PostgreSQL LEFT JOIN
LEFT JOIN
The LEFT JOIN keyword selects ALL records from the "left" table,
and the matching records from the "right" table.
The result is 0 records from the right side if there is no match.
Let's look at an example using our dummy testproducts table:
testproduct_id | product_name
| category_id
----------------+------------------------+-------------
1 | Johns Fruit Cake |
3
2 | Marys Healthy Mix |
9
3 | Peters Scary Stuff |
10
4 | Jims Secret Recipe |
11
5 | Elisabeths Best Apples |
12
6 | Janes Favorite Cheese |
4
7 | Billys Home Made Pizza |
13
8 | Ellas Special Salmon |
8
9 | Roberts Rich Spaghetti |
5
10 |
Mias Popular Ice |
14
(10 rows)
We will try to join the testproducts table
with the categories table:
category_id | category_name |
description
-------------+----------------+------------------------------------------------------------
1 | Beverages | Soft drinks, coffees, teas,
beers, and ales
2 | Condiments | Sweet and savory sauces, relishes,
spreads, and seasonings
3 | Confections | Desserts, candies, and sweet breads
4 | Dairy Products | Cheeses
5 | Grains/Cereals | Breads, crackers, pasta, and cereal
6 | Meat/Poultry | Prepared meats
7 | Produce | Dried fruit and bean
curd
8 |
Seafood | Seaweed and fish
(8
rows)
Note: Many of the products in testproducts have a
category_id that does not match any of the
categories in the categories table.
By using LEFT JOIN we will get all
records from testpoducts, even the ones with no
match in the categories table:
Example
Join testproducts to
categories using the category_id column:
SELECT
testproduct_id, product_name, category_name
FROM testproducts
LEFT JOIN categories ON testproducts.category_id = categories.category_id;
Run Example »
Result
All records from testproducts, and only the matched records from
categories:
testproduct_id | product_name
| category_name
----------------+------------------------+----------------
1 | Johns Fruit Cake | Confections
2 | Marys Healthy Mix |
3 | Peters Scary Stuff |
4 | Jims Secret Recipe |
5 | Elisabeths Best Apples |
6 | Janes Favorite Cheese | Dairy Products
7 | Billys Home Made Pizza |
8 | Ellas Special Salmon | Seafood
9 | Roberts Rich Spaghetti | Grains/Cereals
10 | Mias Popular Ice |
(10 rows)
Note: LEFT JOIN and
LEFT OUTER JOIN
will give the same result.
OUTER is the default join type for
LEFT JOIN, so when you write
LEFT JOIN the parser actually writes
LEFT OUTER JOIN.
