How to Select the First Row of Each Group in a PostgreSQL GROUP BY Query?

Select the first row in PostgreSQL GROUP BY group

In PostgreSQL, selecting the first row of each group in a GROUP BY operation is a common requirement. For this we can use the powerful DISTINCT ON clause.

SELECT DISTINCT ON (customer) id, customer, total
FROM purchases
ORDER BY customer, total DESC

The

DISTINCT ON clause allows us to define which expressions are considered duplicates. In this example, we specified the customer column, which means that only rows with the same customer value will be considered duplicates.

The

ORDER BY clause defines the order in which rows are processed. The first row in each customer group will be the row with the highest total value. If there are multiple rows with the same maximum total value, the deciding factor will be the order in which the id columns are defined.

Key points

• DISTINCT ON is an extension to PostgreSQL that allows us to specify which expressions determine duplicates.
The
• DISTINCT ON expression must match the leading expression in the ORDER BY clause.
The
• ORDER BY clause can be used to prioritize rows within each group.
• You can include expressions in the SELECT list that are not in a DISTINCT ON or ORDER BY clause.
• The index used by a query can significantly affect performance. A multi-column index spanning the customer, total, and id columns, in the same order as the query, will provide the best performance.

Additional Notes

For data with customer high column cardinality (a small number of rows per customer), the DISTINCT ON method is very efficient. However, for customer data with low column cardinality (large number of rows per customer), other query techniques may be more efficient.

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