Variables and SET statements in SQL - Mini Project: Build & Apply

Using Variables and SET Statements in SQL

📖 Scenario: You are managing a small store's sales data. You want to calculate the total sales for a specific product category and store this value in a variable for later use.

🎯 Goal: Create a SQL script that declares a variable, sets it to the total sales amount for the 'Electronics' category, and then updates the variable with a discount amount.

📋 What You'll Learn

Declare a variable named @totalSales of type DECIMAL(10,2).

Set @totalSales to the sum of sales amounts from the Sales table where the category is 'Electronics'.

Declare a variable named @discount of type DECIMAL(10,2) and set it to 100.00.

Update @totalSales by subtracting @discount using a SET statement.

💡 Why This Matters

🌍 Real World

Variables in SQL help store temporary values like totals or counters during data processing, making queries more flexible and readable.

💼 Career

Knowing how to use variables and SET statements is essential for writing dynamic SQL scripts, stored procedures, and performing calculations in database jobs.

1

Declare the @totalSales variable

Declare a variable called @totalSales with the data type DECIMAL(10,2).

SQL

-- Declare the variable @totalSales
-- Your code here

Hint

Use the DECLARE statement to create a variable with the specified name and type.

2

Set @totalSales to sum of sales for 'Electronics'

Write a SET statement to assign @totalSales the sum of the Amount column from the Sales table where the Category is 'Electronics'.

SQL

DECLARE @totalSales DECIMAL(10,2);
-- Set @totalSales to total sales amount for 'Electronics'
-- Your code here

Hint

Use a SELECT statement inside parentheses to calculate the sum and assign it with SET.

3

Declare and set the @discount variable

Declare a variable called @discount with the data type DECIMAL(10,2) and set it to 100.00 using a SET statement.

SQL

DECLARE @totalSales DECIMAL(10,2);
SET @totalSales = (SELECT SUM(Amount) FROM Sales WHERE Category = 'Electronics');
-- Declare @discount and set it to 100.00
-- Your code here

Hint

Declare the variable first, then assign the value with SET.

4

Update @totalSales by subtracting @discount

Use a SET statement to update @totalSales by subtracting the value of @discount.

SQL

DECLARE @totalSales DECIMAL(10,2);
SET @totalSales = (SELECT SUM(Amount) FROM Sales WHERE Category = 'Electronics');
DECLARE @discount DECIMAL(10,2);
SET @discount = 100.00;
-- Subtract @discount from @totalSales
-- Your code here

Hint

Use SET to assign the new value by subtracting @discount from @totalSales.

Practice

1. What is the purpose of using variables in SQL with DECLARE and SET statements?

easy

A. To store temporary values that can be used later in the query

B. To permanently save data in the database

C. To create new tables automatically

D. To delete rows from a table

Solution

1. Step 1: Understand variable usage in SQL

   Variables declared with DECLARE hold temporary values during query execution.

2. Step 2: Identify the purpose of SET

   SET assigns values to these variables for use later in the query.

3. Final Answer:

   To store temporary values that can be used later in the query -> Option A

4. Quick Check:

   Variables = Temporary storage [OK]

Hint: Variables hold temporary data during query execution [OK]

Common Mistakes:

• Thinking variables save data permanently
• Confusing variables with table creation
• Assuming variables delete data

2. Which of the following is the correct syntax to declare an integer variable named @count and set its value to 10 in SQL?

easy

A. DECLARE @count INT; SET @count = 10;

B. SET @count INT; DECLARE @count = 10;

C. DECLARE @count = 10 INT;

D. SET @count = 10 INT;

Solution

1. Step 1: Review correct variable declaration syntax

   In SQL, DECLARE @variable_name TYPE; declares a variable.

2. Step 2: Assign value using SET

   SET @variable_name = value; assigns the value.

3. Final Answer:

   DECLARE @count INT; SET @count = 10; -> Option A

4. Quick Check:

   DECLARE then SET = correct syntax [OK]

Hint: Declare first, then set value with SET [OK]

Common Mistakes:

• Using SET before DECLARE
• Mixing declaration and assignment in wrong order
• Incorrect placement of data type

3. Consider the following SQL code:

DECLARE @x INT;
SET @x = 5;
SET @x = @x + 3;
SELECT @x;

What will be the output of the SELECT statement?

medium

A. 5

B. 8

C. 3

D. NULL

Solution

1. Step 1: Initialize variable @x

   @x is declared as integer and set to 5.

2. Step 2: Update @x by adding 3

   @x = 5 + 3 = 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add values stepwise to variable [OK]

Common Mistakes:

• Forgetting to update variable value
• Assuming variable resets after SET
• Confusing initial and updated values

4. Identify the error in the following SQL code:

DECLARE @total INT
SET total = 100;
SELECT @total;

medium

A. Missing semicolon after DECLARE statement

B. SELECT statement syntax is incorrect

C. Variable name missing '@' in SET statement

D. Variable @total not declared

Solution

1. Step 1: Check variable declaration

   @total is declared correctly with DECLARE @total INT.

2. Step 2: Check SET statement

   SET statement uses total without '@', which is incorrect; it should be @total.

3. Final Answer:

   Variable name missing '@' in SET statement -> Option C

4. Quick Check:

   Variable names need '@' prefix [OK]

Hint: Always use '@' before variable names [OK]

Common Mistakes:

• Omitting '@' in variable usage
• Forgetting semicolons (usually optional)
• Misreading SELECT syntax

5. You want to calculate the total price for 3 items each costing 15, 20, and 25 using variables. Which SQL code correctly declares variables, sets their values, and calculates the total?

hard

A. DECLARE @item1 INT = 15, @item2 INT = 20, @item3 INT = 25; SET @total = @item1 + @item2 + @item3; SELECT @total;

B. DECLARE @total INT; SET total = 15 + 20 + 25; SELECT @total;

C. SET @item1 = 15; SET @item2 = 20; SET @item3 = 25; SET @total = @item1 + @item2 + @item3; SELECT @total;

D. DECLARE @item1 INT; DECLARE @item2 INT; DECLARE @item3 INT; DECLARE @total INT; SET @item1 = 15; SET @item2 = 20; SET @item3 = 25; SET @total = @item1 + @item2 + @item3; SELECT @total;

Solution

1. Step 1: Declare all variables before use

   Variables @item1, @item2, @item3, and @total must be declared first.

2. Step 2: Assign values and calculate total

   Assign each item price with SET, then sum them into @total.

3. Step 3: Select the total value

   Use SELECT to output the total price.

4. Final Answer:

   DECLARE @item1 INT; DECLARE @item2 INT; DECLARE @item3 INT; DECLARE @total INT; SET @item1 = 15; SET @item2 = 20; SET @item3 = 25; SET @total = @item1 + @item2 + @item3; SELECT @total; -> Option D

5. Quick Check:

   Declare then set all variables before calculation [OK]

Hint: Declare all variables first, then assign and calculate [OK]

Common Mistakes:

• Assigning values without declaring variables
• Trying to declare and assign in one line incorrectly
• Missing variable declarations