About
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
Example
Example 1
Example 2
Example 3
| table1 (inner set) | table2 (outer set) |
|
|---|---|---|
| Column ID | Column ID | Column ID_2 |
| A | A | |
| B | B | A |
| D | C | C |
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.