Interface python with sql database.pdf--

Chapter 11 :
Informatics
Practices
Class XII ( As per
CBSE Board)
Interface python
with SQL
Database And
SQL commands
Visit : python.mykvs.in for regular updates
New
Syllabus
2019-20

Interface python with SQL Database
A database is nothing but an organized collection of data. Data is organized into rows, columns and
tables and it is indexed to make it easier to find relevant information. All companies whether large or
small use databases. So it become necessary to develop project/software using any programming
language like python in such a manner which can interface with such databases which support
SQL.Generalised form of Interface of python with SQL Database can be understood with the help of
this diagram.
Form/any user interface designed in any programming language is Front End where as data given
by database as response is known as Back-End database.
SQL is just a query language, it is not a database. To perform SQL queries, we need to install any
database for example Oracle, MySQL, MongoDB, PostGres SQL, SQL Server, DB2 etc.
Using SQL in any of the dbms ,databases and table can be created and data can be accessed,
updated and maintained. The Python standard for database interfaces is the Python DB-API. Python
Database API supports a wide range of database servers, like msql , mysql, postgressql, Informix,
oracle, Sybase etc.

Why choose Python for database programming
Following are the reason to choose python for database
programming
• Programming more efficient and faster compared to other
languages.
• Portability of python programs.
• Support platform independent program development.
• Python supports SQL cursors.
• Python itself take care of open and close of connections.
• Python supports relational database systems.
• Porting of data from one dbms to other is easily possible as it
support large range of APIs for various databases.

SQL Connectors
We must download a separate DB API module for each
database we need to access. Suppose we need to access an
Oracle database as well as a MySQL database, we must
download both the Oracle and the MySQL database modules .
The DB API provides a minimal standard for working with
databases using Python structures and syntax wherever
possible.
This API includes the following −
● Importing the API module.
● Acquiring a connection with the database.
● Issuing SQL statements and stored procedures.
● Closing the connection

Here we are using mysql as back end database because of it is open
source,free and portable and widely used. Any one of mysql-
connector or MySQLdb can be used for database programming.
1. mysql-connector
MySQL-Connector enables Python programs to access MySQL
databases, using an API that is compliant with the Python Database
API Specification v2.0 (PEP 249). It is written in pure Python and does
not have any dependencies except for the Python Standard Library.
Steps to use mysql-connector
1. Download Mysql API ,exe file and install it.(click here to download)
2. Install Mysql-Python Connector (Open command prompt and
execute command) >pip install mysql-connector
3. Now connect Mysql server using python
4. Write python statement in python shell import mysql.connector
If no error message is shown means mysql connector is properly
installed

2. MySQLdb
MySQLdb is an interface for connecting to a MySQL database server
from Python. It implements the Python Database API v2.0 and is built
on top of the MySQL C API.
Steps to use mysqlclient
1. First Upgrade pip command through > python –m pip install –
upgrade pip
2. Install mysqlclient through pip install mysqlclient
3. After successful installation check through import mysqldb
4. If it is installed no error will be displayed otherwise error message will be
displayed
To install MySQLdb module, use the following command −
For Ubuntu, use the following command -
$ sudo apt-get install python-pip python-dev libmysqlclient-dev
For Fedora, use the following command -
$ sudo dnf install python python-devel mysql-devel redhat-rpm-config gc c
For Python command prompt, use the following command -
pip install MySQL-python
Note − Make sure you have root privilege to install above module

Establish connection
For database interface/database programming ,connection must be
established.Before establishing connection there must be mysql installed on
the system and a database and table is already created.In following way we
can establish a connection with mysql database through mysql.connector.
import mysql.connector
mydb=mysql.connector.connect(host="localhost",user="root",passwd="root“,database
=“school”)
print(mydb)
Alternatively we can write the following statement if we are using mysqldb
import MySQLdb
mydb = MySQLdb.connect("localhost",“root",“root",“school" )
print(mydb)
In both way we are specifying host,user,password and database name as
arguments.database is optional argument if we want to create database
through programming later on.
After successful execution of above statements in python following out will
be displayed
<mysql.connector.connection.MySQLConnection object at 0x022624F0>
Otherwise an error message will be shown.

Cursor object :
The MySQLCursor class instantiates objects that can execute operations
such as SQL statements. Cursor objects interact with the MySQL server
using a MySQLConnection object.
How to create cursor object and use it
mydb=mysql.connector.connect(host="localhost",user="root",passwd="root")
mycursor=mydb.cursor()
mycursor.execute("create database if not exists school")
mycursor.execute("show databases")
for x in mycursor:
print(x)
Through line 4 we are creating a database named school if it is already not
created with the help of cursor object.
Line 5 executes the sql query show databases and store result in mycursor
as collection ,whose values are being fetched in x variable one by one.
On execution of above program school database is created and a list of
available databases is shown.

How to create table at run time
Table creation is very easy ,if we are already well versed in sql table creation
then we have to just pass the create table query in execute() method of
cursor object. But before table creation we must open the database.Here we
are opening database school(through connect() method) before student table
creation.
mydb=mysql.connector.connect(host="localhost",user="root",passwd="root"
,database="school")
mycursor.execute("create table student(rollno int(3) primary key,name
varchar(20),age int(2))")
On successful execution of above program a table named student with three
fields rollno,name,age will be created in school database.
We can check student table in mysql shell also,if required.

How to change table structure/(add,edit,remove colum of a table) at run time
To modify the structure of the table we just have to use alter table
query.Below program will add a column mark in the student table.
,database="school")
mycursor.execute("alter table student add (marks int(3))")
mycursor.execute("desc student")
for x in mycursor:
print(x)
Above program will add a column marks in the table student and will display
the structure of the table

How to search records of a table at run time
Below statement demonstrate the use of select query for searching specific
record from a table.
,database="school")
nm=input("enter name")
mycursor.execute("select * from student where name='"+nm+"'")
for x in mycursor:
print (x)
Above statements will prompt a name from user,as user type the name ,that
name is searched into the table student with the help of select query .result
will be shown with the help of mycursor collection.

How to fetch all records of a table at run time
,database="school")
mycursor.execute("select * from student")
myrecords=mycursor.fetchall()
for x in myrecords:
print (x)
MySQLCursor.fetchall() Method
The method fetches all (or all remaining) rows of a query result set and returns a
list of tuples. If no more rows are available, it returns an empty list.

How to fetch one record of a table at run time
,database="school")
mycursor.execute("select * from student")
row=mycursor.fetchone()
while row is not None:
print(row)
row = mycursor.fetchone()
MySQLCursor.fetchone() Method
This method retrieves the next row of a query result set and returns a single
sequence, or None if no more rows are available. By default, the returned tuple
consists of data returned by the MySQL server, converted to Python objects.
MySQLCursor.fetchmany() Method
rows = cursor.fetchmany(size=1)
This method fetches the next set of rows of a query result and returns a list
of tuples. If no more rows are available, it returns an empty list.

How to delete record of a table at run time
,database="school")
mycursor.execute("delete from student where rollno=1")
mydb.commit()
In above program delete query will delete a record with rollno=1.commit()
method is necessary to call for database transaction.
How to update record of a table at run time
,database="school")
mycursor.execute("update student set marks=99 where rollno=2")
mydb.commit()
In above program update query update the marks with 99 of rollno=2
Students are advised to develop menu driven program using above concepts
for better understating of python mysql database interface.

Manage Database Transaction
Database transaction represents a single unit of work. Any operation
which modifies the state of the MySQL database is a transaction.
Python MySQL Connector provides the following method to manage
database transactions.
commit – MySQLConnection.commit() method sends a COMMIT
statement to the MySQL server, committing the current transaction.
rollback – MySQLConnection.rollback revert the changes made by
the current transaction.
AutoCommit – MySQLConnection.autocommit value can be assigned
as True or False to enable or disable the auto-commit feature of
MySQL. By default its value is False.

Manage Database Transaction
try:
conn = mysql.connector.connect(host='localhost',
database='school',
user='root',
password='root')
conn.autocommit = false
cursor = conn.cursor()
sql_update_query = """Update student set marks = 95 where rollno = 2"""
cursor.execute(sql_update_query)
print ("Record Updated successfully ")
#Commit your changes
conn.commit()
except mysql.connector.Error as error :
print("Failed to update record to database rollback: {}".format(error))
#reverting changes because of exception
conn.rollback()
finally:
#closing database connection.
if(conn.is_connected()):
cursor.close()
conn.close()
print("connection is closed")
In above program if update query is successfully executed then commit() method will be executed
otherwise exception error part will be executed where revert of update query will be done due to
error.At finally we are closing cursor as well as connection.To rollback or commit we have to set
autocommit=false,just like conn.autocommit = false in above program otherwise rollback will not work

SQL Commands
Grouping Records in a Query
• Some time it is required to apply a Select query in a group of
records instead of whole table.
• We can group records by using GROUP BY <column> clause
with Select command. A group column is chosen which have
non-distinct (repeating) values like City, Job etc.
• Generally, the following Aggregate Functions [MIN(), MAX(),
SUM(), AVG(), COUNT()] etc. are applied on groups.
Name Purpose
SUM() Returns the sum of given column.
MIN() Returns the minimum value in the given column.
MAX() Returns the maximum value in the given column.
AVG() Returns the Average value of the given column.
COUNT() Returns the total number of values/ records as per given
column.

SQL Commands
Aggregate Functions & NULL
Consider a table Emp having following records as-
Null values are excluded while (avg)aggregate function is
used
SQL Queries
mysql> Select Sum(Sal) from EMP;
mysql> Select Min(Sal) from EMP;
mysql> Select Max(Sal) from EMP;
mysql> Select Count(Sal) from EMP;
mysql> Select Avg(Sal) from EMP;
mysql> Select Count(*) from EMP;
Emp
Code Name Sal
E1 Mohak NULL
E2 Anuj 4500
E3 Vijay NULL
E4 Vishal 3500
E5 Anil 4000
Result of query
12000
3500
4500
3
4000
5

SQL Commands
Aggregate Functions & Group
An Aggregate function may applied on a column with DISTINCT or ALL
keyword. If nothing is given ALL is assumed.
Using SUM (<Column>)
This function returns the sum of values in given column or expression.
mysql> Select Sum(Sal) from EMP;
mysql> Select Sum(DISTINCT Sal) from EMP;
mysql> Select Sum (Sal) from EMP where City=‘Jaipur’;
mysql> Select Sum (Sal) from EMP Group By City;
mysql> Select Job, Sum(Sal) from EMP Group By Job;
Using MIN (<column>)
This functions returns the Minimum value in the given column.
mysql> Select Min(Sal) from EMP;
mysql> Select Min(Sal) from EMP Group By City;
mysql> Select Job, Min(Sal) from EMP Group By Job;

SQL Commands
Aggregate Functions & Group
Using MAX (<Column>)
This function returns the Maximum value in given column.
Using AVG (<column>)
This functions returns the Average value in the given column.
mysql> Select AVG(Sal) from EMP;
mysql> Select AVG(Sal) from EMP Group By City;
Using COUNT (<*|column>)
This functions returns the number of rows in the given
column.
mysql> Select Max(Sal) from EMP;
mysql> Select Max(Sal) from EMP where City=‘Jaipur’;
mysql> Select Max(Sal) from EMP Group By City;
mysql> Select Count ( * ) from EMP;
mysql> Select Count(Sal) from EMP Group By City;
mysql> Select Count(*), Sum(Sal) from EMP Group By Job;

SQL Commands
Aggregate Functions & Conditions
You may use any condition on group, if required. HAVING
<condition> clause is used to apply a condition on a group.
mysql> Select Job,Sum(Pay) from EMP
Group By Job HAVING Sum(Pay)>=8000;
mysql> Select Job, Sum(Pay) from EMP
Group By Job HAVING Avg(Pay)>=7000;
mysql> Select Job, Sum(Pay) from EMP
Group By Job HAVING Count(*)>=5;
mysql> Select Job, Min(Pay),Max(Pay), Avg(Pay) from EMP Group
By Job HAVING Sum(Pay)>=8000;
mysql> Select Job, Sum(Pay) from EMP Where City=‘Jaipur’
Note :- Where clause works in respect of whole table but Having works
on Group only. If Where and Having both are used then Where will be
executed first.

SQL Commands
Ordering Query Result – ORDER BY Clause
A query result can be orders in ascending (A-Z) or
descending (Z-A)
order as per any column. Default is Ascending order.
mysql> SELECT * FROM Student ORDER BY City;
To get descending order use DESC key word.
mysql> SELECT * FROM Student ORDER BY City
DESC;
mysql> SELECT Name, Fname, City FROM Student
Where Name LIKE ‘R%’ ORDER BY Class;

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