目录
- PostgreSQL 教程
- Database
-
PostgreSQL 语法
- PostgreSQL 运算符
- PostgreSQL SELECT
- PostgreSQL
- PostgreSQL WHERE
- PostgreSQL ORDER BY
- PostgreSQL LIMIT
- PostgreSQL MIN and MAX
- PostgreSQL COUNT
- PostgreSQL SUM
- PostgreSQL AVG
- PostgreSQL LIKE
- PostgreSQL IN
- PostgreSQL BETWEEN
- PostgreSQL AS
- PostgreSQL Joins
- PostgreSQL INNER JOIN
- PostgreSQL LEFT JOIN
- PostgreSQL RIGHT JOIN
- PostgreSQL FULL JOIN
- PostgreSQL CROSS JOIN
- PostgreSQL UNION
- PostgreSQL GROUP BY
- PostgreSQL HAVING
- PostgreSQL EXISTS
- PostgreSQL ANY
- PostgreSQL ALL
- PostgreSQL CASE
- 练习
PostgreSQL 左连接
左连接
这个LEFT JOIN关键字从 "left" 表中选择所有记录，并从 "right" 表中选择匹配的记录。如果没有匹配，则结果是从右侧开始 0 条记录。
让我们看一个使用我们的虚拟对象的例子testproducts表格：
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)
我们将努力加入testproducts表与categories表格：
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)
笔记：许多产品在testproducts有一个category_id与中的任何类别都不匹配categories表格。
通过使用LEFT JOIN我们将从中获取所有记录testpoducts，即使是那些没有匹配的categories表格：
示例
加入testproducts到 categories使用category_id柱子：
SELECT testproduct_id, product_name, category_name
FROM testproducts
LEFT JOIN categories ON testproducts.category_id = categories.category_id;
运行示例 »
结果
所有记录来自testproducts，并且只有来自的匹配记录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)
笔记：LEFT JOIN和 LEFT OUTER JOIN会给出相同的结果。
OUTER是默认的连接类型LEFT JOIN，所以当你写LEFT JOIN解析器实际上写的是LEFT OUTER JOIN。
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