violation of Primary Key constraint.

A "Violation of PRIMARY KEY constraint" error occurs when you try to insert a duplicate value into a column that has been defined as a primary key. Primary keys must contain unique values for each row in a table. This error can also occur when trying to update a primary key column to a value that already exists in another row.

Steps to Resolve the Error:

• Identify the Duplicate Data: Find out which value is causing the violation. You can use a query to check for duplicates in the column.

SELECT PrimaryKeyColumn, COUNT(*)
FROM YourTable
GROUP BY PrimaryKeyColumn
HAVING COUNT(*) > 1;

• Check for Existing Data Before Insert: Before inserting a new row, check if the primary key value already exists in the table.

SELECT *
FROM YourTable
WHERE PrimaryKeyColumn = 'ValueYouWantToInsert';

• Correct the Data: Ensure that the data you are trying to insert or update is unique in the primary key column.
• Modify the Insert/Update Logic: Depending on your use case, you may need to modify your logic to handle duplicates appropriately. Here are a few options:

Skip Insertion if Duplicate: Insert only if the primary key value does not already exist.

IF NOT EXISTS (SELECT 1 FROM YourTable WHERE PrimaryKeyColumn = 'ValueYouWantToInsert')
BEGIN
    INSERT INTO YourTable (PrimaryKeyColumn, OtherColumns)
    VALUES ('ValueYouWantToInsert', OtherValues);
END;

Update Existing Row if Duplicate: Update the row if the primary key value already exists.

IF EXISTS (SELECT 1 FROM YourTable WHERE PrimaryKeyColumn = 'ValueYouWantToInsert')
BEGIN
    UPDATE YourTable
    SET OtherColumn = 'NewValue'
    WHERE PrimaryKeyColumn = 'ValueYouWantToInsert';
END
ELSE
BEGIN
    INSERT INTO YourTable (PrimaryKeyColumn, OtherColumns)
    VALUES ('ValueYouWantToInsert', OtherValues);
END;

• Review Application Logic: Ensure that your application logic correctly handles primary key values, avoiding attempts to insert duplicates.

Example:

Let's say you have a table named Employees with EmployeeID as the primary key, and you are trying to insert a new employee:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    Position NVARCHAR(50)
);

-- Attempting to insert a new employee
INSERT INTO Employees (EmployeeID, FirstName, LastName, Position)
VALUES (1, 'John', 'Doe', 'Developer');

-- Attempting to insert another employee with the same EmployeeID
INSERT INTO Employees (EmployeeID, FirstName, LastName, Position)
VALUES (1, 'Jane', 'Smith', 'Manager');

The second insert will fail with a "Violation of PRIMARY KEY constraint" error because EmployeeID 1 already exists.

Handling the Error:

Skip Insertion if Duplicate:

IF NOT EXISTS (SELECT 1 FROM Employees WHERE EmployeeID = 1)
BEGIN
    INSERT INTO Employees (EmployeeID, FirstName, LastName, Position)
    VALUES (1, 'Jane', 'Smith', 'Manager');
END;

Update Existing Row if Duplicate:

IF EXISTS (SELECT 1 FROM Employees WHERE EmployeeID = 1)
BEGIN
    UPDATE Employees
    SET FirstName = 'Jane', LastName = 'Smith', Position = 'Manager'
    WHERE EmployeeID = 1;
END
ELSE
BEGIN
    INSERT INTO Employees (EmployeeID, FirstName, LastName, Position)
    VALUES (1, 'Jane', 'Smith', 'Manager');
END;

Conclusion

By following these steps, you can effectively handle "Violation of PRIMARY KEY constraint" errors, ensuring the integrity and uniqueness of your primary key data in SQL Server.