How to Use the CASE Statement in SQL for Conditional Logic

The CASE statement in SQL allows you to implement conditional logic within queries, making it a powerful tool for handling complex data transformations and classifications.

Basic Syntax

The CASE statement works like an IF-ELSE structure, evaluating conditions and returning corresponding values:

SELECT column_name,
       CASE
           WHEN condition1 THEN result1
           WHEN condition2 THEN result2
           ELSE default_result
       END AS alias_name
FROM table_name;

Example: Categorizing Data

Assume we have an Employees table with an Age column, and we want to categorize employees based on their age groups:

SELECT Name, Age,
       CASE
           WHEN Age < 25 THEN 'Young'
           WHEN Age BETWEEN 25 AND 40 THEN 'Mid-Age'
           ELSE 'Senior'
       END AS AgeCategory
FROM Employees;

Output Example:

Name Age AgeCategory
Alice 22 Young
Bob 30 Mid-Age
Charlie 50 Senior

Using CASE in Aggregations

CASE is often used in aggregate functions to perform conditional counting or summing:

SELECT
    SUM(CASE WHEN Age < 25 THEN 1 ELSE 0 END) AS YoungCount,
    SUM(CASE WHEN Age BETWEEN 25 AND 40 THEN 1 ELSE 0 END) AS MidAgeCount,
    SUM(CASE WHEN Age > 40 THEN 1 ELSE 0 END) AS SeniorCount
FROM Employees;

Using CASE in ORDER BY

You can use CASE to customize sorting order dynamically:

SELECT Name, Age
FROM Employees
ORDER BY
    CASE
        WHEN Age < 25 THEN 1
        WHEN Age BETWEEN 25 AND 40 THEN 2
        ELSE 3
    END;

Conclusion

The CASE statement is a versatile tool in SQL for implementing conditional logic in SELECT, WHERE, ORDER BY, and aggregate functions. It enhances query flexibility, making data classification and transformation more efficient.

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Related

Slow initial load times can drive users away from your React application. One powerful technique to improve performance is lazy loading - loading components only when they're needed.

Let's explore how to implement this in React.

The Problem with Eager Loading

By default, React bundles all your components together, forcing users to download everything upfront. This makes navigation much quicker and more streamlined once this initial download is complete.

However, depending on the size of your application, it could also create a long initial load time.

import HeavyComponent from './HeavyComponent';
import AnotherHeavyComponent from './AnotherHeavyComponent';

function App() {
  return (
    <div>
      {/* These components load even if user never sees them */}
      <HeavyComponent />
      <AnotherHeavyComponent />
    </div>
  );
}

React.lazy() to the Rescue

React.lazy() lets you defer loading components until they're actually needed:

import React, { lazy, Suspense } from 'react';

// Components are now loaded only when rendered
const HeavyComponent = lazy(() => import('./HeavyComponent'));
const AnotherHeavyComponent = lazy(() => import('./AnotherHeavyComponent'));

function App() {
  return (
    <div>
      <Suspense fallback={<div>Loading...</div>}>
        <HeavyComponent />
        <AnotherHeavyComponent />
      </Suspense>
    </div>
  );
}

Route-Based Lazy Loading

Combine with React Router for even better performance:

import React, { lazy, Suspense } from 'react';
import { BrowserRouter, Routes, Route } from 'react-router-dom';

const Home = lazy(() => import('./pages/Home'));
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <BrowserRouter>
      <Suspense fallback={<div>Loading...</div>}>
        <Routes>
          <Route path="/" element={<Home />} />
          <Route path="/dashboard" element={<Dashboard />} />
          <Route path="/settings" element={<Settings />} />
        </Routes>
      </Suspense>
    </BrowserRouter>
  );
}

Implement these techniques in your React application today and watch your load times improve dramatically!

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When working with URLs in C#, encoding is essential to ensure that special characters (like spaces, ?, &, and =) don’t break the URL structure. The recommended way to encode a string for a URL is by using Uri.EscapeDataString(), which converts unsafe characters into their percent-encoded equivalents.

string rawText = "hello world!";
string encodedText = Uri.EscapeDataString(rawText);

Console.WriteLine(encodedText); // Output: hello%20world%21

This method encodes spaces as %20, making it ideal for query parameters.

For ASP.NET applications, you can also use HttpUtility.UrlEncode() (from System.Web), which encodes spaces as +:

using System.Web;

string encodedText = HttpUtility.UrlEncode("hello world!");
Console.WriteLine(encodedText); // Output: hello+world%21

For .NET Core and later, Uri.EscapeDataString() is the preferred choice.

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Reading a file line by line is useful when handling large files without loading everything into memory at once.

✅ Best Practice: Use File.ReadLines() which is more memory efficient.

Example

foreach (string line in File.ReadLines("file.txt"))
{
    Console.WriteLine(line);
}

Why use ReadLines()?

Reads one line at a time, reducing overall memory usage. Ideal for large files (e.g., logs, CSVs).

Alternative: Use StreamReader (More Control)

For scenarios where you need custom processing while reading the contents of the file:

using (StreamReader reader = new StreamReader("file.txt"))
{
    string? line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

Why use StreamReader?

Lets you handle exceptions, encoding, and buffering. Supports custom processing (e.g., search for a keyword while reading).

When to Use ReadAllLines()? If you need all lines at once, use:

string[] lines = File.ReadAllLines("file.txt");

Caution: Loads the entire file into memory—avoid for large files!

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