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:
----------------+------------------------+-------------
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:
-------------+----------------+------------------------------------------------------------
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:
FROM testproducts
LEFT JOIN categories ON testproducts.category_id = categories.category_id;
Result
All records from testproducts
, and only the matched records from
categories
:
----------------+------------------------+----------------
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
.