UPDATE with WHERE clause in MySQL - Step-by-Step Execution

Concept Flow - UPDATE with WHERE clause

Start UPDATE statement

↓

Check WHERE condition for each row

↓

Update row

↓

End UPDATE

The UPDATE command checks each row against the WHERE condition. If true, it updates that row; if false, it leaves it unchanged.

Execution Sample

MySQL

UPDATE employees
SET salary = salary + 500
WHERE department = 'Sales';

This query increases salary by 500 only for employees in the Sales department.

Execution Table

Step	Row ID	Current salary	Department	WHERE condition (department='Sales')	Action	New salary
1	1	3000	Sales	True	Update salary	3500
2	2	3200	HR	False	Skip row	3200
3	3	2800	Sales	True	Update salary	3300
4	4	4000	IT	False	Skip row	4000
5	5	3100	Sales	True	Update salary	3600
6	-	-	-	-	End of table	-

💡 All rows checked; only rows with department 'Sales' updated.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	After Row 4	After Row 5	Final
salary_row_1	3000	3500	3500	3500	3500	3500	3500
salary_row_2	3200	3200	3200	3200	3200	3200	3200
salary_row_3	2800	2800	2800	3300	3300	3300	3300
salary_row_4	4000	4000	4000	4000	4000	4000	4000
salary_row_5	3100	3100	3100	3100	3100	3600	3600

Key Moments - 3 Insights

Why are some rows not updated even though the UPDATE command runs?

Rows not matching the WHERE condition are skipped, as shown in execution_table rows 2 and 4 where the department is not 'Sales'.

Does the UPDATE affect all rows if WHERE is omitted?

No WHERE means all rows match condition by default, so all rows would be updated. Here, only rows with 'Sales' are updated.

What happens if the WHERE condition is always false?

No rows get updated. The execution_table would show all rows skipped, similar to rows 2 and 4 here.

Visual Quiz - 3 Questions

Look at the execution_table, what is the new salary of row 3 after the update?

A 3300

B 2800

C 3500

D 3100

💡 Hint

Check the 'New salary' column for Row ID 3 in the execution_table.

At which step does the WHERE condition evaluate to false for the first time?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'WHERE condition' column in execution_table rows.

If the WHERE clause was removed, how would the action column change in the execution_table?

A All rows would show 'Skip row'.

B All rows would show 'Update salary'.

C Only rows with salary > 3000 would update.

D No rows would be updated.

💡 Hint

Without WHERE, all rows match condition by default, so all get updated.

Concept Snapshot

UPDATE table_name
SET column = value
WHERE condition;

- Only rows matching WHERE condition are updated.
- Rows not matching remain unchanged.
- Omitting WHERE updates all rows.

Full Transcript

The UPDATE statement changes data in a table. It checks each row against the WHERE condition. If the condition is true, it updates that row's columns as specified. If false, it leaves the row unchanged. For example, increasing salary by 500 only for employees in the Sales department updates only those rows. Rows in other departments are skipped. This selective update prevents unwanted changes. If WHERE is omitted, all rows get updated. This trace shows step-by-step how each row is checked and updated or skipped.