python-sql-lessons/python_sql/lessons/python_sql.md

Python + SQL

Lessons

1. Install a Python virtual environment
2. Install psychopg2-binary
3. Connect to Postgres via Python
4. Running Queries with Python
5. SQLite Basics
6. Migrating data to a SQLite
7. CSV Basics
8. Exporting data to CSV

Install a Python virtual environment

python3 -m venv ~/my-env
source ~/my-env/bin/activate

Install psychopg2-binary

python -m pip install psycopg2-binary

Connect to Postgres via Python

import psycopg2
conn = psycopg2.connect(
    database="my_db"
)

# close connection
conn.close()

Running Queries with Python

Select many

cursor = conn.cursor()

cursor.execute("SELECT * FROM people")
print(cursor.fetchall())

# close connection
cursor.close()

Select one

cursor.execute("SELECT * FROM people WHERE id = %s", [24])
print(cursor.fetchone())

Insert

cursor.execute("INSERT INTO people (name, age) VALUES (%s, %s)", ['Matt', 43])
conn.commit()

Delete

cursor.execute("DELETE FROM people WHERE id = %s", [24]);
conn.commit()

Update

cursor.execute("UPDATE people SET name = %s, age = %s WHERE id = %s", ['Matt', 43, 20])
conn.commit()

SQLite Basics

Connect:

import sqlite3
con = sqlite3.connect("mydb.db")

# close connection
con.close()

Create table

cur = con.cursor()
cur.execute("CREATE TABLE people (name, age)")

Select Many

res = cur.execute("SELECT * FROM people")
print(res.fetchall())

Select One

res = cur.execute("SELECT * FROM people WHERE name = 'Matt'")
print(res.fetchone())

Insert

cur.execute("INSERT INTO people (name, age) VALUES (?, ?)", ['Zagthorp', 543])
con.commit()

Delete

cur.execute("DELETE FROM people WHERE name = ?", ['Bilbo'])
con.commit()

Update

cur.execute("UPDATE people SET name = ? WHERE name = ?", ['Bilbo', 'Matthew'])
con.commit()

Check via sqlite3 in the terminal

sqlite3 mydb.db

Migrating data to a SQLite

# PostgreSQL

import psycopg2
conn = psycopg2.connect(
    database="my_db"
)
cursor = conn.cursor()
cursor.execute("SELECT * FROM people")

people = cursor.fetchall()

cursor.close()
conn.close()

# SQLite

import sqlite3

con = sqlite3.connect("mydb.db")
cur = con.cursor()
cur.execute("CREATE TABLE people (id, name, age)")

for person in people:
    cur.execute("INSERT INTO people (id, name, age) VALUES (?, ?, ?)", [person[0], person[1], person[2]])

con.commit()
con.close()

CSV Basics

mydb.csv:

1,matt,43
2,sally,34
3,zagthorp,999

read:

import csv

with open('mydb.csv', newline='') as csvfile:
    people = csv.reader(csvfile, delimiter=',')
    for person in people:
        print(person)

wrte:

import csv
with open('writedb.csv', 'w') as csvfile:
    writer = csv.writer(csvfile)
    writer.writerow([1, 'matt', 43])

Exporting data to CSV

# PostgreSQL

import psycopg2
conn = psycopg2.connect(
    database="my_db"
)
cursor = conn.cursor()

cursor.execute("SELECT * FROM people")

people = cursor.fetchall()

cursor.close()
conn.close()

# CSV

import csv

with open('writedb.csv', 'w') as csvfile:
    writer = csv.writer(csvfile)
    for person in people:
        writer.writerow(person)