Python – MySQL Database Access

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What is Python Database Access?

A database is a collection of tables related to each other via columns. For most real-world projects, a database is a must. We can use SQL (Structured Query Language) to create, access, and manipulate data. We can also make use of normalization to avoid redundancy of data. For database programming, Python supports many database servers- MySQL, Oracle, PostgreSQL, SQLite, Sybase, Microsoft SQL Server, mSQL, Microsoft Access, and many more. It also supports Data Query Statements, Data Definition Language (DDL), and Data Manipulation Language (DML). The standard database interface for Python is Python DB-API. For that, we have the module MySQLdb for MySQL. This is independent of database engines; so we can write Python scripts to access any database engine. So, in this Python Database Access tutorial, we use the module PyMySQL.

The Python standard for database interfaces is the Python DB-API. Most Python database interfaces adhere to this standard. You can choose the right database for your application. Python Database API supports a wide range of database servers such as −

• GadFly
• mSQL
• MySQL
• PostgreSQL
• Microsoft SQL Server 2000
• Informix
• Interbase
• Oracle
• Sybase

Here is the list of available Python database interfaces: Python Database Interfaces and APIs. You must download a separate DB API module for each database you need to access. For example, if you need to access an Oracle database as well as a MySQL database, you 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

We would learn all the concepts using MySQL, so let us talk about MySQLdb module.

What is 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.

How do I Install MySQLdb?

Before proceeding, you make sure you have MySQLdb installed on your machine. Just type the following in your Python script and execute it −

#!/usr/bin/python

import MySQLdb

If it produces the following result, then it means MySQLdb module is not installed −

Traceback (most recent call last):
   File "test.py", line 3, in <module>
      import MySQLdb
ImportError: No module named MySQLdb

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 gcc
For Python command prompt, use the following command -
pip install MySQL-python

Note − Make sure you have root privilege to install above module.

Can Python interact with database?

Python supports various databases like MySQL, Oracle, Sybase, PostgreSQL, etc. Python also supports Data Definition Language (DDL), Data Manipulation Language (DML) and Data Query Statements. For database programming, the Python DB API is a widely used module that provides a database application programming interface.

How import SQL database in Python?

sql is an export copy of a database i.e. list of SQL commands required to create copy of that database, then what you have to do is to run SQLite prompt, execute all commands from that copy and save database in SQLite format.Then you can connect to this newly created file using python sqlite lib

How does Python deal with database?

Create a cursor to communicate with the data. Manipulate the data using SQL (interact). Tell the connection to either apply the SQL manipulations to the data and make them permanent (commit), or tell it to abort those manipulations (rollback), thus returning the data to the state before the interactions occurred

Performing Transactions

Before connecting to a MySQL database, make sure of the followings −

• You have created a database TESTDB.
• You have created a table EMPLOYEE in TESTDB.
• This table has fields FIRST_NAME, LAST_NAME, AGE, SEX and INCOME.
• User ID “testuser” and password “test123” are set to access TESTDB.
• Python module MySQLdb is installed properly on your machine.
• You have gone through MySQL tutorial to understand MySQL Basics.

Example

Following is the example of connecting with MySQL database “TESTDB”

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# execute SQL query using <em>execute()</em> method.
cursor.execute("SELECT VERSION()")

# Fetch a single row using <em>fetchone()</em> method.
data = cursor.fetchone()
print "Database version : %s " % data

# disconnect from server
db.close()

While running this script, it is producing the following result in my Linux machine.

Database version : 5.0.45

If a connection is established with the datasource, then a Connection Object is returned and saved into db for further use, otherwise db is set to None. Next, db object is used to create a cursor object, which in turn is used to execute SQL queries. Finally, before coming out, it ensures that database connection is closed and resources are released.

Creating Database Table

Once a database connection is established, we are ready to create tables or records into the database tables using execute method of the created cursor.

Example

Let us create Database table EMPLOYEE −

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# Drop table if it already exist using <em>execute()</em> method.
cursor.execute("DROP TABLE IF EXISTS EMPLOYEE")

# Create table as per requirement
sql = """CREATE TABLE EMPLOYEE (
         FIRST_NAME  CHAR(20) NOT NULL,
         LAST_NAME  CHAR(20),
         AGE INT,  
         SEX CHAR(1),
         INCOME FLOAT )"""

cursor.execute(sql)

# disconnect from server
db.close()

Creating a cursor object

The cursor object can be defined as an abstraction specified in the Python DB-API 2.0. It facilitates us to have multiple separate working environments through the same connection to the database. We can create the cursor object by calling the ‘cursor’ function of the connection object. The cursor object is an important aspect of executing queries to the databases.

The syntax to create the cursor object is given below.

<my_cur>  = conn.cursor()  

Example

<strong>import</strong> mysql.connector
  #Create the connection object myconn = mysql.connector.connect(host = "localhost", 
user = "root",passwd = "google", database = "mydb")   #printing the connection object    <strong>print</strong>(myconn)   #creating the cursor object  cur = myconn.cursor()  <strong>print</strong>(cur)  

Output:

<mysql.connector.connection.
MySQLConnection object at 0x7faa17a15748> 
 MySQLCursor: (Nothing executed yet)

INSERT Operation

It is required when you want to create your records into a database table.

Example

The following example, executes SQL INSERT statement to create a record into EMPLOYEE table −

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# Prepare SQL query to INSERT a record into the database.
sql = """INSERT INTO EMPLOYEE(FIRST_NAME,
         LAST_NAME, AGE, SEX, INCOME)
         VALUES ('Mac', 'Mohan', 20, 'M', 2000)"""
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Above example can be written as follows to create SQL queries dynamically −

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# Prepare SQL query to INSERT a record into the database.
sql = "INSERT INTO EMPLOYEE(FIRST_NAME, \
       LAST_NAME, AGE, SEX, INCOME) \
       VALUES ('%s', '%s', '%d', '%c', '%d' )" % \
       ('Mac', 'Mohan', 20, 'M', 2000)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Example

Following code segment is another form of execution where you can pass parameters directly −

..................................
user_id = "test123"
password = "password"

con.execute('insert into Login values("%s", "%s")' % \
             (user_id, password))
..................................

READ Operation

READ Operation on any database means to fetch some useful information from the database. Once our database connection is established, you are ready to make a query into this database. You can use either fetchone() method to fetch single record or fetchall() method to fetech multiple values from a database table.

• fetchone() − It fetches the next row of a query result set. A result set is an object that is returned when a cursor object is used to query a table.
• fetchall() − It fetches all the rows in a result set. If some rows have already been extracted from the result set, then it retrieves the remaining rows from the result set.
• rowcount − This is a read-only attribute and returns the number of rows that were affected by an execute() method.

Example

The following procedure queries all the records from EMPLOYEE table having salary more than 1000 −

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

sql = "SELECT * FROM EMPLOYEE \
       WHERE INCOME &gt; '%d'" % (1000)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Fetch all the rows in a list of lists.
   results = cursor.fetchall()
   for row in results:
      fname = row[0]
      lname = row[1]
      age = row[2]
      sex = row[3]
      income = row[4]
      # Now print fetched result
      print "fname=%s,lname=%s,age=%d,sex=%s,income=%d" % \
             (fname, lname, age, sex, income )
except:
   print "Error: unable to fecth data"

# disconnect from server
db.close()

This will produce the following result −

fname=Mac, lname=Mohan, age=20, sex=M, income=2000

Update Operation

UPDATE Operation on any database means to update one or more records, which are already available in the database .The following procedure updates all the records having SEX as ‘M’. Here, we increase AGE of all the males by one year.

Example

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# Prepare SQL query to UPDATE required records
sql = "UPDATE EMPLOYEE SET AGE = AGE + 1
                          WHERE SEX = '%c'" % ('M')
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

DELETE Operation

DELETE operation is required when you want to delete some records from your database. Following is the procedure to delete all the records from EMPLOYEE where AGE is more than 20 −

Example

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using <em>cursor()</em> method
cursor = db.cursor()

# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE &gt; '%d'" % (20)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Performing Transactions

Transactions are a mechanism that ensures data consistency. Transactions have the following four properties −

• Atomicity − Either a transaction completes or nothing happens at all.
• Consistency − A transaction must start in a consistent state and leave the system in a consistent state.
• Isolation − Intermediate results of a transaction are not visible outside the current transaction.
• Durability − Once a transaction was committed, the effects are persistent, even after a system failure.

The Python DB API 2.0 provides two methods to either commit or rollback a transaction.

Example

You already know how to implement transactions. Here is again similar example −

# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

COMMIT Operation

Commit is the operation, which gives a green signal to database to finalize the changes, and after this operation, no change can be reverted back. Here is a simple example to call commit method.

db.commit()

ROLLBACK Operation

If you are not satisfied with one or more of the changes and you want to revert back those changes completely, then use rollback() method. Here is a simple example to call rollback() method.

db.rollback()

Disconnecting Database

To disconnect Database connection, use close() method.

db.close()

If the connection to a database is closed by the user with the close() method, any outstanding transactions are rolled back by the DB. However, instead of depending on any of DB lower level implementation details, your application would be better off calling commit or rollback explicitly.

Handling Errors

There are many sources of errors. A few examples are a syntax error in an executed SQL statement, a connection failure, or calling the fetch method for an already canceled or finished statement handle. The DB API defines a number of errors that must exist in each database module. The following table lists these exceptions.

Sr.No.	Exception & Description
1	WarningUsed for non-fatal issues. Must subclass StandardError.
2	ErrorBase class for errors. Must subclass StandardError.
3	InterfaceErrorUsed for errors in the database module, not the database itself. Must subclass Error.
4	DatabaseErrorUsed for errors in the database. Must subclass Error.
5	DataErrorSubclass of DatabaseError that refers to errors in the data.
6	OperationalErrorSubclass of DatabaseError that refers to errors such as the loss of a connection to the database. These errors are generally outside of the control of the Python scripter.
7	IntegrityErrorSubclass of DatabaseError for situations that would damage the relational integrity, such as uniqueness constraints or foreign keys.
8	InternalErrorSubclass of DatabaseError that refers to errors internal to the database module, such as a cursor no longer being active.
9	ProgrammingErrorSubclass of DatabaseError that refers to errors such as a bad table name and other things that can safely be blamed on you.
10	NotSupportedErrorSubclass of DatabaseError that refers to trying to call unsupported functionality.

Your Python scripts should handle these errors, but before using any of the above exceptions, make sure your MySQLdb has support for that exception. You can get more information about them by reading the DB API 2.0 specification.