When you need to return all rows that are present in both tables, you use a full outer join. This type of join combines the results of both left and right outer joins and returns all rows from both tables, with matching rows from both sides where available. If there is no match, the result is NULL on the side of the join where there is no match.
References: Information about joins and their use in querying data in a lakehouse can be typically found in the SQL and data processing documentation of the Fabric tenant or lakehouse solutions.