Database Querying

In the field of data engineering, querying databases is a fundamental task. It involves retrieving and manipulating data stored in a database using a query language such as SQL (Structured Query Language).

SQL allows you to extract specific data from tables and perform operations on the data to meet your requirements. With SQL, you can use various clauses and functions to filter, sort, aggregate, and join data.

Here's an example of querying a SQLite database using Python:

1import sqlite3
2
3# Connect to the database
4conn = sqlite3.connect('mydatabase.db')
5cursor = conn.cursor()
6
7# Execute a SQL query
8query = "SELECT * FROM customers"
9cursor.execute(query)
10
11# Fetch all the results
12results = cursor.fetchall()
13
14# Print the results
15for row in results:
16    print(row)
17
18# Close the connection
19cursor.close()
20conn.close()

In this example, we connect to a SQLite database, execute a SELECT query to fetch all the rows from the 'customers' table, and then print the results.

It's important to have a solid understanding of SQL syntax and concepts such as table structures, join operations, and data manipulation functions. This knowledge enables you to effectively retrieve and analyze data from databases in a data engineering role.

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import sqlite3

​

# Connect to the database

conn = sqlite3.connect('mydatabase.db')

cursor = conn.cursor()

​

# Execute a SQL query

query = "SELECT * FROM customers"

cursor.execute(query)

​

# Fetch all the results

results = cursor.fetchall()

​

# Print the results

for row in results:

    print(row)

​

# Close the connection

cursor.close()

conn.close()