A Left Outer is a join that shows:
- All data from the left table
- and any data that matches the join predicate with the right table.
In some databases, LEFT JOIN is called LEFT OUTER JOIN.
- Causes the data of the shared records found in the second source to be merged with the records found in the first source
- Ensures that all records from the first data source are processed
- May cause some records within the second data source to be ignored or dropped
| table1 |
|Column ID||Column ID||Column ID_2|
select table1.id "ID_table1", table2.id "ID_table2" from table1 LEFT OUTER JOIN table2 on table1.id = table2.id;
ID_table1 ID_table2 ---------- ---------- A A B B D
All the data from the table1 appears even if for the value D, we don't have any match with the table1.