Python and Oracle – Selecting Data

When performing an operation on an Oracle database, such as selecting, inserting, updating and deleting data, the first thing that needs to be done is to connect to the database, as previously described.

In order to retrieve data, as well as insert, update and delete data, from an Oracle database, SQL, or Structured Query Language needs to be used, more details of which can be found here. Retrieving data is done via the ‘Select’ statement.

The following table of data, called ‘person’, will be used in the example below for selecting data.

id firstname lastname title dob
1 Bob Smith Mr 1980-01-20
2 George Jones Mr 1997-12-15
3 Fred Bloggs Mr 1975-05-07
4 Alan White Mr 1989-03-20

Note that by default, dates in an Oracle database are stored in the format YYYY-MM-DD (four digit year, two digit month and two digit day).

The example below selects four items of data from the ‘person’ table, in last name, first name and date of birth order. The resulting data is stored in a variable, which is then used in a ‘for’ loop to output details of each record to the console in the format: “id: lastname, firstname (dob)”. Note that the second ‘try-except’ block has a ‘finally’ section that closes the database connection, regardless of whether the data retrieval is successful or not.

import cx_Oracle

# Database connection variable.
connect = None

try:

    # Connect to database.
    connect = cx_Oracle.connect('DemoUN/DemoPW@localhost')

except cx_Oracle.DatabaseError as e:

    # Confirm unsuccessful connection and stop program execution.
    print("Database connection unsuccessful.")
    quit()

# Cursor to execute query.
cursor = connect.cursor()

# SQL to select data from the person table.
sqlSelect = \
    "SELECT id, firstname, lastname, dob \
     FROM person \
     ORDER BY lastname, firstname, dob"

try:

    # Execute query.
    cursor.execute(sqlSelect)

    # Fetch the data returned.
    results = cursor.fetchall()

    # Display person information in the console.
    for row in results:

        print("{}: {}, {} ({})".format(
            row[0], row[2], row[1], row[3].strftime('%d/%m/%Y')))

except cx_Oracle.DatabaseError as e:

    # Confirm error retrieving person information and stop program execution.
    print("Error retrieving person information.")
    quit()

finally:

    # Close database connection.
    connect.close()

The resulting output to the console is as follows.

3: Bloggs, Fred (07/05/1975)
2: Jones, George (15/12/1997)
1: Smith, Bob (20/01/1980)
4: White, Alan (20/03/1989)

Often it isn’t necessary to return all records from a database table. Where this is the case, parameters need to be introduced into the query. In the following example, the records returned are limited to those with a date of birth between 1 January 1980 and 31 December 1989. Within the SQL, text preceded by ‘:’ symbols are used to signify that parameters need to be incorporated. The parameter values are then bound into the SQL statement before it is executed. Binding the parameters in this way helps prevent SQL injection, where hackers try to insert malicious code to either do damage to the database or access more data than should be allowed.

import datetime
import cx_Oracle

# Database connection variable.
connect = None

try:

    # Connect to database.
    connect = cx_Oracle.connect('DemoUN/DemoPW@localhost')

except cx_Oracle.DatabaseError as e:

    # Confirm unsuccessful connection and stop program execution.
    print("Database connection unsuccessful.")
    quit()

# Cursor to execute query.
cursor = connect.cursor()

# Query parameters.
dobLower = datetime.datetime.strptime('1980-01-01 00:00:00', '%Y-%m-%d %H:%M:%S')
dobUpper = datetime.datetime.strptime('1989-12-31 23:59:59', '%Y-%m-%d %H:%M:%S')

# SQL to select data from the person table.
sqlSelect = \
    "SELECT id, firstname, lastname, dob \
     FROM person \
     WHERE dob BETWEEN :dobLower AND :dobUpper \
     ORDER BY lastname, firstname, dob"

try:

    # Execute query.
    cursor.execute(sqlSelect, [dobLower, dobUpper])

    # Fetch the data returned.
    results = cursor.fetchall()

    # Display person information in the console.
    for row in results:

        print("{}: {}, {} ({})".format(
            row[0], row[2], row[1], row[3].strftime('%d/%m/%Y')))

except cx_Oracle.DatabaseError as e:

    # Confirm error retrieving person information and stop program execution.
    print("Error retrieving person information.")
    quit()

finally:

    # Close database connection.
    connect.close()

The resulting output to the console is as follows.

1: Smith, Bob (20/01/1980)
4: White, Alan (20/03/1989)