Table of Contents

• 1  Introduction
• 2  Load the Libraries
• 3  jupyter magic for postgres
• 4  Create Tables
• 5  Create pandas dataframes
• 6  Exercise Questions
  • 6.1  Select the names of all the products in the store.
  • 6.2  Select the names and the prices of all the products in the store.
  • 6.3  Select the name of the products with a price less than or equal to 200.
  • 6.4  Select all the products with a price between 60 and 120.
  • 6.5  Select the name and price in cents (i.e., the price must be multiplied by 100)
  • 6.6  Compute the average price of all the products.
  • 6.7  Compute the average price of all products with manufacturer code equal to 2.
  • 6.8  Compute the number of products with a price larger than or equal to 180
  • 6.9  Select the name and price of all products with a price larger than or equal to 180, and sort first by price (in descending order), and then by name (in ascending order).
  • 6.10  Select all the data from the products, including all the data for each product's manufacturer.
  • 6.11  Select the product name, price, and manufacturer name of all the products.
  • 6.12  Select the average price of each manufacturer's products, showing only the manufacturer's code.
  • 6.13  Select the average price of each manufacturer's products, showing the manufacturer's name.
  • 6.14  Select the names of manufacturer whose products have an average price larger than or equal to 150.
  • 6.15  Select the name and price of the cheapest product.
  • 6.16  Select the name of each manufacturer along with the name and price of its most expensive product.
  • 6.17  Add a new product: Loudspeakers, 70, manufacturer 2.
  • 6.18  Update the name of product 8 to "Laser Printer".
  • 6.19  Apply a 10 percent discount to all products.
  • 6.20  Apply a 10% discount to all products with a price larger than or equal to 120.
• 7  Time Taken

Introduction

• This is wikibooks SQL Exercise "The Computer Store"
• https://en.wikibooks.org/wiki/SQL_Exercises/The_computer_store
• I have conda environment Spark3 to run sql commands
• Always run PgAdmin4 before running postgres in notebooks.

Relational Schema for Computer Store Database

Load the Libraries

import time
time_start_notebook = time.time()

# my local library
import sys
sys.path.append("/Users/poudel/Dropbox/a00_Bhishan_Modules/")
sys.path.append("/Users/poudel/Dropbox/a00_Bhishan_Modules/bhishan")
from bhishan import bp

import numpy as np
import pandas as pd
import os
import json

%load_ext watermark
%watermark -a "Bhishan Poudel" -d -v -m
print()
%watermark -iv

The watermark extension is already loaded. To reload it, use:
  %reload_ext watermark
Author: Bhishan Poudel

Python implementation: CPython
Python version       : 3.7.7
IPython version      : 7.16.1

Compiler    : Clang 4.0.1 (tags/RELEASE_401/final)
OS          : Darwin
Release     : 19.6.0
Machine     : x86_64
Processor   : i386
CPU cores   : 4
Architecture: 64bit


json    : 2.0.9
bhishan : 0.4
pandas  : 1.0.5
numpy   : 1.17.5
sys     : 3.7.7 (default, May  6 2020, 04:59:01) 
[Clang 4.0.1 (tags/RELEASE_401/final)]
autopep8: 1.5.4

jupyter magic for postgres

• first we install ipython-sql, pyscopg2
• Then we get cell magic %%sql and inline magic %sql.

%load_ext sql
%config SqlMagic.displaycon=False 
%config SqlMagic.feedback=False  # no Done, rows affected
%config SqlMagic.autopandas=True

%sql sqlite://

The sql extension is already loaded. To reload it, use:
  %reload_ext sql

Create Tables

We do in only once. Once we added these tables to our database dvdrental, we are done. We can now use these table now for all the future times.

%%sql

DROP TABLE IF EXISTS Manufacturers;

%%sql

CREATE TABLE Manufacturers (Code INTEGER, Name VARCHAR(255) NOT NULL,
                                                            PRIMARY KEY (Code));

%%sql

INSERT INTO Manufacturers(Code, Name)
VALUES(1,'Sony');


INSERT INTO Manufacturers(Code, Name)
VALUES(2,'Creative Labs');


INSERT INTO Manufacturers(Code, Name)
VALUES(3,'Hewlett-Packard');


INSERT INTO Manufacturers(Code, Name)
VALUES(4,'Iomega');


INSERT INTO Manufacturers(Code, Name)
VALUES(5,'Fujitsu');


INSERT INTO Manufacturers(Code, Name)
VALUES(6,'Winchester');


INSERT INTO Manufacturers(Code, Name)
VALUES(7,'Bose');

%%sql

SELECT *
FROM Manufacturers;

	Code	Name
0	1	Sony
1	2	Creative Labs
2	3	Hewlett-Packard
3	4	Iomega
4	5	Fujitsu
5	6	Winchester
6	7	Bose

%%sql

DROP TABLE IF EXISTS Products;

%%sql

CREATE TABLE Products (Code INTEGER, Name VARCHAR(255) NOT NULL,
                                                       Price DECIMAL NOT NULL,
                                                                     Manufacturer INTEGER NOT NULL,
                                                                                          PRIMARY KEY (Code),
                       FOREIGN KEY (Manufacturer) REFERENCES Manufacturers(Code)) ;

%%sql

INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(1, 'Hard drive', 240, 5);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(2, 'Memory', 120, 6);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(3, 'ZIP drive', 150, 4);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(4, 'Floppy disk', 5, 6);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(5, 'Monitor', 240, 1);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(6, 'DVD drive', 180, 2);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(7, 'CD drive', 90, 2);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(8, 'Printer', 270, 3);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(9, 'Toner cartridge', 66, 3);


INSERT INTO Products(Code, Name, Price, Manufacturer)
VALUES(10, 'DVD burner', 180, 2);

Create pandas dataframes

%%sql

SELECT *
FROM Manufacturers;

	Code	Name
0	1	Sony
1	2	Creative Labs
2	3	Hewlett-Packard
3	4	Iomega
4	5	Fujitsu
5	6	Winchester
6	7	Bose

dfm = pd.DataFrame({'code': [1, 2, 3, 4, 5, 6,7],
          'name': ['Sony', 'Creative Labs', 'Hewlett-Packard', 'Iomega', 'Fujitsu', 'Winchester','Bose']})
dfm

	code	name
0	1	Sony
1	2	Creative Labs
2	3	Hewlett-Packard
3	4	Iomega
4	5	Fujitsu
5	6	Winchester
6	7	Bose

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

x = %sql select * from Products;
dfp = x  # x.DataFrame() we need x.DataFrame() if we dont have %config SqlMagic.autopandas=True
dfp.columns = dfp.columns.str.lower()

dfp.dtypes

code             int64
name            object
price            int64
manufacturer     int64
dtype: object

dfp.infer_objects().dtypes

code             int64
name            object
price            int64
manufacturer     int64
dtype: object

dfp['price'] = dfp['price'].astype(int)
dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

df = dfp.merge(dfm,left_on='manufacturer', right_on='code',suffixes=['_product','_manufacturer'])
df

	code_product	name_product	price	manufacturer	code_manufacturer	name_manufacturer
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	4	Floppy disk	5	6	6	Winchester
3	3	ZIP drive	150	4	4	Iomega
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	10	DVD burner	180	2	2	Creative Labs
8	8	Printer	270	3	3	Hewlett-Packard
9	9	Toner cartridge	66	3	3	Hewlett-Packard

Exercise Questions

Select the names of all the products in the store.

Question: 1. Select the names of all the products in the store.

%%sql

SELECT Name
FROM Products;

	Name
0	Hard drive
1	Memory
2	ZIP drive
3	Floppy disk
4	Monitor
5	DVD drive
6	CD drive
7	Printer
8	Toner cartridge
9	DVD burner

# dfp['name'] # series
dfp[['name']] # dataframe

	name
0	Hard drive
1	Memory
2	ZIP drive
3	Floppy disk
4	Monitor
5	DVD drive
6	CD drive
7	Printer
8	Toner cartridge
9	DVD burner

Select the names and the prices of all the products in the store.

Question: 2. Select the names and the prices of all the products in the store.

dfp.columns

Index(['code', 'name', 'price', 'manufacturer'], dtype='object')

%%sql
-- select name,price from Products;

SELECT Name,
       Price
FROM Products;

	Name	Price
0	Hard drive	240
1	Memory	120
2	ZIP drive	150
3	Floppy disk	5
4	Monitor	240
5	DVD drive	180
6	CD drive	90
7	Printer	270
8	Toner cartridge	66
9	DVD burner	180

dfp[['name','price']]

	name	price
0	Hard drive	240
1	Memory	120
2	ZIP drive	150
3	Floppy disk	5
4	Monitor	240
5	DVD drive	180
6	CD drive	90
7	Printer	270
8	Toner cartridge	66
9	DVD burner	180

Select the name of the products with a price less than or equal to 200.

Question: 3. Select the name of the products with a price less than or equal to $200.

dfp.head(1)

	code	name	price	manufacturer
0	1	Hard drive	240	5

dfp.loc[dfp.price <=200, 'name']

1             Memory
2          ZIP drive
3        Floppy disk
5          DVD drive
6           CD drive
8    Toner cartridge
9         DVD burner
Name: name, dtype: object

dfp.query("price <= 200")#['name']

	code	name	price	manufacturer
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
5	6	DVD drive	180	2
6	7	CD drive	90	2
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

%%sql

SELECT name
FROM Products
WHERE price <= 200;

	Name
0	Memory
1	ZIP drive
2	Floppy disk
3	DVD drive
4	CD drive
5	Toner cartridge
6	DVD burner

Select all the products with a price between 60 and 120.

Question: 4. Select all the products with a price between \$60 and \$120.

dfp.query("60<= price <= 120")

	code	name	price	manufacturer
1	2	Memory	120	6
6	7	CD drive	90	2
8	9	Toner cartridge	66	3

dfp.loc[ (dfp.price>=60) & (dfp.price<=120)]

	code	name	price	manufacturer
1	2	Memory	120	6
6	7	CD drive	90	2
8	9	Toner cartridge	66	3

# dfp.loc[ 60<= dfp.price <=120] # syntax error.

dfp.loc[dfp.price.between(60,120)]

	code	name	price	manufacturer
1	2	Memory	120	6
6	7	CD drive	90	2
8	9	Toner cartridge	66	3

%%sql

SELECT *
FROM Products
WHERE price BETWEEN 60 AND 120;

	Code	Name	Price	Manufacturer
0	2	Memory	120	6
1	7	CD drive	90	2
2	9	Toner cartridge	66	3

%%sql

SELECT *
FROM Products
WHERE price >= 60
  AND Price <= 120;

	Code	Name	Price	Manufacturer
0	2	Memory	120	6
1	7	CD drive	90	2
2	9	Toner cartridge	66	3

Select the name and price in cents (i.e., the price must be multiplied by 100)

Question: 5. Select the name and price in cents (i.e., the price must be multiplied by 100)

# dfp['price_cents'] = dfp['price'] * 100
# dfp[['name','price_cents']]

dfp.assign(
price_cents = dfp['price']*100
)[['name','price_cents']]

	name	price_cents
0	Hard drive	24000
1	Memory	12000
2	ZIP drive	15000
3	Floppy disk	500
4	Monitor	24000
5	DVD drive	18000
6	CD drive	9000
7	Printer	27000
8	Toner cartridge	6600
9	DVD burner	18000

%%sql

SELECT name,
       price * 100 AS price_cents
FROM Products;

	Name	price_cents
0	Hard drive	24000
1	Memory	12000
2	ZIP drive	15000
3	Floppy disk	500
4	Monitor	24000
5	DVD drive	18000
6	CD drive	9000
7	Printer	27000
8	Toner cartridge	6600
9	DVD burner	18000

Compute the average price of all the products.

Question: 6. Compute the average price of all the products.

dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

dfp.shape[0], dfp['name'].nunique()

(10, 10)

dfp.groupby('name')['price'].mean()

name
CD drive            90
DVD burner         180
DVD drive          180
Floppy disk          5
Hard drive         240
Memory             120
Monitor            240
Printer            270
Toner cartridge     66
ZIP drive          150
Name: price, dtype: int64

dfp['price'].mean()

154.1

%%sql
select avg(price) from Products;

	avg(price)
0	154.1

Compute the average price of all products with manufacturer code equal to 2.

Question: 7. Compute the average price of all products with manufacturer code equal to 2.

dfp.price[dfp.manufacturer==2].mean()

150.0

# Never do:
dfp[dfp.manufacturer==2]['price'].mean()
# never select whole dataframe, if you just need one series.

150.0

%%sql

SELECT avg(price)
FROM Products
WHERE manufacturer=2;

	avg(price)
0	150.0

Compute the number of products with a price larger than or equal to 180

Question: 8. Compute the number of products with a price larger than or equal to $180.

dfp[dfp.price >= 180].shape[0]

5

dfp.price[dfp.price >= 180].shape[0]

5

%%sql

SELECT count(*)
FROM Products
WHERE price >= 180;

	count(*)
0	5

Select the name and price of all products with a price larger than or equal to 180, and sort first by price (in descending order), and then by name (in ascending order).

Question: 9. Select the name and price of all products with a price larger than or equal to $180, and sort first by price (in descending order), and then by name (in ascending order).

dfp[['name','price']].query("price >= 180")\
.sort_values(['price','name'],ascending=[False,True])

	name	price
7	Printer	270
0	Hard drive	240
4	Monitor	240
9	DVD burner	180
5	DVD drive	180

%%sql

SELECT name,
       price
FROM Products
WHERE price >= 180
ORDER BY price DESC,
         name ASC;

	Name	Price
0	Printer	270
1	Hard drive	240
2	Monitor	240
3	DVD burner	180
4	DVD drive	180

Select all the data from the products, including all the data for each product's manufacturer.

Question: 10. Select all the data from the products, including all the data for each product's manufacturer.

dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

dfm

	code	name
0	1	Sony
1	2	Creative Labs
2	3	Hewlett-Packard
3	4	Iomega
4	5	Fujitsu
5	6	Winchester
6	7	Bose

dfp.merge(dfm,left_on='manufacturer', right_on='code',how='left')

	code_x	name_x	price	manufacturer	code_y	name_y
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	3	ZIP drive	150	4	4	Iomega
3	4	Floppy disk	5	6	6	Winchester
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	8	Printer	270	3	3	Hewlett-Packard
8	9	Toner cartridge	66	3	3	Hewlett-Packard
9	10	DVD burner	180	2	2	Creative Labs

%%sql

SELECT *
FROM Products p,
     Manufacturers m
WHERE p.Manufacturer = m.Code;

	Code	Name	Price	Manufacturer	Code	Name
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	3	ZIP drive	150	4	4	Iomega
3	4	Floppy disk	5	6	6	Winchester
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	8	Printer	270	3	3	Hewlett-Packard
8	9	Toner cartridge	66	3	3	Hewlett-Packard
9	10	DVD burner	180	2	2	Creative Labs

%%sql

SELECT *
FROM Products p
INNER JOIN Manufacturers m ON p.Manufacturer = m.Code;

	Code	Name	Price	Manufacturer	Code	Name
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	3	ZIP drive	150	4	4	Iomega
3	4	Floppy disk	5	6	6	Winchester
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	8	Printer	270	3	3	Hewlett-Packard
8	9	Toner cartridge	66	3	3	Hewlett-Packard
9	10	DVD burner	180	2	2	Creative Labs

%%sql

SELECT *
FROM Products p
LEFT JOIN Manufacturers m ON p.Manufacturer = m.Code;

	Code	Name	Price	Manufacturer	Code	Name
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	3	ZIP drive	150	4	4	Iomega
3	4	Floppy disk	5	6	6	Winchester
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	8	Printer	270	3	3	Hewlett-Packard
8	9	Toner cartridge	66	3	3	Hewlett-Packard
9	10	DVD burner	180	2	2	Creative Labs

Select the product name, price, and manufacturer name of all the products.

Question: 11. Select the product name, price, and manufacturer name of all the products.

dfp.head(2)

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6

dfm.head(2)

	code	name
0	1	Sony
1	2	Creative Labs

dfp.merge(dfm,left_on='manufacturer',right_on='code',
          suffixes=['_product','_manufacturer']
         )[['name_product','price','name_manufacturer']]

	name_product	price	name_manufacturer
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	Floppy disk	5	Winchester
3	ZIP drive	150	Iomega
4	Monitor	240	Sony
5	DVD drive	180	Creative Labs
6	CD drive	90	Creative Labs
7	DVD burner	180	Creative Labs
8	Printer	270	Hewlett-Packard
9	Toner cartridge	66	Hewlett-Packard

# dfm.set_index('code')
# dfm.set_index('code')['name']
# dfp['manufacturer'].map(dfm.set_index('code')['name'])

dfp[['name','price']].assign(
name_manufacturer = dfp['manufacturer'].map(dfm.set_index('code')['name']))

	name	price	name_manufacturer
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	ZIP drive	150	Iomega
3	Floppy disk	5	Winchester
4	Monitor	240	Sony
5	DVD drive	180	Creative Labs
6	CD drive	90	Creative Labs
7	Printer	270	Hewlett-Packard
8	Toner cartridge	66	Hewlett-Packard
9	DVD burner	180	Creative Labs

# using two tables and where

%%sql

SELECT p.name,
       p.price,
       m.name AS manufacturer
FROM Products p,
     Manufacturers m
WHERE p.manufacturer=m.Code;

	Name	Price	manufacturer
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	ZIP drive	150	Iomega
3	Floppy disk	5	Winchester
4	Monitor	240	Sony
5	DVD drive	180	Creative Labs
6	CD drive	90	Creative Labs
7	Printer	270	Hewlett-Packard
8	Toner cartridge	66	Hewlett-Packard
9	DVD burner	180	Creative Labs

# using inner join and ON

%%sql

SELECT p.name,
       p.price,
       m.name AS manufacturer
FROM Products p
INNER JOIN Manufacturers m ON p.manufacturer=m.code;

	Name	Price	manufacturer
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	ZIP drive	150	Iomega
3	Floppy disk	5	Winchester
4	Monitor	240	Sony
5	DVD drive	180	Creative Labs
6	CD drive	90	Creative Labs
7	Printer	270	Hewlett-Packard
8	Toner cartridge	66	Hewlett-Packard
9	DVD burner	180	Creative Labs

Select the average price of each manufacturer's products, showing only the manufacturer's code.

Question: 12. Select the average price of each manufacturer's products, showing only the manufacturer's code.

dfp.head(2)

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6

dfm.head(2)

	code	name
0	1	Sony
1	2	Creative Labs

dfp.groupby('manufacturer')['price'].mean()

manufacturer
1    240.0
2    150.0
3    168.0
4    150.0
5    240.0
6     62.5
Name: price, dtype: float64

dfp['manufacturer'].unique()

array([5, 6, 4, 1, 2, 3])

%%sql

SELECT manufacturer,
       round(avg(price), 2) AS avg_price
FROM Products
GROUP BY manufacturer;

	Manufacturer	avg_price
0	1	240.0
1	2	150.0
2	3	168.0
3	4	150.0
4	5	240.0
5	6	62.5

%%sql

SELECT manufacturer,
       round(avg(price), 2) AS avg_price
FROM Products
GROUP BY manufacturer
ORDER BY manufacturer;

	Manufacturer	avg_price
0	1	240.0
1	2	150.0
2	3	168.0
3	4	150.0
4	5	240.0
5	6	62.5

dfp.groupby('manufacturer')['price'].mean()

manufacturer
1    240.0
2    150.0
3    168.0
4    150.0
5    240.0
6     62.5
Name: price, dtype: float64

Select the average price of each manufacturer's products, showing the manufacturer's name.

Question: 13. Select the average price of each manufacturer's products, showing the manufacturer's name.

dfp.merge(dfm,left_on='manufacturer', right_on='code',
          suffixes=['_product','_manufacturer'])\
.groupby('name_manufacturer')['price'].mean()

name_manufacturer
Creative Labs      150.0
Fujitsu            240.0
Hewlett-Packard    168.0
Iomega             150.0
Sony               240.0
Winchester          62.5
Name: price, dtype: float64

%%sql

SELECT m.name,
       round(avg(price), 2) AS avg_price
FROM Products p
INNER JOIN Manufacturers m ON p.manufacturer = m.code
GROUP BY m.name;

	Name	avg_price
0	Creative Labs	150.0
1	Fujitsu	240.0
2	Hewlett-Packard	168.0
3	Iomega	150.0
4	Sony	240.0
5	Winchester	62.5

%%sql

SELECT m.name,
       round(avg(price), 2) AS avg_price
FROM Products p,
     Manufacturers m
WHERE p.manufacturer = m.code
GROUP BY m.name;

	Name	avg_price
0	Creative Labs	150.0
1	Fujitsu	240.0
2	Hewlett-Packard	168.0
3	Iomega	150.0
4	Sony	240.0
5	Winchester	62.5

Select the names of manufacturer whose products have an average price larger than or equal to 150.

Question: 14. Select the names of manufacturer whose products have an average price larger than or equal to $150.

dfp.head(2)

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6

dfp.merge(dfm,left_on='manufacturer', right_on='code',
         suffixes=['_product','_manufacturer'])\
.groupby('name_manufacturer')['price'].mean()\
.rename('avg_price').rename_axis('manufacturer')\
.loc[lambda x: x>=150]\
.reset_index()

	manufacturer	avg_price
0	Creative Labs	150.0
1	Fujitsu	240.0
2	Hewlett-Packard	168.0
3	Iomega	150.0
4	Sony	240.0

%%sql

SELECT m.name AS manufacturer,
       round(avg(price), 2) AS avg_price
FROM Products p
INNER JOIN Manufacturers m ON p.manufacturer = m.code
GROUP BY m.name
HAVING avg(price) >= 150;

	manufacturer	avg_price
0	Creative Labs	150.0
1	Fujitsu	240.0
2	Hewlett-Packard	168.0
3	Iomega	150.0
4	Sony	240.0

Select the name and price of the cheapest product.

Question: 15. Select the name and price of the cheapest product.

dfp.head(2)

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6

dfp.loc[dfp.price==dfp.price.min(), ['name','price']]

	name	price
3	Floppy disk	5

m = dfp.price.min()
dfp.query("price == @m")[['name','price']]

	name	price
3	Floppy disk	5

dfp.query("price == @dfp.price.min()")[['name','price']]

	name	price
3	Floppy disk	5

# %%sql
# select name, price
# from Products
# where price = min(price); # this does not work

%%sql

SELECT name,
       price
FROM Products
WHERE price =
    (SELECT min(price)
     FROM Products);

	Name	Price
0	Floppy disk	5

%%sql

SELECT name,
       price
FROM Products
ORDER BY price ASC
LIMIT 1;

	Name	Price
0	Floppy disk	5

dfp.nsmallest(1,'price')[['name','price']]

	name	price
3	Floppy disk	5

dfp.sort_values('price').head(1)[['name','price']]

	name	price
3	Floppy disk	5

dfp.sort_values('price').iloc[[0]][['name','price']]

	name	price
3	Floppy disk	5

Select the name of each manufacturer along with the name and price of its most expensive product.

Question: 16. Select the name of each manufacturer along with the name and price of its most expensive product.

dfp.merge(dfm,left_on='manufacturer', right_on='code',
         suffixes=['_product','_manufacturer'])\
.nlargest(1,'price')[['name_manufacturer','price']]

	name_manufacturer	price
8	Hewlett-Packard	270

%%sql

SELECT m.name,
       price
FROM Products p,
     Manufacturers m
WHERE (p.manufacturer=m.code
       AND price =
         (SELECT max(price)
          FROM Products));

	Name	Price
0	Hewlett-Packard	270

%%sql

SELECT m.name,
       price
FROM Products p
INNER JOIN Manufacturers m ON p.manufacturer=m.code
ORDER BY price DESC
LIMIT 1;

	Name	Price
0	Hewlett-Packard	270

%%sql

SELECT p.name,
       p.price,
       m.name
FROM Products p,
     Manufacturers m
WHERE p.manufacturer = m.code
  AND p.price =
    (SELECT max(p.price)
     FROM Products p
     WHERE p.Manufacturer = m.Code );

	Name	Price	Name
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	ZIP drive	150	Iomega
3	Monitor	240	Sony
4	DVD drive	180	Creative Labs
5	Printer	270	Hewlett-Packard
6	DVD burner	180	Creative Labs

%%sql
-- here, subquery has P and M and it has only one max value.

SELECT P.name,
       P.price,
       M.name
FROM Products P,
     Manufacturers M
WHERE P.manufacturer = M.code
  AND P.price =
    (SELECT max(P.price)
     FROM Products P,
          Manufacturers M
     WHERE P.Manufacturer = M.Code );

	Name	Price	Name
0	Printer	270	Hewlett-Packard

%%sql

SELECT P.Name,
       P.Price,
       M.Name
FROM Products P
INNER JOIN Manufacturers M ON P.Manufacturer = M.Code
AND P.Price =
  (SELECT max(P.Price)
   FROM Products P
   WHERE P.Manufacturer = M.Code );

	Name	Price	Name
0	Hard drive	240	Fujitsu
1	Memory	120	Winchester
2	ZIP drive	150	Iomega
3	Monitor	240	Sony
4	DVD drive	180	Creative Labs
5	Printer	270	Hewlett-Packard
6	DVD burner	180	Creative Labs

%%sql

SELECT max(P.Price)
FROM Products P,
     Manufacturers M
WHERE P.Manufacturer = M.Code;

	max(P.Price)
0	270

df = dfp.merge(dfm,left_on='manufacturer', right_on='code',suffixes=['_product','_manufacturer'])
df

	code_product	name_product	price	manufacturer	code_manufacturer	name_manufacturer
0	1	Hard drive	240	5	5	Fujitsu
1	2	Memory	120	6	6	Winchester
2	4	Floppy disk	5	6	6	Winchester
3	3	ZIP drive	150	4	4	Iomega
4	5	Monitor	240	1	1	Sony
5	6	DVD drive	180	2	2	Creative Labs
6	7	CD drive	90	2	2	Creative Labs
7	10	DVD burner	180	2	2	Creative Labs
8	8	Printer	270	3	3	Hewlett-Packard
9	9	Toner cartridge	66	3	3	Hewlett-Packard

# for each manufactures, seleect rows with max price for that manufacturer.

df['max_price_manufacturer'] = df.groupby('name_manufacturer')['price'].transform(max)

df

	code_product	name_product	price	manufacturer	code_manufacturer	name_manufacturer	max_price_manufacturer
0	1	Hard drive	240	5	5	Fujitsu	240
1	2	Memory	120	6	6	Winchester	120
2	4	Floppy disk	5	6	6	Winchester	120
3	3	ZIP drive	150	4	4	Iomega	150
4	5	Monitor	240	1	1	Sony	240
5	6	DVD drive	180	2	2	Creative Labs	180
6	7	CD drive	90	2	2	Creative Labs	180
7	10	DVD burner	180	2	2	Creative Labs	180
8	8	Printer	270	3	3	Hewlett-Packard	270
9	9	Toner cartridge	66	3	3	Hewlett-Packard	270

df[df.price==df.max_price_manufacturer]

	code_product	name_product	price	manufacturer	code_manufacturer	name_manufacturer	max_price_manufacturer
0	1	Hard drive	240	5	5	Fujitsu	240
1	2	Memory	120	6	6	Winchester	120
3	3	ZIP drive	150	4	4	Iomega	150
4	5	Monitor	240	1	1	Sony	240
5	6	DVD drive	180	2	2	Creative Labs	180
7	10	DVD burner	180	2	2	Creative Labs	180
8	8	Printer	270	3	3	Hewlett-Packard	270

Add a new product: Loudspeakers, 70, manufacturer 2.

Question: 17 Add a new product: Loudspeakers, $70, manufacturer 2.

dfp.tail(2)

	code	name	price	manufacturer
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2

dfp.loc[len(dfp)] = [dfp.code.iloc[-1]+1, 'Loudspeakers', 70, 2]

dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

%%sql

INSERT INTO Products(code, name, price, manufacturer)
VALUES (11, 'Loudspeakers',70, 2);

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

Update the name of product 8 to "Laser Printer".

Question: 18. Update the name of product 8 to "Laser Printer".

dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

dfp.loc[dfp.code==8,'name'] = 'Laser Printer'
dfp

	code	name	price	manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Laser Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

%%sql

UPDATE Products
SET name = 'Laser Printer'
WHERE code = 8;

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Laser Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

Apply a 10 percent discount to all products.

Question: 19. Apply a 10% discount to all products.

dfp['price'] *= 0.9
dfp

	code	name	price	manufacturer
0	1	Hard drive	216.0	5
1	2	Memory	108.0	6
2	3	ZIP drive	135.0	4
3	4	Floppy disk	4.5	6
4	5	Monitor	216.0	1
5	6	DVD drive	162.0	2
6	7	CD drive	81.0	2
7	8	Laser Printer	243.0	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	162.0	2
10	11	Loudspeakers	63.0	2

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	240	5
1	2	Memory	120	6
2	3	ZIP drive	150	4
3	4	Floppy disk	5	6
4	5	Monitor	240	1
5	6	DVD drive	180	2
6	7	CD drive	90	2
7	8	Laser Printer	270	3
8	9	Toner cartridge	66	3
9	10	DVD burner	180	2
10	11	Loudspeakers	70	2

%%sql

UPDATE Products
SET price = price * 0.9;

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	216.0	5
1	2	Memory	108.0	6
2	3	ZIP drive	135.0	4
3	4	Floppy disk	4.5	6
4	5	Monitor	216.0	1
5	6	DVD drive	162.0	2
6	7	CD drive	81.0	2
7	8	Laser Printer	243.0	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	162.0	2
10	11	Loudspeakers	63.0	2

Apply a 10% discount to all products with a price larger than or equal to 120.

Question: 20. Apply a 10% discount to all products with a price larger than or equal to $120.

dfp

	code	name	price	manufacturer
0	1	Hard drive	216.0	5
1	2	Memory	108.0	6
2	3	ZIP drive	135.0	4
3	4	Floppy disk	4.5	6
4	5	Monitor	216.0	1
5	6	DVD drive	162.0	2
6	7	CD drive	81.0	2
7	8	Laser Printer	243.0	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	162.0	2
10	11	Loudspeakers	63.0	2

dfp.loc[dfp.price >= 120, 'price'] *= 0.9
dfp

	code	name	price	manufacturer
0	1	Hard drive	194.4	5
1	2	Memory	108.0	6
2	3	ZIP drive	121.5	4
3	4	Floppy disk	4.5	6
4	5	Monitor	194.4	1
5	6	DVD drive	145.8	2
6	7	CD drive	81.0	2
7	8	Laser Printer	218.7	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	145.8	2
10	11	Loudspeakers	63.0	2

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	216.0	5
1	2	Memory	108.0	6
2	3	ZIP drive	135.0	4
3	4	Floppy disk	4.5	6
4	5	Monitor	216.0	1
5	6	DVD drive	162.0	2
6	7	CD drive	81.0	2
7	8	Laser Printer	243.0	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	162.0	2
10	11	Loudspeakers	63.0	2

%%sql

UPDATE Products
SET price = price * 0.9
WHERE price >= 120;

%%sql

SELECT *
FROM Products;

	Code	Name	Price	Manufacturer
0	1	Hard drive	194.4	5
1	2	Memory	108.0	6
2	3	ZIP drive	121.5	4
3	4	Floppy disk	4.5	6
4	5	Monitor	194.4	1
5	6	DVD drive	145.8	2
6	7	CD drive	81.0	2
7	8	Laser Printer	218.7	3
8	9	Toner cartridge	59.4	3
9	10	DVD burner	145.8	2
10	11	Loudspeakers	63.0	2

Time Taken

time_taken = time.time() - time_start_notebook
h,m = divmod(time_taken,60*60)
print('Time taken to run whole notebook: {:.0f} hr '\
      '{:.0f} min {:.0f} secs'.format(h, *divmod(m,60)))

Time taken to run whole notebook: 0 hr 0 min 2 secs