PostgreSQL LEFT JOIN

LEFT JOIN

LEFT JOIN 关键字从“左”表中选择所有记录，并从“右”表中选择匹配的记录。如果没有匹配项，则右侧的结果为 0 条记录。

让我们使用虚拟的 testproducts 作为实例：

 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)

我们将尝试把 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 中的许多产品都有一个与 categories 表中的任何类别都不匹配的 category_id。

通过使用 LEFT JOIN，我们将从 testproducts 中获取所有记录，甚至是 categories 表中不匹配的记录：

实例

使用 category_id 列将 testproduct 与 categories 连接：

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

运行实例

结果

从 testproducts 中选择所有记录，并且只从 categories 中选择匹配的记录：

 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)

注意：LEFT JOIN 和 LEFT OUTER JOIN 将给出相同的结果。

OUTER 是 LEFT JOIN 的默认连接类型，因此当您编写 LEFT JOIN 时，解析器实际上会编写 LEFT OUTER JOIN。