Where clause filtering in C Sharp (C#) - Mini Project: Build & Apply

Filtering Products with a WHERE Clause in C#

📖 Scenario: You work at a small store that sells products. You have a list of products with their prices. You want to find only the products that cost less than $20 to decide what to put on sale.

🎯 Goal: Build a simple C# program that stores product names and prices, then uses a WHERE clause to filter and show only products cheaper than $20.

📋 What You'll Learn

Create a dictionary called products with these exact entries: "Pen": 5, "Notebook": 15, "Backpack": 45, "Pencil": 3, "Eraser": 2

Create an integer variable called priceLimit and set it to 20

Use a LINQ Where clause with priceLimit to filter products for items with price less than priceLimit

Print the filtered products in the format "Product: Price"

💡 Why This Matters

🌍 Real World

Filtering products by price is common in stores to create sales or promotions.

💼 Career

Understanding how to filter data with WHERE clauses is essential for database queries and data processing in many programming jobs.

1

Create the products dictionary

Create a dictionary called products with these exact entries: "Pen": 5, "Notebook": 15, "Backpack": 45, "Pencil": 3, "Eraser": 2

C Sharp (C#)

# Create the products dictionary with exact entries
// Your code here

Hint

Use Dictionary<string, int> and initialize it with the exact product-price pairs.

2

Add the price limit variable

Create an integer variable called priceLimit and set it to 20

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Dictionary products = new Dictionary
        {
            {"Pen", 5},
            {"Notebook", 15},
            {"Backpack", 45},
            {"Pencil", 3},
            {"Eraser", 2}
        };

        // Create priceLimit variable and set to 20
        // Your code here
    }
}

Hint

Declare int priceLimit = 20; inside the Main method.

3

Filter products using a WHERE clause

Use a LINQ Where clause with priceLimit to filter products for items with price less than priceLimit. Store the result in a variable called filteredProducts

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        Dictionary products = new Dictionary
        {
            {"Pen", 5},
            {"Notebook", 15},
            {"Backpack", 45},
            {"Pencil", 3},
            {"Eraser", 2}
        };

        int priceLimit = 20;

        // Use LINQ Where clause to filter products cheaper than priceLimit
        // Your code here
    }
}

Hint

Use var filteredProducts = products.Where(product => product.Value < priceLimit);

4

Print the filtered products

Use a foreach loop to print each product and its price from filteredProducts in the format "Product: Price"

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        Dictionary products = new Dictionary
        {
            {"Pen", 5},
            {"Notebook", 15},
            {"Backpack", 45},
            {"Pencil", 3},
            {"Eraser", 2}
        };

        int priceLimit = 20;

        var filteredProducts = products.Where(product => product.Value < priceLimit);

        // Print each filtered product and price
        // Your code here
    }
}

Hint

Use foreach (var product in filteredProducts) and inside the loop print Console.WriteLine($"{product.Key}: {product.Value}");

Practice

1.

What does the WHERE clause do in a SQL query?

easy

A. Groups rows by a column

B. Filters rows based on a condition

C. Joins two tables together

D. Sorts the rows in ascending order

Solution

1. Step 1: Understand the purpose of WHERE clause

   The WHERE clause is used to select only rows that meet a specific condition.

2. Step 2: Compare with other SQL clauses

   Sorting is done by ORDER BY, joining by JOIN, grouping by GROUP BY, so WHERE is for filtering rows.

3. Final Answer:

   Filters rows based on a condition -> Option B

4. Quick Check:

   WHERE clause = filter rows [OK]

Hint: WHERE filters rows by condition, not sorting or joining [OK]

Common Mistakes:

• Confusing WHERE with ORDER BY
• Thinking WHERE joins tables
• Mixing WHERE with GROUP BY

2.

Which of the following is the correct syntax to filter rows where Age is greater than 30?

SELECT * FROM Users WHERE ___;

easy

A. Age > 30

B. Age = > 30

C. Age >> 30

D. Age >= 30

Solution

1. Step 1: Identify correct comparison operator

   The operator for 'greater than' is >, so 'Age > 30' is correct.

2. Step 2: Check other options for syntax errors

   'Age = > 30' and 'Age >> 30' are invalid syntax. 'Age >= 30' means 'greater or equal', not strictly greater.

3. Final Answer:

   Age > 30 -> Option A

4. Quick Check:

   Use > for greater than [OK]

Hint: Use > for greater than, >= for greater or equal [OK]

Common Mistakes:

• Using = > instead of >
• Using >> which is invalid
• Confusing > with >= operator

3.

Given the table Employees with columns Name and Salary, what rows will this query return?

SELECT Name FROM Employees WHERE Salary < 50000;

medium

A. Employees with salary less than 50000

B. Employees with salary greater than 50000

C. All employees regardless of salary

D. Employees with salary equal to 50000

Solution

1. Step 1: Understand the WHERE condition

   The condition Salary < 50000 means select rows where salary is less than 50000.

2. Step 2: Interpret the query result

   The query returns only the Name column for employees meeting that condition.

3. Final Answer:

   Employees with salary less than 50000 -> Option A

4. Quick Check:

   WHERE Salary < 50000 filters salaries below 50000 [OK]

Hint: Less than means <, so Salary < 50000 filters lower salaries [OK]

Common Mistakes:

• Confusing < with >
• Thinking it returns all employees
• Assuming it returns salary column too

4.

Identify the error in this query that tries to select users with age 18 or older:

SELECT * FROM Users WHERE Age => 18;

medium

A. WHERE clause cannot use numeric comparisons

B. Missing quotes around 18

C. The operator => is invalid; should be >= instead

D. SELECT * is not allowed with WHERE

Solution

1. Step 1: Check the comparison operator

   The operator => is not valid SQL syntax; the correct operator for 'greater or equal' is >=.

2. Step 2: Verify other parts of the query

   Numeric values like 18 do not need quotes, WHERE supports numeric comparisons, and SELECT * works with WHERE.

3. Final Answer:

   The operator => is invalid; should be >= instead -> Option C

4. Quick Check:

   Use >= for greater or equal, not => [OK]

Hint: Use >= for greater or equal, not => [OK]

Common Mistakes:

• Using => instead of >=
• Adding quotes around numbers
• Thinking WHERE can't compare numbers

5.

You want to select all products from a Products table where the Price is between 10 and 20 inclusive. Which WHERE clause is correct?

hard

A. Price BETWEEN 10 AND 20 EXCLUSIVE

B. Price > 10 AND Price < 20

C. Price >= 10 OR Price <= 20

D. Price >= 10 AND Price <= 20

Solution

1. Step 1: Understand inclusive range filtering

   Inclusive means including 10 and 20, so use >= and <= operators.

2. Step 2: Analyze each option

   Price > 10 AND Price < 20 excludes 10 and 20 (strictly greater and less). Price >= 10 OR Price <= 20 uses OR, which selects too many rows. Price BETWEEN 10 AND 20 EXCLUSIVE is invalid syntax.

3. Final Answer:

   Price >= 10 AND Price <= 20 -> Option D

4. Quick Check:

   Inclusive range uses >= and <= with AND [OK]

Hint: Use >= and <= with AND for inclusive ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using OR instead of AND
• Trying invalid BETWEEN syntax