AFTER UPDATE triggers in MySQL - Step-by-Step Execution

Concept Flow - AFTER UPDATE triggers

UPDATE statement executed

↓

Row(s) updated in table

↓

AFTER UPDATE trigger fires

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Trigger code runs using updated data

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Trigger completes, control returns to UPDATE

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UPDATE statement finishes

When an UPDATE changes rows, the AFTER UPDATE trigger runs immediately after the change, using the new data.

Execution Sample

MySQL

CREATE TRIGGER trg_after_update
AFTER UPDATE ON employees
FOR EACH ROW
BEGIN
  INSERT INTO audit_log(emp_id, action) VALUES (NEW.id, 'updated');
END;

This trigger logs an entry into audit_log every time an employee row is updated.

Execution Table

Step	Action	Row Data Before	Row Data After	Trigger Fired?	Trigger Action
1	UPDATE employees SET salary = 60000 WHERE id = 1	{id:1, salary:50000}	{id:1, salary:60000}	No	No action yet
2	Row updated in employees table	{id:1, salary:50000}	{id:1, salary:60000}	Yes	Trigger inserts audit_log(emp_id=1, action='updated')
3	Trigger completes	{id:1, salary:60000}	{id:1, salary:60000}	Yes	Audit log entry created
4	UPDATE statement finishes	{id:1, salary:60000}	{id:1, salary:60000}	Yes	All done

💡 UPDATE completes after trigger runs for each updated row

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
salary	50000	60000	60000	60000
trigger_fired	No	No	Yes	Yes
audit_log_entries	0	0	1	1

Key Moments - 2 Insights

Why does the trigger see the updated data, not the old data?

Because AFTER UPDATE triggers run after the row is changed, so NEW contains the new values as shown in execution_table step 2.

Does the trigger run once per statement or once per row?

It runs once for each row updated, as indicated by 'FOR EACH ROW' and the execution_table showing trigger firing after the row update.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the salary value in the updated row?

A 60000

B 50000

C NULL

D 55000

💡 Hint

Check the 'Row Data After' column in step 2 of execution_table.

At which step does the trigger insert a new audit_log entry?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Trigger Action' column in execution_table rows.

If the UPDATE affects multiple rows, how many times does the AFTER UPDATE trigger run?

A Once per statement

B Never

C Once per row updated

D Only if rows have changed

💡 Hint

Refer to key_moments about 'FOR EACH ROW' and trigger firing.

Concept Snapshot

AFTER UPDATE triggers run after a row is updated.
They see the new data in NEW.
Trigger runs once per updated row.
Use FOR EACH ROW to specify per-row execution.
Commonly used for logging or cascading changes.

Full Transcript

When you run an UPDATE statement, the database changes the row data first. Then, if there is an AFTER UPDATE trigger, it runs immediately after each row is updated. The trigger can access the new row values using NEW. This trigger runs once for every row that the UPDATE changes. For example, if you update one employee's salary, the trigger logs this change in an audit table. The execution table shows the update happening, then the trigger firing, and finally the update finishing. This helps keep track of changes automatically.