SUM function in SQL - Step-by-Step Execution

Concept Flow - SUM function

Start with empty total = 0

↓

Read next row's value

↓

Add value to total

↓

More rows?

Yes→Repeat read and add

No↓

Return total sum

The SUM function adds up values from each row one by one until all rows are processed, then returns the total.

Execution Sample

SQL

SELECT SUM(sales) FROM orders;

This query calculates the total sales by adding the sales values from all rows in the orders table.

Execution Table

Step	Row sales value	Running total	Action
1	100	0 + 100 = 100	Add 100 to total
2	200	100 + 200 = 300	Add 200 to total
3	50	300 + 50 = 350	Add 50 to total
4	NULL	350 + 0 = 350	NULL treated as 0, no change
5	150	350 + 150 = 500	Add 150 to total
6	-	500	No more rows, return total

💡 All rows processed, final sum is 500

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
total	0	100	300	350	350	500	500

Key Moments - 2 Insights

Why does the total not change when the sales value is NULL?

In the execution_table row 4, NULL is ignored by SUM, so it does not add to the total.

What happens if there are no rows in the table?

SUM returns NULL or zero depending on the database, but logically it means no values to add, so total stays at start value.

Visual Quiz - 3 Questions

Look at the execution_table, what is the running total after step 3?

A 250

B 300

C 350

D 100

💡 Hint

Check the 'Running total' column at step 3 in the execution_table

At which step does the SUM function encounter a NULL value?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look for 'NULL' in the 'Row sales value' column in the execution_table

If the sales value at step 5 was 100 instead of 150, what would be the final total?

A 500

B 450

C 400

D 350

💡 Hint

Subtract 50 from the final total in variable_tracker to adjust for the change at step 5

Concept Snapshot

SUM(column_name) adds all values in that column from all rows.
NULL values are ignored (treated as zero).
Returns one total number.
Used to find totals like total sales or total quantity.
Syntax: SELECT SUM(column) FROM table;

Full Transcript

The SUM function in SQL adds up all the values in a specified column from multiple rows. It starts with a total of zero, then reads each row's value and adds it to the total. If a value is NULL, it is ignored and does not change the total. After processing all rows, SUM returns the final total. For example, if sales values are 100, 200, 50, NULL, and 150, the total sum is 500. This function helps find totals like total sales or total quantity easily with a simple query.