Join Two or More Tables – Хоёр ба түүнээс дээш хүснэгтийг нэгтгэх #
You can combine rows from two or more tables, based on a related column between them, by using a JOIN statement.
Consider you have a “users” table and a “products” table:
users
Хэрэглэгч
{ 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
Бүтээгдэхүүн
{ id: 154, name: 'Chocolate Heaven' },
{ id: 155, name: 'Tasty Lemons' },
{ id: 156, name: 'Vanilla Dreams' }
These two tables can be combined by using users’ fav field and products’ id field.
Example – Жишээ #
Join users and products to see the name of the users favorite product:
import mysql.connector
mydb = mysql.connector.connect(
host="localhost",
user="yourusername",
password="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)
Note: You can use JOIN instead of INNER JOIN. They will both give you the same result.
LEFT JOIN #
In the example above, Hannah, and Michael were excluded from the result, that is because INNER JOIN only shows the records where there is a match.
If you want to show all users, even if they do not have a favorite product, use the LEFT JOIN statement:
Example #
Select all users and their favorite product:
sql = "SELECT \
users.name AS user, \
products.name AS favorite \
FROM users \
LEFT JOIN products ON users.fav = products.id"
RIGHT JOIN #
If you want to return all products, and the users who have them as their favorite, even if no user have them as their favorite, use the RIGHT JOIN statement:
Example #
Select all products, and the user(s) who have them as their favorite:
sql = "SELECT \
users.name AS user, \
products.name AS favorite \
FROM users \
RIGHT JOIN products ON users.fav = products.id"
Note: Hannah and Michael, who have no favorite product, are not included in the result.
