Node.js MySQL 关联

关联两张或更多的表

您可以使用 JOIN 语句，根据两个或多个表之间的相关列，组合这些表中的行。

假设您有一个 "users" 表和一个 "products" 表。:

users

[
 { id: 1, name: 'John',  favorite_product: 154},
 { id: 2, name: 'Peter', favorite_product: 154},
 { id: 3, name: 'Amy',  favorite_product: 155},
 { id: 4, name: 'Hannah', favorite_product:},
 { id: 5, name: 'Michael',  favorite_product:}
]

products

[
 { id: 154, name:  'Chocolate Heaven' },
 { id: 155, name: 'Tasty Lemons' },
 { id: 156, name: 'Vanilla Dreams' }
]

这两个表可以通过使用 user 表中的 favorite_product 字段和 product 表中的 id 字段进行组合。

实例

选择两个表中都匹配的记录:

var mysql = require('mysql');
var con = mysql.createConnection({
 host: "localhost",
 user: "yourusername",
 password: "yourpassword",
 database: "mydb"
});
con.connect(function(err) {
 if (err) throw err;
 var  sql = "SELECT users.name AS user, products.name AS favorite FROM users <strong> JOIN products ON  users.favorite_product = products.id</strong>";
  con.query(sql, function (err, result) {
   if (err) throw err;
  console.log(result);
 });
});

将上述代码保存在名为 "demo_db_join.js" 的文件中，然后运行该文件：:

运行 "demo_db_join.js"

结果如下:

[
 { user: 'John', favorite:  'Chocolate Heaven' },
 { user: 'Peter', favorite: 'Chocolate Heaven' },
 { user: 'Amy', favorite: 'Tasty Lemons' }
]

从上面的结果可以看出，只返回两个表中都匹配的记录。

左关联

无论他们是否有喜爱的产品，都返回 所有 用户，请使用 LEFT JOIN 语句：:

实例

选择所有用户及其喜爱的产品:

 SELECT users.name AS user,
products.name AS favorite
FROM users<strong>
LEFT JOIN</strong> products ON users.favorite_product = products.id

结果如下:

[
 { user: 'John', favorite:  'Chocolate Heaven' },
 { user: 'Peter', favorite: 'Chocolate Heaven' },
 { user: 'Amy', favorite: 'Tasty Lemons' },
 { user: 'Hannah', favorite: null },
 { user: 'Michael', favorite: null }
]

右关联

如果要返回所有产品以及将其作为最喜爱商品的用户（即使没有用户将其作为最喜爱商品），请使用正确的 JOIN 语句:

实例

选择所有产品以及将其作为最喜爱商品的用户:

 SELECT users.name AS user,
products.name AS favorite
FROM users<strong>
RIGHT JOIN</strong> products ON users.favorite_product = products.id

结果如下:

[
 { user: 'John', favorite:  'Chocolate Heaven' },
 { user: 'Peter', favorite: 'Chocolate Heaven' },
 { user: 'Amy', favorite: 'Tasty Lemons' },
 { user: null, favorite: 'Vanilla Dreams' }
]