In PostgreSQL, the DISTINCT ON clause is a powerful tool used in the SELECT statement to retrieve unique rows based on a specified set of columns. It is often combined with the ORDER BY clause to determine which unique rows to select.

I. How DISTINCT ON Works

  1. Data Sorting: The data is sorted based on the columns specified in the ORDER BY clause.

  2. Selecting Unique Rows: Only one unique row is selected for each group of unique values in the specified column or columns.

  3. Returning Results: The selected unique rows are returned in the result set of the SELECT statement.

II. When to Use DISTINCT ON

  1. When You Need Only One Unique Row per Group: DISTINCT ON is useful when you want to return only one unique row for each group of unique values in a column or columns. For example, you can use it to select the most recent record for each user in a history table of information changes.

  2. When Sorting Results is Necessary: DISTINCT ON is often combined with ORDER BY to determine the unique row based on a specific criterion, such as the most recent timestamp or the highest value.

  3. When Removing Duplicate Values is Required: DISTINCT ON helps to eliminate duplicate rows based on the specified columns, helping to clean and refine the query results.

III. Example:

Let’s say you have an orders table containing information about orders, and you want to return only the most recent order for each customer:

SELECT DISTINCT ON (customer_id) *
FROM orders
ORDER BY customer_id, order_date DESC;

In this example, DISTINCT ON is used to return only one row for each customer (based on customer_id), and the results are sorted by customer_id and order_date DESC, ensuring that only the most recent order is selected for each customer.

IV. Conclusion

DISTINCT ON in PostgreSQL provides a powerful way to select unique rows based on specified criteria. By understanding how it works and when to use it, you can efficiently retrieve the desired data while maintaining accuracy and clarity in your queries.