In PROC SQL, what does a left join accomplish?

A left join in PROC SQL is designed to combine records from two tables while retaining all records from the left table regardless of whether there are matching records in the right table. This means that for every record in the left table, the query attempts to find a corresponding match in the right table based on the specified join condition. If there is no match found, the resulting dataset will still include the records from the left table, but the fields from the right table for those unmatched records will be filled with missing values.

This behavior makes the left join particularly useful when the primary focus is to ensure all entries from the left dataset are represented in the output, while simultaneously allowing for matches where they exist. In situations where the left table holds all necessary records, a left join guarantees that no information is lost from it, even if some entries in the right table do not correspond with it.

A better grasp of this concept can help in scenarios where it is critical to maintain the complete dataset from one side of a relationship while still acquiring additional data from the other side, reflecting the desired insight without omitting important information from the primary dataset.