However, you can use a WHERE clause to filter the results.Ī NATURAL JOIN is identical to an explicit JOIN on the common columns of the two tables, except that the common columns are included only once in the output. A CROSS JOIN cannot be combined with an ON condition clause. Cartesian product), the joined table contains a row consisting of all columns in o1 followed by all columns in o2. Returns all joined rows, plus one row for each unmatched left side row (extended with nulls on the right), plus one row for each unmatched right side row (extended with nulls on the left).įor every possible combination of rows from o1 and o2 (i.e. The result columns referencing o1 contain null. The result of the inner join is augmented with a row for each row of o2 that has no matches in o1. The result columns referencing o2 contain null. The result of the inner join is augmented with a row for each row of o1 that has no matches in o2. For an example, see the examples section below.) If you use INNER JOIN without the ON clause (or if you use comma without a WHERE clause), the result is the same as using CROSS JOIN: a Cartesian product (every row of o1 paired with every row of o2). (Note that you can also use a comma to specify an inner join. O2 for object_ref1 and object_ref2, respectively).įor each row of o1, a row is produced for each row of o2 that matches according to the ON condition subclause. The semantics of joins are as follows (for brevity, this topic uses o1 and INNER or OUTER) to specify the type of join. Use the JOIN keyword to specify that the tables should be joined. In part III of the series, I'll continue with some examples, this time explaining how we can combine multiple joins in the same query.Each object reference is a table or table-like data source. This will produce exactly the same SQL and output as before: (:bookings) If you're on rails 6 you could write a more semantic version for this solution using the ActiveRecord's missing method. The SQL: SELECT COUNT(*) FROM "accommodations" LEFT OUTER JOIN "bookings" ON "bookings"."accommodation_id" = "accommodations"."id" WHERE "bookings"."id" IS NULL Accommodation.left_joins(:bookings).where(bookings: ) So knowing that, we can use where to get only the accommodations with a null bookings.id. So how can we filter that list to return only the no-bookings accommodations? Looking at the left outer join table from the earlier exercise, we can say that what differentiates the two groups is that the accommodations with bookings will have an attribute bookings.id with an integer value but the no-bookings accommodations will have a null value for that same attribute. We now know that left_joins will include the accommodations that have no bookings but it will also include all the others that do. Let's now say that we only want to get those accommodations that do not have any bookings. Get all the accommodations with no bookings Will have to be written as: Accommodation.joins('LEFT OUTER JOIN bookings ON bookings.accommodation_id = acommodations.id") This means that, Accommodation.left_joins(:bookings) If you're working with earlier versions of rails you'll have to customize the rails joins method by passing an SQL string with the desired join method. Left_joins and left_outer_joins methods are only available from rails 5.2.3 onwards. Here's our domain model: class Accommodation 2, "Charming House"=>1, "Lisbon Flat"=>0, "Le Petit Chalet"=>1, "Farm House"=>0} Let's again follow along with a couple of exercises: Contrarily to an inner join, the left outer join will always return all the entries of the left table even if the join condition does not find any match on the right table. What is a left outer join and when should it be used?Ī left outer join (or just left join) is used to query a table based on matching and non-matching entries from a related table.
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