PostgreSQL LEFT JOIN
LEFT JOIN
TheLEFT 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 dummytestproducts 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 thetestproducts table with thecategories 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 intestproducts have acategory_id that does not match any of the categories in thecategories table.
By usingLEFT JOIN we will get all records fromtestpoducts, even the ones with no match in thecategories table:
Example
Jointestproducts to categories using thecategory_id column:
FROM testproducts
LEFT JOIN categories ON testproducts.category_id = categories.category_id;
Result
All records fromtestproducts, and only the matched records fromcategories:
----------------+------------------------+----------------
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 andLEFT OUTER JOINwill give the same result.
OUTER is the default join type forLEFT JOIN, so when you writeLEFT JOIN the parser actually writesLEFT OUTER JOIN.
