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Python MySQL Join

Python基础 2022-06-06 12:58:34小码哥的IT人生shichen

Python MySQL Join

组合两张或更多表

您可以使用 JOIN 语句,根据它们之间的相关列组合两个或多个表中的行。

假设您有 "users" 表和 "products" 表:

users

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

products

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

可以使用 users 的 fav 字段和 products 的 id 字段来组合这两个表。

示例代码:

组合用户和产品,查看用户最喜欢的产品名称:

import mysql.connector
mydb = mysql.connector.connect(
  host="localhost",
  user="yourusername",
  passwd="yourpassword",
  database="mydatabase"
)
mycursor = mydb.cursor()
sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  INNER JOIN products ON users.fav = products.id"
mycursor.execute(sql)
myresult = mycursor.fetchall()
for x in myresult:
  print(x)

完整实例:

import mysql.connector
mydb = mysql.connector.connect(
  host="localhost",
  user="myusername",
  passwd="mypassword",
  database="mydatabase"
)
mycursor = mydb.cursor()
sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  INNER JOIN products ON users.fav = products.id"
mycursor.execute(sql)
myresult = mycursor.fetchall()
for x in myresult:
  print(x)

注释:您可以使用 JOIN 而不是 INNER JOIN。您都会得到相同的结果。

LEFT JOIN

在上例中,Hannah 和 Michael 被排除在结果之外,这是因为 INNER JOIN 只显示匹配的记录。

如果希望显示所有用户,即使他们没有喜欢的产品,请使用 LEFT JOIN 语句:

示例代码:

选择所有用户及其喜爱的产品:

sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  LEFT JOIN products ON users.fav = products.id"

完整实例:

import mysql.connector
mydb = mysql.connector.connect(
  host="localhost",
  user="myusername",
  passwd="mypassword",
  database="mydatabase"
)
mycursor = mydb.cursor()
sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  LEFT JOIN products ON users.fav = products.id"
mycursor.execute(sql)
myresult = mycursor.fetchall()
for x in myresult:
  print(x)

RIGHT JOIN

如果您想要返回所有产品以及喜欢它们的用户,即使没有用户喜欢这些产品,请使用 RIGHT JOIN 语句:

示例代码:

选择所有产品以及喜欢它们的用户:

sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  RIGHT JOIN products ON users.fav = products.id"

完整实例:

import mysql.connector
mydb = mysql.connector.connect(
  host="localhost",
  user="myusername",
  passwd="mypassword",
  database="mydatabase"
)
mycursor = mydb.cursor()
sql = "SELECT \
  users.name AS user, \
  products.name AS favorite \
  FROM users \
  RIGHT JOIN products ON users.fav = products.id"
mycursor.execute(sql)
myresult = mycursor.fetchall()
for x in myresult:
  print(x)

注释:不对任何产品感兴趣的 Hannah 和 Michael 不包括在结果中。

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