PostgreSQL 左关联（LEFT JOIN）

LEFT JOIN

LEFT JOIN 关键字从 "left" 表中选择所有记录，从 "right" 表中选中匹配的记录。如果没有匹配项，则结果为右侧的 0 条记录。

让我们看一个使用我们的 testproducts 表的例子:

 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)

我们将尝试将 testproducts 表与 categories 表连接起来:

 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)

注意：testproducts 中的许多产品的 category_id 与 categories 表中的任何类别都不匹配。

通过使用 LEFT JOIN，我们将从 testpoducts 中获取所有记录，即使是 categories 表中没有匹配项的记录:

实例

使用 category_id 列将 testproducts 表和 categories 关联起来:

SELECT testproduct_id, product_name, category_name
FROM testproducts
LEFT JOIN categories ON testproducts.category_id = categories.category_id;

结果

来自 testproducts 的所有记录，以及仅来自 categories 的匹配记录:

 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)