This tutorial shows you how to query data from a
MySQL database in Python by using MySQL Connector/Python API such
as fetchone() , fetchmany() , and fetchall() .

To query data in a MySQL database from Python, you need to do the
following steps:

  1. Connect to the MySQL
    Database,
    you get a MySQLConnection object.
  2. Instantiate a  MySQLCursor object from the
    the MySQLConnection object.
  3. Use the cursor to execute a query by calling
    its  execute() method.
  4. Use fetchone() ,  fetchmany() or  fetchall() method to fetch
    data from the result set.
  5. Close the cursor as well as the database connection by calling
    the  close() method of the corresponding object.

We will show you how to use fetchone() , fetchmany() ,
and  fetchall() methods in more detail in the following sections.

Querying data with fetchone

The  fetchone() method returns the next row of a query result set
or None in case there is no row left. Let’s take a look at the
following code:

 

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from mysql.connector import MySQLConnection, Error
from python_mysql_dbconfig import read_db_config
 
 
def query_with_fetchone():
    try:
        dbconfig = read_db_config()
        conn = MySQLConnection(**dbconfig)
        cursor = conn.cursor()
        cursor.execute("SELECT * FROM books")
 
        row = cursor.fetchone()
 
        while row is not None:
            print(row)
            row = cursor.fetchone()
 
    except Error as e:
        print(e)
 
    finally:
        cursor.close()
        conn.close()
 
 
if __name__ == ‘__main__’:
    query_with_fetchone()

Let’s examine the code in detail:

  1. First, we connected to the database by create a
    new  MySQLConnection object
  2. Second, from the  MySQLConnectionhttp://www.mysqltutorial.org/python-mysql-query/。 object, we instantiated a
    new  MySQLCursorhttp://www.mysqltutorial.org/python-mysql-query/。 object
  3. http://www.mysqltutorial.org/python-mysql-query/。Third, we executed a query that selects all rows from
    the bookshttp://www.mysqltutorial.org/python-mysql-query/。 table.
  4. http://www.mysqltutorial.org/python-mysql-query/。Fourth, we called  fetchone() method to fetch the next row in the
    result set. In the  while loop block, we printed out the content
    of the row and move to the next row until all rows are fetched.
  5. Fifth, we closed both cursor and connection objects by invoking
    the  close() method of the corresponding object.

Querying data with fetchall

 In case the number of rows in the table is small, you can use
the  fetchall() method to fetch all rows from the database table.  See
the following code.

 

 

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from mysql.connector import MySQLConnection, Error
from python_mysql_dbconfig import read_db_config
 
 
def query_with_fetchall():
    try:
        dbconfig = read_db_config()
        conn = MySQLConnection(**dbconfig)
        cursor = conn.cursor()
        cursor.execute("SELECT * FROM books")
        rows = cursor.fetchall()
 
        print(‘Total Row(s):’, cursor.rowcount)
        for row in rows:
            print(row)
 
    except Error as e:
        print(e)
 
    finally:
        cursor.close()
        conn.close()
 
 
if __name__ == ‘__main__’:
    query_with_fetchall()

The logic is similar to the example with the  fetchone() method except
for the  fetchall()method call part. Because we fetched all rows from
the books table into the memory, we can get the total rows returned by
using the  rowcount property of the cursor object.

Querying data with fetchmany

For a relatively big table, it takes time to fetch all rows and return
the result set. In addition, fetchall() needs to allocate enough
memory to store the entire result set in the memory. This is inefficient
and not a good practice.

MySQL Connector/Python provides us with the  fetchmany() method that
returns the next number of rows (n) of the result set, which allows us
to balance between time and memory space. Let’s take a look at how do we
use  fetchmany() method.

First, we develop a generator that chunks the database calls into a
series of  fetchmany() calls as follows:

 

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def iter_row(cursor, size=10):
    while True:
        rows = cursor.fetchmany(size)
        if not rows:
            break
        for row in rows:
            yield row

Second, we can use the  iter_row() generator to fetch 10 rows at a
time as shown below:

 

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def query_with_fetchmany():
    try:
        dbconfig = read_db_config()
        conn = MySQLConnection(**dbconfig)
        cursor = conn.cursor()
 
        cursor.execute("SELECT * FROM books")
 
        for row in iter_row(cursor, 10):
            print(row)
 
    except Error as e:
        print(e)
 
    finally:
        cursor.close()
        conn.close()

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