5.2 Creating Tables Using Connector/Python

All DDL (Data Definition Language) statements are executed using a handle structure known as a cursor. The following examples show how to create the tables of the Employee Sample Database. You need them for the other examples.

In a MySQL server, tables are very long-lived objects, and are often accessed by multiple applications written in different languages. You might typically work with tables that are already set up, rather than creating them within your own application. Avoid setting up and dropping tables over and over again, as that is an expensive operation. The exception is temporary tables, which can be created and dropped quickly within an application.

from __future__ import print_function

import mysql.connector
from mysql.connector import errorcode

DB_NAME = 'employees'

TABLES = {}
TABLES['employees'] = (
    "CREATE TABLE `employees` ("
    "  `emp_no` int(11) NOT NULL AUTO_INCREMENT,"
    "  `birth_date` date NOT NULL,"
    "  `first_name` varchar(14) NOT NULL,"
    "  `last_name` varchar(16) NOT NULL,"
    "  `gender` enum('M','F') NOT NULL,"
    "  `hire_date` date NOT NULL,"
    "  PRIMARY KEY (`emp_no`)"
    ") ENGINE=InnoDB")

TABLES['departments'] = (
    "CREATE TABLE `departments` ("
    "  `dept_no` char(4) NOT NULL,"
    "  `dept_name` varchar(40) NOT NULL,"
    "  PRIMARY KEY (`dept_no`), UNIQUE KEY `dept_name` (`dept_name`)"
    ") ENGINE=InnoDB")

TABLES['salaries'] = (
    "CREATE TABLE `salaries` ("
    "  `emp_no` int(11) NOT NULL,"
    "  `salary` int(11) NOT NULL,"
    "  `from_date` date NOT NULL,"
    "  `to_date` date NOT NULL,"
    "  PRIMARY KEY (`emp_no`,`from_date`), KEY `emp_no` (`emp_no`),"
    "  CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) "
    "     REFERENCES `employees` (`emp_no`) ON DELETE CASCADE"
    ") ENGINE=InnoDB")

TABLES['dept_emp'] = (
    "CREATE TABLE `dept_emp` ("
    "  `emp_no` int(11) NOT NULL,"
    "  `dept_no` char(4) NOT NULL,"
    "  `from_date` date NOT NULL,"
    "  `to_date` date NOT NULL,"
    "  PRIMARY KEY (`emp_no`,`dept_no`), KEY `emp_no` (`emp_no`),"
    "  KEY `dept_no` (`dept_no`),"
    "  CONSTRAINT `dept_emp_ibfk_1` FOREIGN KEY (`emp_no`) "
    "     REFERENCES `employees` (`emp_no`) ON DELETE CASCADE,"
    "  CONSTRAINT `dept_emp_ibfk_2` FOREIGN KEY (`dept_no`) "
    "     REFERENCES `departments` (`dept_no`) ON DELETE CASCADE"
    ") ENGINE=InnoDB")

TABLES['dept_manager'] = (
    "  CREATE TABLE `dept_manager` ("
    "  `dept_no` char(4) NOT NULL,"
    "  `emp_no` int(11) NOT NULL,"
    "  `from_date` date NOT NULL,"
    "  `to_date` date NOT NULL,"
    "  PRIMARY KEY (`emp_no`,`dept_no`),"
    "  KEY `emp_no` (`emp_no`),"
    "  KEY `dept_no` (`dept_no`),"
    "  CONSTRAINT `dept_manager_ibfk_1` FOREIGN KEY (`emp_no`) "
    "     REFERENCES `employees` (`emp_no`) ON DELETE CASCADE,"
    "  CONSTRAINT `dept_manager_ibfk_2` FOREIGN KEY (`dept_no`) "
    "     REFERENCES `departments` (`dept_no`) ON DELETE CASCADE"
    ") ENGINE=InnoDB")

TABLES['titles'] = (
    "CREATE TABLE `titles` ("
    "  `emp_no` int(11) NOT NULL,"
    "  `title` varchar(50) NOT NULL,"
    "  `from_date` date NOT NULL,"
    "  `to_date` date DEFAULT NULL,"
    "  PRIMARY KEY (`emp_no`,`title`,`from_date`), KEY `emp_no` (`emp_no`),"
    "  CONSTRAINT `titles_ibfk_1` FOREIGN KEY (`emp_no`)"
    "     REFERENCES `employees` (`emp_no`) ON DELETE CASCADE"
    ") ENGINE=InnoDB")

The preceding code shows how we are storing the CREATE statements in a Python dictionary called TABLES. We also define the database in a global variable called DB_NAME, which enables you to easily use a different schema.

cnx = mysql.connector.connect(user='scott')
cursor = cnx.cursor()

A single MySQL server can manage multiple databases. Typically, you specify the database to switch to when connecting to the MySQL server. This example does not connect to the database upon connection, so that it can make sure the database exists, and create it if not:

def create_database(cursor):
    try:
        cursor.execute(
            "CREATE DATABASE {} DEFAULT CHARACTER SET 'utf8'".format(DB_NAME))
    except mysql.connector.Error as err:
        print("Failed creating database: {}".format(err))
        exit(1)

try:
    cnx.database = DB_NAME    
except mysql.connector.Error as err:
    if err.errno == errorcode.ER_BAD_DB_ERROR:
        create_database(cursor)
        cnx.database = DB_NAME
    else:
        print(err)
        exit(1)

We first try to change to a particular database using the database property of the connection object cnx. If there is an error, we examine the error number to check if the database does not exist. If so, we call the create_database function to create it for us.

On any other error, the application exits and displays the error message.

After we successfully create or change to the target database, we create the tables by iterating over the items of the TABLES dictionary:

for name, ddl in TABLES.iteritems():
    try:
        print("Creating table {}: ".format(name), end='')
        cursor.execute(ddl)
    except mysql.connector.Error as err:
        if err.errno == errorcode.ER_TABLE_EXISTS_ERROR:
            print("already exists.")
        else:
            print(err.msg)
    else:
        print("OK")

cursor.close()
cnx.close()

To handle the error when the table already exists, we notify the user that it was already there. Other errors are printed, but we continue creating tables. (The example shows how to handle the table already exists condition for illustration purposes. In a real application, we would typically avoid the error condition entirely by using the IF NOT EXISTS clause of the CREATE TABLE statement.)

The output would be something like this:

Creating table employees: already exists.
Creating table salaries: already exists.
Creating table titles: OK
Creating table departments: already exists.
Creating table dept_manager: already exists.
Creating table dept_emp: already exists.

To populate the employees tables, use the dump files of the Employee Sample Database. Note that you only need the data dump files that you will find in an archive named like employees_db-dump-files-1.0.5.tar.bz2. After downloading the dump files, execute the following commands, adding connection options to the mysql commands if necessary:

shell> tar xzf employees_db-dump-files-1.0.5.tar.bz2
shell> cd employees_db
shell> mysql employees < load_employees.dump
shell> mysql employees < load_titles.dump
shell> mysql employees < load_departments.dump
shell> mysql employees < load_salaries.dump
shell> mysql employees < load_dept_emp.dump
shell> mysql employees < load_dept_manager.dump