An outer join indicates that one table has more rows the other table for the joining columns. You define an outer join by specifying which table is the outer table in the original equi-join.
The outer table contains the column for which you want to return all values, even if there is no matching column in the other table. You specify the outer table from the Edit Join dialog box for the selected join.
When writing SQL directly (versus generating it using a universe), different database systems define "outer" differently, and the syntax of the statement also differs. For example, depending on the underlying database system, the outer join may be on the left or right table.
But a universe is not database-specific, so outer joins must remain generic. The database driver chosen when creating the connection determines whether the outer join is left or right. Therefore, in a universe, the outer join is usually placed on the table that contains all the data.
Outer Join Best Practices
SAP recommends that you place outer joins at the end of the flow of data, otherwise ambiguous outer join errors may occur. These errors have the potential to cause the SQL to try to match the equality of a NULL value, which it cannot do.
If you do place outer joins in the middle of a table path, make the subsequent joins in the path outer joins to avoid errors.
Be aware that outer joins may cause the query to run slower than a query with no outer joins.