UPDATE with expressions in SQL - Step-by-Step Execution

Concept Flow - UPDATE with expressions

Start UPDATE

↓

Identify rows to update

↓

Calculate new values using expressions

↓

Apply new values to rows

↓

Finish UPDATE

The UPDATE command finds rows to change, calculates new values using expressions, then applies these new values to the rows.

Execution Sample

SQL

UPDATE employees
SET salary = salary * 1.1
WHERE department = 'Sales';

This increases salary by 10% for employees in the Sales department.

Execution Table

Step	Row ID	Old salary	Condition (department='Sales')	Expression evaluation	New salary	Action
1	101	50000	True	50000 * 1.1 = 55000	55000	Update salary
2	102	60000	False	Skip	60000	No change
3	103	55000	True	55000 * 1.1 = 60500	60500	Update salary
4	104	70000	False	Skip	70000	No change
5	105	48000	True	48000 * 1.1 = 52800	52800	Update salary
6	-	-	-	-	-	End of rows

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

Variable Tracker

Row ID	salary (start)	salary (after update)
101	50000	55000
102	60000	60000
103	55000	60500
104	70000	70000
105	48000	52800

Key Moments - 2 Insights

Why are some salaries not changed even though the UPDATE runs on the whole table?

Only rows where the condition (department='Sales') is true get updated, as shown in execution_table rows 2 and 4 where condition is false and action is 'No change'.

How is the new salary calculated for each row?

The new salary is computed by multiplying the old salary by 1.1, as shown in the 'Expression evaluation' column in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the new salary for Row ID 103 after the update?

A 55000

B 60500

C 60000

D 6050

💡 Hint

Check the 'New salary' column for Row ID 103 in the execution_table.

At which step does the condition department='Sales' evaluate to false?

A Step 2

B Step 1

C Step 3

D Step 5

💡 Hint

Look at the 'Condition' column in the execution_table for each step.

If the expression changed to salary = salary + 500, what would be the new salary for Row ID 105?

A 48500

B 53000

C 53300

D 52800

💡 Hint

Add 500 to the old salary 48000 from variable_tracker for Row ID 105.

Concept Snapshot

UPDATE table_name
SET column = expression
WHERE condition;

- Finds rows matching condition
- Calculates new values using expressions
- Updates rows with new values

Expressions can use old column values.

Full Transcript

The UPDATE statement changes data in a table. It first finds rows that match a condition. Then it calculates new values using expressions, often involving old column values. Finally, it updates those rows with the new values. For example, increasing salary by 10% for employees in Sales updates only those rows where department is 'Sales'. The execution table shows each row's old salary, whether it meets the condition, the calculation done, and the new salary applied. Rows not matching the condition remain unchanged. This step-by-step process helps understand how UPDATE with expressions works.