How does PROC SQL determine which records to keep in a left join?

A left join in PROC SQL is designed to keep all records from the left table and only those records from the right table that match based on a specified key. This means that when you perform a left join, PROC SQL examines the records in the left table and attempts to find corresponding records in the right table using the defined join condition—usually a common variable or key that exists in both tables.

When a match is found, the combined data from both tables is returned. However, if there is no match for a record in the left table, that record will still be included in the result set, but the fields from the right table will return as missing (null) for those unmatched records. This feature of the left join allows for comprehensive analysis, ensuring that all relevant data from the left table remains accessible even when complementary data in the right table is absent.

The other options do not accurately describe the behavior of a left join. For instance, including all records regardless of matches does not reflect the specific nature of a left join; while it keeps all records from the left table, it does not incorporate all records from the right table. Similarly, excluding all unmatched records or creating duplicates for unmatched records fails to characterize the left join, as these actions contradict