SUM function in MySQL - Step-by-Step Execution

Concept Flow - SUM function

Start Query

↓

Scan Table Rows

↓

Extract Numeric Column Values

↓

Add Values Together

↓

Return Total Sum

↓

End Query

The SUM function scans all rows, extracts the numeric values from a column, adds them up, and returns the total.

Execution Sample

MySQL

SELECT SUM(sales) FROM orders;

This query calculates the total sales by adding all values in the sales column of the orders table.

Execution Table

Step	Action	Current Row sales	Running Total	Notes
1	Start query execution	-	0	Initialize sum to zero
2	Read first row	100	100	Add 100 to total
3	Read second row	200	300	Add 200 to total
4	Read third row	150	450	Add 150 to total
5	Read fourth row	50	500	Add 50 to total
6	No more rows	-	500	Return final sum

💡 All rows processed, sum of sales is 500

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
running_total	0	100	300	450	500	500

Key Moments - 2 Insights

Why does the SUM function start with zero instead of the first row's value?

The SUM function initializes the total to zero to ensure it correctly adds all values, including when there are no rows. This is shown in execution_table row 1 where running_total starts at 0.

What happens if the column contains NULL values?

SUM ignores NULL values and does not add them. This means the running total only increases with actual numbers, similar to the values shown in execution_table rows 2-5.

Visual Quiz - 3 Questions

Look at the execution table, what is the running total after reading the third row?

A 150

B 300

C 450

D 500

💡 Hint

Check the 'Running Total' column at step 4 in the execution_table.

At which step does the SUM function return the final total?

A Step 6

B Step 4

C Step 5

D Step 2

💡 Hint

Look for the step where 'No more rows' is noted in the 'Action' column.

If the first row's sales value was NULL, what would be the running total after step 2?

A 100

B 0

C NULL

D Cannot determine

💡 Hint

Recall that SUM ignores NULL values and starts from zero as shown in variable_tracker.

Concept Snapshot

SUM(column_name) adds all numeric values in the column.
Ignores NULLs.
Returns a single total value.
Used in SELECT queries.
Example: SELECT SUM(sales) FROM orders;

Full Transcript

The SUM function in SQL adds up all the numbers in a specified column from all rows in a table. It starts with zero and adds each row's value one by one. If a row has NULL, it skips it. After processing all rows, it returns the total sum. For example, SELECT SUM(sales) FROM orders; adds all sales values in the orders table and returns the total.