From SQL to REST: Demystifying REST API

Does the thought of handling large, unstructured datasets keep you up at night? Do you feel overwhelmed by REST APIs and SQL queries? You're not alone. Many data engineers, analysts, and developers struggle to bridge the gap between unorganized API data and well-structured SQL tables for analysis and decision-making.

This article will demystify the relationship between REST APIs and SQL queries, helping you move from chaotic datasets to structured, queryable information. We’ll explore how REST API query parameters relate to SQL WHERE, SELECT, JOIN, and ORDER BY clauses. By the end, you’ll be equipped with the knowledge to seamlessly transform raw API data into actionable business insights.

🚀 What is a REST API?

A REST API (Representational State Transfer) is a web service that allows applications to access and interact with remote data via HTTP. Unlike SQL queries that interact directly with a database, a REST API serves as a middleman, allowing applications to request data from web servers.

When you access data from a REST API, you typically make a GET request like this:

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GET https://api.example.com/employees?department=sales&limit=10

This query retrieves 10 employee records from the Sales department. The data is often returned in JSON format, which is unstructured and can be hard to work with directly. By contrast, SQL lets you define a schema (tables and columns), making it much easier to query data.

Key Components of a REST API

ComponentDefinitionBase URLThe main URL endpoint (e.g., https://api.example.com/)EndpointThe specific resource (e.g., /employees)Query ParametersFilters the result set (e.g., ?department=sales)HTTP MethodsMethods like GET, POST, PUT, and DELETE for CRUD operationsResponseThe data, typically in JSON or XML format

🔍 REST API vs SQL Queries

If you're familiar with SQL queries, you'll notice some similarities between API query parameters and SQL WHERE clauses. Both are designed to filter and retrieve data. The main difference is where the data resides. SQL queries operate directly on a database, while REST API calls request data from a web service.

Mapping REST API Query Parameters to SQL

REST API Query ParameterSQL EquivalentExampleFilter (?department=sales)WHERE clauseSELECT * FROM employees WHERE department='sales'Sort (?sort=asc)ORDER BY clauseSELECT * FROM employees ORDER BY name ASCLimit (?limit=10)LIMIT clauseSELECT * FROM employees LIMIT 10Fields (?fields=name,age)SELECT clauseSELECT name, age FROM employeesPagination (?page=2&limit=10)OFFSET and LIMITSELECT * FROM employees LIMIT 10 OFFSET 10

📘 Step 1: Data Retrieval from a REST API

Let’s start with a simple API request to get data about employees.
API URL:

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https://api.example.com/employees?department=Engineering&limit=5

Response (JSON Format)

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[ {"id": 1, "name": "John Doe", "department": "Engineering", "salary": 90000}, {"id": 2, "name": "Jane Smith", "department": "Engineering", "salary": 85000}, {"id": 3, "name": "Alice Johnson", "department": "Engineering", "salary": 92000}, {"id": 4, "name": "Bob Brown", "department": "Engineering", "salary": 88000}, {"id": 5, "name": "Charlie White", "department": "Engineering", "salary": 87000} ]

📘 Step 2: Transform API Data into a SQL Table

We’ll now convert the JSON response into a relational table. The table below represents how this data might be structured in an SQL database.

idnamedepartmentsalary1John DoeEngineering900002Jane SmithEngineering850003Alice JohnsonEngineering920004Bob BrownEngineering880005Charlie WhiteEngineering87000

📘 Step 3: SQL Queries to Retrieve and Filter Data

Let’s see how to convert REST API query parameters into SQL queries.

1️⃣ Filter by Department

REST API Call:

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https://api.example.com/employees?department=Engineering

SQL Equivalent:

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SELECT * FROM employees WHERE department = 'Engineering';

2️⃣ Select Specific Columns

REST API Call:

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https://api.example.com/employees?fields=name,salary

SQL Equivalent:

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SELECT name, salary FROM employees;

3️⃣ Sort Data

REST API Call:

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https://api.example.com/employees?sort=salary&order=desc

SQL Equivalent:

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SELECT * FROM employees ORDER BY salary DESC;

4️⃣ Pagination (Limit and Offset)

REST API Call:

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https://api.example.com/employees?page=2&limit=5

SQL Equivalent:

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SELECT * FROM employees LIMIT 5 OFFSET 5;

5️⃣ Combine Filters, Sorting, and Pagination

REST API Call:

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https://api.example.com/employees?department=Engineering&sort=salary&order=desc&limit=5

SQL Equivalent:

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SELECT * FROM employees WHERE department = 'Engineering' ORDER BY salary DESC LIMIT 5;

This query combines a WHERE clause, ORDER BY clause, and LIMIT clause to achieve a complex, multi-parameter filter.

📘 Step 4: Building a REST API from SQL Queries

Want to create a REST API that returns SQL data? Here’s a simple Python Flask API that runs an SQL query and returns the result as JSON.

Flask REST API (app.py)

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from flask import Flask, request, jsonify import sqlite3 app = Flask(__name__) @app.route('/employees', methods=['GET']) def get_employees(): department = request.args.get('department') limit = request.args.get('limit', 10) sort = request.args.get('sort', 'id') order = request.args.get('order', 'asc') query = f""" SELECT * FROM employees WHERE department = '{department}' ORDER BY {sort} {order} LIMIT {limit} """ conn = sqlite3.connect('database.db') cursor = conn.cursor() cursor.execute(query) rows = cursor.fetchall() conn.close() return jsonify(rows) if __name__ == '__main__': app.run(debug=True)

How it works:

  • The /employees endpoint reads the query parameters from the URL (department, limit, sort, and order).

  • It dynamically constructs an SQL query and retrieves employee records from the SQLite database.

📘 Best Practices for REST API and SQL Integration

  1. Sanitize User Inputs: Avoid SQL injection by using parameterized queries.

  2. Limit API Pagination: Avoid exposing large datasets by setting a max page size.

  3. Index Your Tables: Use SQL indexes to optimize WHERE, ORDER BY, and LIMIT queries.

  4. Log and Monitor API Usage: Log API requests to detect abnormal behavior.

🎉 Final Thoughts

The connection between REST API query parameters and SQL clauses is more intuitive than it seems. Both are designed to filter, sort, and paginate data. By mastering the mapping between the two, you can:

  • Build smarter APIs.

  • Query large datasets with precision.

  • Enable better data-driven decisions.

Want to go deeper? Learn to build REST APIs with Python Flask and implement advanced SQL optimizations to improve performance. 🚀

Call to Action: Start exploring public APIs like JSONPlaceholder or OpenWeather API. Practice transforming their raw JSON data into well-structured SQL queries. Your next data-driven app will be unstoppable!

API, Rest API, REST APIFrancesca Tabor