SUM function in SQL

Introduction

The SUM function adds up all the numbers in a column to give you the total. It helps you quickly find the total amount or quantity.

When you want to find the total sales made in a day.

When you need to calculate the total number of items sold.

When you want to add up all the expenses in a budget.

When you want to find the total score of a player in a game.

When you want to sum up all the hours worked by employees.

Syntax

SQL

SELECT SUM(column_name) FROM table_name;

The column_name should be a numeric column like numbers or amounts.

You can use SUM with WHERE to add only certain rows.

Examples

Adds up all the prices in the products table.

SQL

SELECT SUM(price) FROM products;

Adds up quantities only for completed orders.

SQL

SELECT SUM(quantity) FROM orders WHERE status = 'completed';

Adds sales for each category separately.

SQL

SELECT category, SUM(sales) FROM sales_data GROUP BY category;

Sample Program

This creates a sales table, adds some sales amounts, and then sums all the amounts.

SQL

CREATE TABLE sales (
  id INT,
  product VARCHAR(50),
  amount INT
);

INSERT INTO sales (id, product, amount) VALUES
(1, 'Apple', 10),
(2, 'Banana', 15),
(3, 'Apple', 5),
(4, 'Orange', 20);

SELECT SUM(amount) FROM sales;

Important Notes

SUM ignores NULL values in the column.

If there are no rows, SUM returns NULL, not zero.

Use GROUP BY with SUM to get totals for groups.

Summary

SUM adds all numbers in a column to get a total.

It works only on numeric columns.

You can filter rows or group them before summing.

Practice

1. What does the SQL SUM() function do?

easy

A. Adds all numbers in a numeric column to get a total

B. Counts the number of rows in a table

C. Finds the highest value in a column

D. Deletes duplicate rows from a table

Solution

1. Step 1: Understand the purpose of SUM()

   The SUM() function is designed to add up all values in a numeric column.

2. Step 2: Compare with other functions

   Counting rows is done by COUNT(), highest value by MAX(), and deleting duplicates is unrelated.

3. Final Answer:

   Adds all numbers in a numeric column to get a total -> Option A

4. Quick Check:

   SUM() = total of numbers [OK]

Hint: SUM() always adds numbers in a column [OK]

Common Mistakes:

• Confusing SUM() with COUNT()
• Thinking SUM() works on text columns
• Assuming SUM() deletes or filters rows

2. Which of the following is the correct syntax to get the total sales from a table named Orders with a column Amount?

easy

A. SELECT COUNT(Amount) FROM Orders;

B. SELECT SUM(Amount) FROM Orders;

C. SELECT TOTAL(Amount) FROM Orders;

D. SELECT ADD(Amount) FROM Orders;

Solution

1. Step 1: Identify the correct aggregate function

   The function to add values is SUM(), so SUM(Amount) is correct.

2. Step 2: Check syntax correctness

   SUM() is standard SQL; ADD() and TOTAL() are invalid, COUNT() counts rows, not sums.

3. Final Answer:

   SELECT SUM(Amount) FROM Orders; -> Option B

4. Quick Check:

   SUM() syntax correct [OK]

Hint: SUM() is the only valid function to add column values [OK]

Common Mistakes:

• Using ADD() or TOTAL() which are not SQL functions
• Using COUNT() instead of SUM()
• Missing parentheses after SUM

3. Given the table Sales with rows:
Product | Quantity
Apple | 10
Banana | 5
Apple | 15
What is the result of the query:
SELECT SUM(Quantity) FROM Sales WHERE Product = 'Apple';

medium

A. 10

B. 15

C. 25

D. 30

Solution

1. Step 1: Filter rows where Product = 'Apple'

   Rows matching: Apple with Quantity 10 and Apple with Quantity 15.

2. Step 2: Sum the Quantity values for these rows

   10 + 15 = 25.

3. Final Answer:

   25 -> Option C

4. Quick Check:

   10 + 15 = 25 [OK]

Hint: Sum only filtered rows matching condition [OK]

Common Mistakes:

• Summing all rows ignoring WHERE clause
• Adding only one matching row
• Confusing Quantity values

4. Consider this query:
SELECT SUM(Price) FROM Products WHERE Category = 'Electronics'
It returns NULL instead of a number. What is the most likely cause?

medium

A. There are no rows with Category = 'Electronics'

B. The Price column contains text values

C. SUM() cannot be used with WHERE clause

D. The table Products does not exist

Solution

1. Step 1: Understand SUM() behavior with no matching rows

   If no rows match the WHERE condition, SUM() returns NULL.

2. Step 2: Check other options

   Price with text would cause error, SUM() works with WHERE, and table missing causes error, not NULL.

3. Final Answer:

   There are no rows with Category = 'Electronics' -> Option A

4. Quick Check:

   SUM() returns NULL if no rows match [OK]

Hint: SUM() returns NULL if no rows match WHERE [OK]

Common Mistakes:

• Assuming SUM() returns 0 when no rows match
• Thinking SUM() fails with WHERE clause
• Ignoring NULL result meaning no data

5. You have a table Orders with columns CustomerID, OrderAmount. How do you write a query to find the total order amount for each customer?

hard

A. SELECT SUM(OrderAmount) FROM Orders GROUP BY CustomerID;

B. SELECT CustomerID, SUM(OrderAmount) FROM Orders;

C. SELECT CustomerID, TOTAL(OrderAmount) FROM Orders GROUP BY CustomerID;

D. SELECT CustomerID, SUM(OrderAmount) FROM Orders GROUP BY CustomerID;

Solution

1. Step 1: Use GROUP BY to group rows by CustomerID

   This groups all orders per customer so we can sum their amounts.

2. Step 2: Use SUM(OrderAmount) to add orders per group

   SUM() calculates total order amount for each customer group.

3. Final Answer:

   SELECT CustomerID, SUM(OrderAmount) FROM Orders GROUP BY CustomerID; -> Option D

4. Quick Check:

   GROUP BY + SUM() = total per group [OK]

Hint: Use GROUP BY with SUM() to total per group [OK]

Common Mistakes:

• Missing GROUP BY causes error or wrong result
• Using TOTAL() which is not standard SQL
• Selecting SUM() without grouping CustomerID