Database Connection:
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 is having 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 cursor() method cursor = db.cursor() # execute SQL query using execute() method. cursor.execute("SELECT VERSION()") # Fetch a single row using fetchone() 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 intodb 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:
First, let's create Database table EMPLOYEE:
#!/usr/bin/python import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Drop table if it already exist using execute() 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()
INSERT Operation:
INSERT operation is required when you want to create your records into a database table.
EXAMPLE:
Following is the example, which 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 cursor() 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 cursor() 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 databasse means to fetch some useful information from the database.
Once our database connection is established, we are ready to make a query into this database. We can use either fetchone() method to fetch single record or fetchall() method to fetech multiple values from a database table.
- fetchone(): This method 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(): This method fetches all the rows in a result set. If some rows have already been extracted from the result set, the fetchall() method 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:
Following is the procedure to query 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 cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = "SELECT * FROM EMPLOYEE \ WHERE INCOME > '%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 databasse means to update one or more records, which are already available in the database. Following is the procedure to update all the records having SEX as 'M'. Here, we will 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 cursor() 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 cursor() method cursor = db.cursor() # 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() # disconnect from server db.close()
Performing Transactions:
Transactions are a mechanism that ensures data consistency. Transactions should 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 have seen how we have implemented transations. 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.
| Exception | Description |
|---|---|
| Warning | Used for non-fatal issues. Must subclass StandardError. |
| Error | Base class for errors. Must subclass StandardError. |
| InterfaceError | Used for errors in the database module, not the database itself. Must subclass Error. |
| DatabaseError | Used for errors in the database. Must subclass Error. |
| DataError | Subclass of DatabaseError that refers to errors in the data. |
| OperationalError | Subclass 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. |
| IntegrityError | Subclass of DatabaseError for situations that would damage the relational integrity, such as uniqueness constraints or foreign keys. |
| InternalError | Subclass of DatabaseError that refers to errors internal to the database module, such as a cursor no longer being active. |
| ProgrammingError | Subclass of DatabaseError that refers to errors such as a bad table name and other things that can safely be blamed on you. |
| NotSupportedError | Subclass 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.
댓글 없음:
댓글 쓰기