BEFORE trigger execution in SQL - Step-by-Step Execution

Concept Flow - BEFORE trigger execution

Start: Insert/Update/Delete operation

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BEFORE Trigger fires

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Trigger code runs: can modify NEW row

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Original operation continues with possibly modified data

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Operation completes, AFTER triggers may run

When a data change is requested, the BEFORE trigger runs first, allowing changes before the actual data operation happens.

Execution Sample

SQL

CREATE TRIGGER before_insert_example
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
  SET NEW.salary = NEW.salary * 1.1;
END;

This trigger increases the salary by 10% before inserting a new employee row.

Execution Table

Step	Action	NEW.salary before	Trigger modifies NEW.salary	NEW.salary after	Operation continues
1	Insert employee with salary 1000 requested	N/A	No	N/A	Waiting for trigger
2	BEFORE INSERT trigger fires	1000	Yes, multiply by 1.1	1100	Trigger ends
3	Insert operation continues with NEW.salary=1100	1100	No	1100	Row inserted with updated salary

💡 Trigger finishes, operation continues with modified NEW data

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
NEW.salary	N/A	1000	1100	1100

Key Moments - 2 Insights

Why can the NEW.salary value be changed inside the BEFORE trigger?

Because the BEFORE trigger runs before the actual insert, it can modify the NEW row values that will be saved, as shown in execution_table step 2.

Does the original insert operation use the old or modified NEW.salary?

It uses the modified NEW.salary after the trigger runs, as shown in execution_table step 3 where the operation continues with salary 1100.

Visual Quiz - 3 Questions

Look at the execution_table, what is NEW.salary after the BEFORE trigger modifies it?

A 1000

B 1100

C 900

D 1200

💡 Hint

Check the 'NEW.salary after' column in step 2 of the execution_table.

At which step does the actual insert operation use the modified salary?

A Step 1

B Step 2

C Step 3

D After all steps

💡 Hint

Look at the 'Operation continues' column in step 3 of the execution_table.

If the BEFORE trigger did not modify NEW.salary, what would the final salary be?

A 1000

B 1100

C 0

D NULL

💡 Hint

Refer to the 'NEW.salary before' value in step 2 and imagine no modification.

Concept Snapshot

BEFORE triggers run before data changes happen.
They can modify NEW row data.
Changes affect the actual operation.
Useful for validation or automatic adjustments.
Runs once per affected row before insert/update/delete.

Full Transcript

When you insert, update, or delete data, a BEFORE trigger runs first. It can change the data before it is saved. For example, if you insert a salary of 1000, the BEFORE trigger can increase it to 1100. Then the insert uses this new value. This lets you adjust or check data automatically before saving. The execution table shows the steps: request insert, trigger runs and changes salary, then insert continues with new salary.