In an left outer join, all rows from the first table mentioned in the SQL query is selected, regardless whether there is a matching row on the second table mentioned in the SQL query. Let's assume that we have the following two tables,
Table Store_Information
store_name | Sales | Date |
Los Angeles | $1500 | Jan-05-1999 |
San Diego | $250 | Jan-07-1999 |
Los Angeles | $300 | Jan-08-1999 |
Boston | $700 | Jan-08-1999 |
Table Geography
region_name | store_name |
East | Boston |
East | New York |
West | Los Angeles |
West | San Diego |
We want to find out sales by store, and we want to see the results for all stores regardless whether there is a sale in the Store_Information table. To do this, we can use the following SQL statement using LEFT OUTER JOIN:
SELECT A1.store_name STORE, SUM(A2.Sales) SALES
FROM Geography A1
LEFT OUTER JOIN Store_Information A2
ON A1.store_name = A2.store_name
GROUP BY A1.store_name
Result:
STORE | SALES | |
Los Angeles | $1800 | |
San Diego | $250 | |
New York | NULL | |
Boston | $700 |
By using LEFT OUTER JOIN, all four rows in the Geography table is listed. Since there is no match for "New York" in the Store_Information table, the Sales total for "New York" is NULL. Note that it is NULL and not 0, as NULL indicates there is no match.