What is a left join in Rust Diesel?

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A left join in Rust Diesel is a type of SQL join where all the records from one table (the left table) are returned and matched with the corresponding records from a second table (the right table), if there are any matches. If there are no matches in the right table, NULL values are returned. It is represented in Diesel by the left_join method on a QuerySource struct, which creates a new query that includes the left table and the matched records from the right table.