Adventures in Machine Learning

Connecting to Databases with Python SQL Libraries: A Step-by-Step Guide

Connecting to Databases with Python SQL Libraries

If you are someone who is looking to connect to a database using Python, then you have come to the right place. Connecting to a database is a crucial first step in applications that require the ability to read, write and manipulate data.

In this article, we will focus on how to connect to SQLite, MySQL and PostgreSQL using Python SQL libraries.

Connecting to SQLite

SQLite is a lightweight, serverless database that is highly preferred when you need to st

ore small to medium data sets that need to be accessed locally. It is an embedded database that does not require a separate server process and allows quick and easy integration into the application.

To connect to SQLite in Python, SQLite3, a Python SQL library, is used. To connect to an SQLite database in Python, you will need to follow these steps:

Step 1: First, ensure that you have the SQLite3 Python library installed

“`

pip install sqlite3

“`

Step 2: Create a connection to the database using the .connect() method

“`

imp

ort sqlite3

conn = sqlite3.connect(‘example.db’)

“`

Step 3: Once the connection is established, create a curs

or object to execute SQL statements using .curs

or() method

“`

curs

or = conn.curs

or()

“`

With these simple steps, you will be able to connect to an SQLite database in Python.

Connecting to MySQL

MySQL is a popular open-source relational database management system that is widely used f

or web applications. To connect to a MySQL database in Python, you will need to use a Python SQL driver such as PyMySQL

or mysql-connect

or-python.

To connect to a MySQL database in Python, you will need to follow these steps:

Step 1: Ensure you have one of the MySQL drivers installed

“`

pip install PyMySQL

“`

or

“`

pip install mysql-connect

or-python

“`

Step 2: Create a connection object by providing the necessary credentials as parameters

“`

imp

ort pymysql

conn = pymysql.connect(host=’localhost’, user=’root’, passw

ord=’passw

ord’, db=’test_db’)

“`

Step 3: Once the connection is established, create a curs

or object to execute SQL statements using the .curs

or() method

“`

curs

or = conn.curs

or()

“`

With these simple steps, you will be able to connect to MySQL in Python.

Connecting to PostgreSQL

PostgreSQL is an object-relational database management system that is commonly used in large enterprise settings. It is open-source, and its features include supp

ort f

or ACID transactions, f

oreign keys, and the ability to handle large amounts of data.

To connect to a PostgreSQL database in Python, you will need to use a Python SQL driver such as psycopg2. To connect to a PostgreSQL database in Python, you will need to follow these steps:

Step 1: Ensure you have psycopg2 installed

“`

pip install psycopg2

“`

Step 2: Create a connection object by providing the necessary credentials as parameters

“`

imp

ort psycopg2

conn = psycopg2.connect(

host=’localhost’,

database=’test_db’,

user=’root’,

passw

ord=’passw

ord’

)

“`

Step 3: Once the connection is established, create a curs

or object to execute SQL statements using the .curs

or() method

“`

curs

or = conn.curs

or()

“`

With these simple steps, you will be able to connect to PostgreSQL in Python.

Creating Tables in Databases

Once you have successfully established a connection to your database of choice, the next logical step would be to create tables. A table is a set of data elements that is

organized in a tabular f

ormat.

Tables are the backbone of database systems because they st

ore the data that is retrieved, updated,

or removed from the database. In this section, we will focus on how to create tables in SQLite, MySQL and PostgreSQL.

Creating Tables in SQLite

When creating tables in SQLite, there are a few guidelines that you need to follow. The first step is to create a connection object as we have seen earlier.

Next, create a curs

or object to execute SQL statements using the .curs

or() method and call the curs

or.execute() method to run the SQL statement. Here is an example of how to create a table in SQLite using Python:

“`

imp

ort sqlite3

conn = sqlite3.connect(‘example.db’)

curs

or = conn.curs

or()

# Creating a table

curs

or.execute(”’CREATE TABLE stocks (date text, trans text, symbol text, qty real, price real)”’)

“`

In this example, we create a table named ‘stocks’ that has five columns – ‘date’, ‘trans’, ‘symbol’, ‘qty’ and ‘price’ all with different data types.

Creating Tables in MySQL

To create a table in MySQL, you will need to use the same guidelines that were used f

or SQLite. The only difference will be the syntax used in the SQL query.

When creating the table, you can also define various constraints such as setting a primary key, defining default values, and specifying data types. Here is an example of how to create a table in MySQL using Python:

“`

imp

ort pymysql

conn = pymysql.connect(host=’localhost’, user=’root’, passw

ord=’passw

ord’, db=’test_db’)

curs

or = conn.curs

or()

# Creating a table

curs

or.execute(”’CREATE TABLE stocks (date TEXT, trans TEXT, symbol TEXT, qty REAL, price REAL, PRIMARY KEY (symbol))”’)

“`

In this example, we create a table named ‘stocks’ that has five columns – ‘date’, ‘trans’, ‘symbol’, ‘qty’ and ‘price’.

The PRIMARY KEY clause specifies that the ‘symbol’ column will be the primary key f

or the table.

Creating Tables in PostgreSQL

Creating tables in PostgreSQL follows the same guidelines that were used f

or SQLite and MySQL. The only difference will be the syntax used in the SQL query.

“`

imp

ort psycopg2

conn = psycopg2.connect(

host=’localhost’,

database=’test_db’,

user=’root’,

passw

ord=’passw

ord’

)

curs

or = conn.curs

or()

# Creating a table

curs

or.execute(”’CREATE TABLE stocks (date TEXT, trans TEXT, symbol TEXT, qty REAL, price REAL, PRIMARY KEY (symbol))”’)

“`

In this example, we create a table named ‘stocks’ that has five columns – ‘date’, ‘trans’, ‘symbol’, ‘qty’ and ‘price’.

Conclusion

In this article, we have expl

ored how to connect to SQLite, MySQL and PostgreSQL using their respective Python SQL libraries. We have also seen how to create tables in each of these databases.

I hope you found this article inf

ormative and useful. With these skills, you can easily start manipulating data in your database using Python.

Inserting Rec

ords into Tables

Once you have created the tables in your database, the next step is to insert rec

ords into them. Rec

ords are the data entries st

ored within the tables.

When inserting data into a table, you need to specify the values f

or each column in the table. In this section, we will look at how to insert rec

ords into SQLite, MySQL and PostgreSQL tables.

Inserting Rec

ords into SQLite Tables

To insert data into an SQLite table, you need to use the INSERT INTO statement. This statement will allow you to add new rec

ords to the table.

You can add a single rec

ord at a time,

or you can add multiple rec

ords at once using the executemany() method. Here’s an example of how to insert rec

ords into an SQLite table using Python:

“`

imp

ort sqlite3

conn = sqlite3.connect(‘example.db’)

curs

or = conn.curs

or()

# Inserting a single rec

ord

curs

or.execute(“INSERT INTO stocks VALUES (‘2022-11-03’, ‘BUY’, ‘AAPL’, 100, 135.2)”)

# Inserting multiple rec

ords

data = [(‘2022-11-05’, ‘SELL’, ‘GOOG’, 50, 1230.4), (‘2022-11-07’, ‘BUY’, ‘MSFT’, 75, 250.6)]

curs

or.executemany(‘INSERT INTO stocks VALUES (?, ?, ?, ?, ?)’, data)

conn.commit()

“`

In this example, we first create a connection object to the database and create a curs

or object to execute SQL statements using the .curs

or() method. We then insert a single rec

ord and multiple rec

ords using the curs

or.execute() and curs

or.executemany() methods, respectively.

Finally, we commit the changes using the commit() method. Inserting Rec

ords into MySQL Tables

To insert data into a MySQL table, you can use the same INSERT INTO statement that we used f

or SQLite.

However, the syntax f

or the statement will be slightly different. Here’s an example of how to insert rec

ords into a MySQL table using Python:

“`

imp

ort pymysql

conn = pymysql.connect(host=’localhost’, user=’root’, passw

ord=’passw

ord’, db=’test_db’)

curs

or = conn.curs

or()

# Inserting a single rec

ord

curs

or.execute(“INSERT INTO stocks VALUES (‘2022-11-03’, ‘BUY’, ‘AAPL’, 100, 135.2)”)

# Inserting multiple rec

ords

data = [(‘2022-11-05’, ‘SELL’, ‘GOOG’, 50, 1230.4), (‘2022-11-07’, ‘BUY’, ‘MSFT’, 75, 250.6)]

curs

or.executemany(‘INSERT INTO stocks VALUES (%s,%s,%s,%s,%s)’, data)

conn.commit()

“`

In this example, we establish a connection to the database and create a curs

or object using the .curs

or() method.

We insert a single rec

ord and multiple rec

ords using curs

or.execute() and curs

or.executemany(), respectively. Note that the placeholders f

or values in the executemany() method use the ‘%s’ syntax instead of the ‘?’ syntax used by SQLite.

Finally, we commit the changes using commit(). Inserting Rec

ords into PostgreSQL Tables

To insert data into a PostgreSQL table, we use the same INSERT INTO statement as we did f

or SQLite and MySQL.

But, again, the syntax f

or this statement in PostgreSQL is slightly different. Here’s an example of how to insert rec

ords into a PostgreSQL table using Python:

“`

imp

ort psycopg2

conn = psycopg2.connect(

host=’localhost’,

database=’test_db’,

user=’root’,

passw

ord=’passw

ord’

)

curs

or = conn.curs

or()

# Inserting a single rec

ord

curs

or.execute(“INSERT INTO stocks VALUES (‘2022-11-03’, ‘BUY’, ‘AAPL’, 100, 135.2)”)

# Inserting multiple rec

ords

data = [(‘2022-11-05’, ‘SELL’, ‘GOOG’, 50, 1230.4), (‘2022-11-07’, ‘BUY’, ‘MSFT’, 75, 250.6)]

curs

or.executemany(‘INSERT INTO stocks VALUES (%s,%s,%s,%s,%s)’, data)

conn.commit()

“`

In this example, we create a connection to the test_db database and create a curs

or object using .curs

or().

We insert a single rec

ord and multiple rec

ords using curs

or.execute() and curs

or.executemany() methods, respectively. The placeholders f

or the values in psycopg2 would also be ‘%s’, similar to MySQL.

Selecting Rec

ords from Tables

Selecting rec

ords from a table is one of the most imp

ortant and commonly used operations in databases. It allows you to retrieve specific data from the table that matches certain criteria.

In this section, we will learn how to select rec

ords from SQLite, MySQL and PostgreSQL tables. Selecting Rec

ords from SQLite Tables

To select rec

ords from an SQLite table in Python, you need to use the SELECT statement.

This statement allows you to retrieve data from specific columns of a table based on certain criteria, such as filtering by a specific value

or s

orting by a specific column.

Here’s an example of how to select rec

ords from an SQLite table using Python:

“`

imp

ort sqlite3

conn = sqlite3.connect(‘example.db’)

curs

or = conn.curs

or()

# Selecting all rec

ords

curs

or.execute(“SELECT * FROM stocks”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

# Selecting specific rec

ords based on a condition

curs

or.execute(“SELECT * FROM stocks WHERE symbol=’AAPL'”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

“`

In this example, we first create a connection object to the database and create a curs

or object to execute SQL statements using the .curs

or() method. We then select all rec

ords from the ‘stocks’ table using the SELECT statement and fetch all the results using the fetchall() method.

We finally print out all the rows using a f

or loop. We do the same f

or selecting specific rec

ords based on the condition that the ‘symbol’ column equals ‘AAPL’.

Selecting Rec

ords from MySQL Tables

To select rec

ords from a MySQL table, you need to use the same SELECT statement that we used f

or SQLite. However, the syntax of the statement will be slightly different.

Here’s an example of how to select rec

ords from a MySQL table using Python:

“`

imp

ort pymysql

conn = pymysql.connect(host=’localhost’, user=’root’, passw

ord=’passw

ord’, db=’test_db’)

curs

or = conn.curs

or()

# Selecting all rec

ords

curs

or.execute(“SELECT * FROM stocks”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

# Selecting specific rec

ords based on a condition

curs

or.execute(“SELECT * FROM stocks WHERE symbol=’AAPL'”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

“`

In this example, we establish a connection to the database and create a curs

or object using the .curs

or() method. We then select all rec

ords from the ‘stocks’ table and fetch all the results using fetchall().

We do the same f

or selecting specific rec

ords based on the condition that the ‘symbol’ column equals ‘AAPL’. Selecting Rec

ords from PostgreSQL Tables

To select rec

ords from a PostgreSQL table in Python, we use the same SELECT statement as we did f

or SQLite and MySQL.

But, again, the syntax f

or the statement is slightly different. Here’s an example of how to select rec

ords from a PostgreSQL table using Python:

“`

imp

ort psycopg2

conn = psycopg2.connect(

host=’localhost’,

database=’test_db’,

user=’root’,

passw

ord=’passw

ord’

)

curs

or = conn.curs

or()

# Selecting all rec

ords

curs

or.execute(“SELECT * FROM stocks”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

# Selecting specific rec

ords based on a condition

curs

or.execute(“SELECT * FROM stocks WHERE symbol=’AAPL'”)

rows = curs

or.fetchall()

f

or row in rows:

print(row)

“`

In this example, we create a connection to the test_db database and create a curs

or object using .curs

or().

We select all rec

ords from the ‘stocks’ table and fetch all the results using the fetchall() method. We then print all the rows, and do the same f

or selecting specific rec

ords based on the condition the ‘symbol’ column equals ‘AAPL’.

Conclusion

In this article, we have examined how to insert rec

ords into tables and select rec

ords from tables using Python SQL libraries. We saw how the same basic principles apply to all three databases – SQLite, MySQL, and PostgreSQL.

By following the guidelines outlined in this article, you should be able to insert and select rec

ord data from your database tables effectively from Python. Updating Rec

ords in Tables

Updating rec

ords in a database is necessary when you want to make changes to the existing data.

F

or instance, you may want to update a customer’s address, an employee’s salary,

or any other rec

ord in the system. In this section, we will cover how to update rec

ords in SQLite, MySQL, and PostgreSQL databases using Python SQL libraries.

Updating Rec

ords in SQLite Tables

To update data in an SQLite table, you need to use the UPDATE statement along with the SET clause. The UPDATE statement allows you to update existing rec

ords, while the SET clause lets you specify the particular columns and their new values.

Here’s an example of how to update rec

ords in an SQLite table using Python:

“`

imp

ort sqlite3

conn = sqlite3.connect(‘example.db’)

curs

or = conn.curs

or()

# Updating a single rec

ord

curs

or.execute(“UPDATE stocks SET price=137.5 WHERE symbol=’AAPL’ AND qty=100”)

# Updating multiple rec

ords

data = [(130, ‘GOOG’), (240, ‘MSFT’)]

curs

or.executemany(“UPDATE stocks SET price = ? WHERE symbol = ?”, data)

conn.commit()

“`

In this example, we create a connection object to the database and then update a single rec

ord where the stock symbol is ‘

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