Handling duplicate records in SQL is a common task that often requires a few steps to effectively identify and remove or consolidate these duplicates. The first part of the process is to identify duplicates based on specified criteria, such as unique columns that should not have repeating values. This can be achieved using SQL queries with the GROUP BY clause combined with aggregate functions. For instance, you can use a query like SELECT column_a, COUNT(*) FROM table_name GROUP BY column_a HAVING COUNT(*) > 1 to find all records in the table that have duplicates in column_a.
Once you’ve identified the duplicates, you need to decide how to handle them. There are several approaches depending on your requirements. If you want to keep one instance of each duplicate and delete the rest, you can use a CTE (Common Table Expression) or a temporary table to store unique records and then delete the duplicates. For example, using a CTE, you can use a query like this:
WITH CTE AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY column_a ORDER BY id) AS rn
FROM table_name
)
DELETE FROM CTE WHERE rn > 1;
This query will keep the first occurrence based on the specified order and remove the remaining duplicates.
Another option is to consolidate duplicate records into a single entry. This might involve aggregating data from the duplicates into one record. For example, if you have multiple records for the same customer with different order amounts, you could sum those amounts together. A query for this could look like:
INSERT INTO new_table_name (column_a, total_order_amount)
SELECT column_a, SUM(order_amount)
FROM table_name
GROUP BY column_a;
This would create a new table that contains unique customer entries with their total order amounts. Choosing the right method depends on your specific data and application needs, but SQL provides flexible tools to manage duplicate records effectively.