Techniques for Inserting Unique Rows in SQL Server (SQL Server 2008+ and Alternatives)

1. Using NOT EXISTS with INSERT:

   This method combines a SELECT statement with an INSERT statement. The SELECT acts like a check, looking for existing data based on specific criteria. The INSERT only proceeds if the SELECT returns no rows (meaning the data doesn't exist).

   Here's an example:

   INSERT INTO MyTable (Column1, Column2)
   SELECT 'New Data', 'Value'
   WHERE NOT EXISTS (SELECT * FROM MyTable WHERE Column1 = 'New Data');
   

   In this example, we're trying to insert a new row with 'New Data' in Column1 and 'Value' in Column2. The NOT EXISTS checks if there's already a row with 'New Data' in Column1. If none exists, the INSERT happens.

2. Using MERGE (SQL Server 2008 and above):

   SQL Server 2008 introduced the MERGE statement, which offers a more concise way to handle insert-if-not-exists logic. MERGE combines checking for existing data and inserting or updating in a single statement.

   Here's an example using MERGE:

   MERGE MyTable AS target
   USING (SELECT 'New Data', 'Value') AS source (Column1, Column2)
   ON (target.Column1 = source.Column1) -- Check if Column1 matches
   WHEN NOT MATCHED THEN INSERT (Column1, Column2) VALUES (source.Column1, source.Column2);
   

   This MERGE statement achieves the same outcome as the previous example. It checks for a matching Column1 value and inserts the new row if there's no match.

Using NOT EXISTS with INSERT:

INSERT INTO MyTable (Column1, Column2)
SELECT 'New Data', 'Value'
WHERE NOT EXISTS (SELECT * FROM MyTable WHERE Column1 = 'New Data');

This code attempts to insert a new row into the MyTable table. It inserts two columns:

• Column1: Contains the value 'New Data'
• Column2: Contains the value 'Value'

The WHERE clause uses NOT EXISTS to check if a row already exists in MyTable where the Column1 value is 'New Data'. If no such row exists (i.e., NOT EXISTS returns true), the INSERT statement proceeds and adds the new row.

Using MERGE (SQL Server 2008 and above):

MERGE MyTable AS target
USING (SELECT 'New Data', 'Value') AS source (Column1, Column2)
ON (target.Column1 = source.Column1) -- Check if Column1 matches
WHEN NOT MATCHED THEN INSERT (Column1, Column2) VALUES (source.Column1, source.Column2);

This code utilizes the MERGE statement, available in SQL Server 2008 and later versions. It combines checking and inserting into a single statement.

Here's a breakdown:

• MERGE MyTable AS target: Defines the target table as MyTable and assigns it the alias target.
• USING (SELECT 'New Data', 'Value') AS source (Column1, Column2): Creates a virtual table named source with two columns (Column1 and Column2) containing the values we want to insert.
• ON (target.Column1 = source.Column1): Specifies the join condition for checking existing data. It compares Column1 values between the target table and the source virtual table.
• WHEN NOT MATCHED THEN INSERT (Column1, Column2) VALUES (source.Column1, source.Column2): This part defines the action to take if no match is found. It inserts a new row into MyTable with the values from the source virtual table.

Both code snippets achieve the same result: inserting a new row into MyTable only if there's no existing row with the specified value in Column1. Choose the method that best suits your SQL Server version and coding preference.

1. Temporary Table and INSERT...SELECT:

   This method involves creating a temporary table to hold the data you want to insert. Then, you can use an INSERT...SELECT statement to insert the data from the temporary table into the main table, but only if no duplicates exist.

   Here's a basic example:

   DECLARE @tempTable TABLE (Column1 VARCHAR(255), Column2 VARCHAR(255));
   
   INSERT INTO @tempTable (Column1, Column2)
   VALUES ('New Data', 'Value');
   
   INSERT INTO MyTable (Column1, Column2)
   SELECT * FROM @tempTable
   WHERE NOT EXISTS (SELECT * FROM MyTable WHERE Column1 = (SELECT Column1 FROM @tempTable));
   
   DROP TABLE @tempTable;
   

   This approach separates the data preparation (inserting into the temporary table) from the actual insert-if-not-exists logic. It might be useful if you need to perform additional processing on the data before insertion.

2. Transactional INSERT with CHECK CONSTRAINT:

   This method utilizes transactions and a check constraint to ensure data uniqueness. You wrap the insert statement in a transaction and define a check constraint on the relevant column(s) to prevent duplicate insertion.

   Here's a breakdown:

   • Transaction: Begin a transaction before the insert statement.
   • INSERT: Attempt to insert the data.
   • CHECK CONSTRAINT: If a duplicate value is encountered due to the constraint violation, the transaction automatically rolls back, preventing the insertion.
   • Commit Transaction: If the insert succeeds without violating the constraint, commit the transaction to finalize the change.

   Note: This method requires creating and managing the check constraint, which might add complexity compared to other options.

Remember, the best alternative depends on your specific needs and database version. Consider factors like:

• SQL Server Version: MERGE is only available in SQL Server 2008 and later.
• Readability: Choose the method that promotes clear and maintainable code.
• Performance: For large datasets, temporary tables might introduce overhead.