Inserting Only New Records: Your Guide to Avoiding Duplicates in SQL Server

sql server Inserting Only New Data in SQL Server: Understanding "IF NOT EXISTS"

1. WHERE NOT EXISTS with SELECT...INSERT:

This method uses a subquery to check if the data exists and an INSERT statement to add it if not.

INSERT INTO MyTable (col1, col2)
SELECT ..., ...
FROM YourData
WHERE NOT EXISTS (
  SELECT 1 FROM MyTable WHERE col1 = ... AND col2 = ...
);

Example: Adding a customer only if their email doesn't exist:

INSERT INTO Customers (Name, Email)
SELECT 'foo', '[email protected]'
FROM MyData
WHERE NOT EXISTS (
  SELECT 1 FROM Customers WHERE Email = '[email protected]'
);

2. MERGE Statement (SQL Server 2008 R2 and later):

The MERGE statement combines checking and inserting/updating in one go.

MERGE INTO MyTable AS target
USING YourData AS source
ON target.col1 = source.col1 AND target.col2 = source.col2
WHEN MATCHED THEN
  INSERT (col1, col2) VALUES (source.col1, source.col2);
WHEN NOT MATCHED THEN
  INSERT (col1, col2) VALUES (source.col1, source.col2);

3. IF EXISTS...ELSE INSERT Logic (Stored Procedures):

For more complex scenarios, you can use stored procedures with conditional logic.

CREATE PROCEDURE AddCustomer (@Name nvarchar(50), @Email nvarchar(100))
AS
BEGIN
  IF NOT EXISTS (SELECT 1 FROM Customers WHERE Email = @Email)
  BEGIN
    INSERT INTO Customers (Name, Email) VALUES (@Name, @Email);
  END
END

Related Issues and Solutions:

  • Performance: The WHERE NOT EXISTS method can be inefficient for large datasets. Consider MERGE or stored procedures for better performance.
  • Uniqueness: Ensure your table has appropriate unique constraints to avoid unintended duplicates.
  • Error Handling: Add error handling in your chosen method to gracefully handle unexpected situations.

Remember: Choose the method that best suits your needs, considering factors like SQL Server version, data size, and complexity. Happy querying!