What is a SQL Left (Outer) Join?

Data System Architecture


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 1

Left Join Result Discard

Example 2


Example 3

(inner set)
(outer set)
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

All the data from the table1 appears even if for the value D, we don't have any match with the table1.


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