With the usual join of two tables, the result table contains only those rows for which the join condition is satisfied. The outer join also permits rows to be included in the result table for which there are no corresponding rows in the second table. These kinds of rows are supplemented in the outer join by NULL values in the columns which, properly speaking, should come from the other table. When joining two tables, the NULL value can be added for the "left" or the "right" table or also for both.
To be able to execute outer joins over more than two tables with a clearly defined semantics, ADABAS offers the possibility of formulating a further SELECT instead of a table name in the FROM part of the SELECT statement. In this way, in the case of outer joins, the processing sequence is laid down implicitly.