PostgreSQL Left Join with Distinct On - SQL

In PostgreSQL, you can use a left join with the distinct on clause to return unique rows from a table while joining it with another table. The distinct on clause is used to filter out duplicate rows based on specific columns.

Syntax

The syntax for a left join with distinct on in PostgreSQL is:

SELECT DISTINCT ON (table1.column1) table1.column1, table2.column2
FROM table1
LEFT JOIN table2 ON table1.column1 = table2.column1

The DISTINCT ON clause is followed by the column or columns that you want to use to filter out duplicate rows. In the example above, we are using the table1.column1 column to filter out duplicate rows.

Example

For example, let's say we have two tables - users and orders.

users
-------
id   name
1    John
2    Jane
3    Bob

orders
-------
id   user_id   product
1    1         Apple
2    1         Banana
3    2         Orange
4    3         Pear

We want to join these tables and return only the unique user_id values along with their corresponding name values from the users table.

SELECT DISTINCT ON (users.id) users.id, users.name, orders.product
FROM users
LEFT JOIN orders ON users.id = orders.user_id;

This will return:

id   name   product
-------------------
1    John   Apple
2    Jane   Orange
3    Bob    Pear

Note that the left join ensures that all rows from the users table are included in the result, even if there are no matching rows in the orders table. By using the distinct on clause, we remove any duplicate rows from the final result.

Conclusion

In this tutorial, we learned how to use a left join with distinct on in PostgreSQL to return unique rows while joining two tables. This can be useful in situations where you have duplicate rows in your tables and you only want to return unique rows based on certain criteria.