left join keyword from the left table (table_name1) where all rows are returned, even in the right table (table_name2) no matching rows.
LEFT JOIN keyword syntax
select column_name(s)
from table_name1
left join table_name2
on table_name1.column_name=table_name2.column_nameNote: In some databases, left join called a left join outer join.
example:
"Persons" 表：
| Id_P | LastName | FirstName | Address | City |
|---|---|---|---|---|
| 1 | Adams | John | Oxford Street | London |
| 2 | Bush | George | Fifth Avenue | New York |
| 3 | Carter | Thomas | Changan Street | Beijing |
"Orders" 表：
| Id_O | OrderNo | Id_P |
|---|---|---|
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 65 |
Left connecting (LEFT JOIN) Example
Now we want to list all of them, and their orders - if any.
You can use the following SELECT statement:
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
LEFT JOIN Orders
ON Persons.Id_P=Orders.Id_P
ORDER BY Persons.LastNameThe results set:
| LastName | FirstName | OrderNo |
|---|---|---|
| Adams | John | 22456 |
| Adams | John | 24562 |
| Carter | Thomas | 77895 |
| Carter | Thomas | 44678 |
| Bush | George |
LEFT JOIN keyword from the left table (Persons) where all rows are returned, even in the right table (Orders) no matching rows.
