BEFORE trigger behavior in PostgreSQL

Introduction

A BEFORE trigger lets you run some checks or changes before data is saved in the database. It helps keep data clean and correct.

You want to check or change data before it is inserted or updated.

You need to stop a wrong or unwanted change before it happens.

You want to automatically fill in missing information before saving.

You want to log or audit changes before they are made.

You want to modify data to a standard format before saving.

Syntax

PostgreSQL

CREATE TRIGGER trigger_name
BEFORE INSERT OR UPDATE ON table_name
FOR EACH ROW
EXECUTE FUNCTION function_name();

BEFORE triggers run before the data change happens.

You can modify the data in the trigger before it is saved.

Examples

This trigger runs before a new user is added to check their age.

PostgreSQL

CREATE TRIGGER check_age
BEFORE INSERT ON users
FOR EACH ROW
EXECUTE FUNCTION check_age_function();

This trigger updates a timestamp before an order is changed.

PostgreSQL

CREATE TRIGGER update_timestamp
BEFORE UPDATE ON orders
FOR EACH ROW
EXECUTE FUNCTION update_timestamp_function();

Sample Program

This example creates a BEFORE INSERT trigger that stops adding users younger than 18.

PostgreSQL

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name TEXT,
  age INT
);

CREATE OR REPLACE FUNCTION check_age_function()
RETURNS TRIGGER AS $$
BEGIN
  IF NEW.age < 18 THEN
    RAISE EXCEPTION 'Age must be 18 or older';
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER check_age
BEFORE INSERT ON users
FOR EACH ROW
EXECUTE FUNCTION check_age_function();

-- Try to insert a user with age 16
INSERT INTO users (name, age) VALUES ('Alice', 16);

Important Notes

BEFORE triggers can change the data by modifying the NEW record.

If a BEFORE trigger raises an error, the data change is stopped.

BEFORE triggers run once for each row affected.

Summary

BEFORE triggers run before data is inserted or updated.

They let you check or change data before saving.

You can stop bad data by raising errors in BEFORE triggers.

Practice

1. What is the main purpose of a BEFORE trigger in PostgreSQL?

easy

A. To run code after data is inserted or updated

B. To delete rows automatically

C. To run custom code before data is inserted or updated

D. To create new tables dynamically

Solution

1. Step 1: Understand trigger timing

   BEFORE triggers execute before the actual data change happens in the table.

2. Step 2: Identify trigger purpose

   They allow checking or modifying data before it is saved, preventing bad data if needed.

3. Final Answer:

   To run custom code before data is inserted or updated -> Option C

4. Quick Check:

   BEFORE trigger = runs before data change [OK]

Hint: BEFORE triggers run before saving data [OK]

Common Mistakes:

• Confusing BEFORE with AFTER triggers
• Thinking triggers create or delete tables
• Assuming triggers run only after data changes

2. Which of the following is the correct syntax to create a BEFORE INSERT trigger on a table named users?

easy

A. CREATE TRIGGER trg BEFORE INSERT ON users CALL func();

B. CREATE TRIGGER trg AFTER INSERT ON users EXECUTE FUNCTION func();

C. CREATE TRIGGER trg BEFORE INSERT INTO users EXECUTE FUNCTION func();

D. CREATE TRIGGER trg BEFORE INSERT ON users EXECUTE FUNCTION func();

Solution

1. Step 1: Check trigger timing and event

   The trigger must be BEFORE INSERT on the table users.

2. Step 2: Verify syntax for calling function

   PostgreSQL uses EXECUTE FUNCTION to call the trigger function.

3. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON users EXECUTE FUNCTION func(); -> Option D

4. Quick Check:

   Correct syntax uses BEFORE INSERT ON and EXECUTE FUNCTION [OK]

Hint: Use BEFORE INSERT ON table EXECUTE FUNCTION func() [OK]

Common Mistakes:

• Using AFTER instead of BEFORE
• Writing INTO instead of ON
• Using CALL instead of EXECUTE FUNCTION

3. Consider this BEFORE INSERT trigger function that changes the new row's status to 'active':

CREATE FUNCTION set_status() RETURNS trigger AS $$
BEGIN
  NEW.status := 'active';
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_set_status BEFORE INSERT ON accounts
FOR EACH ROW EXECUTE FUNCTION set_status();

INSERT INTO accounts (id, status) VALUES (1, 'pending');
SELECT status FROM accounts WHERE id = 1;

What will be the output of the SELECT query?

medium

A. active

B. NULL

C. pending

D. Error: cannot insert

Solution

1. Step 1: Understand BEFORE INSERT trigger effect

   The trigger sets NEW.status to 'active' before the row is inserted.

2. Step 2: Check inserted data

   Even though 'pending' was given, the trigger changes it to 'active' before saving.

3. Final Answer:

   active -> Option A

4. Quick Check:

   BEFORE trigger modifies data before insert [OK]

Hint: BEFORE triggers can modify NEW row data before insert [OK]

Common Mistakes:

• Assuming original value 'pending' is saved
• Thinking trigger runs after insert
• Expecting NULL or error without reason

4. Given this trigger function:

CREATE FUNCTION check_age() RETURNS trigger AS $$
BEGIN
  IF NEW.age < 18 THEN
    RAISE EXCEPTION 'Age must be 18 or older';
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Which problem will occur if you create a BEFORE INSERT trigger using this function and try to insert a row with age = 16?

medium

A. An error will be raised and insertion will stop

B. The trigger will silently ignore the age check

C. The row will be inserted with age 16

D. The age will be automatically set to 18

Solution

1. Step 1: Analyze trigger logic

   If NEW.age is less than 18, the trigger raises an exception.

2. Step 2: Understand effect of RAISE EXCEPTION

   Raising an exception stops the insert and returns an error to the client.

3. Final Answer:

   An error will be raised and insertion will stop -> Option A

4. Quick Check:

   RAISE EXCEPTION stops insert with error [OK]

Hint: RAISE EXCEPTION in BEFORE trigger stops insert with error [OK]

Common Mistakes:

• Thinking the row inserts anyway
• Assuming age auto-corrects
• Ignoring that exceptions stop execution

5. You want to create a BEFORE UPDATE trigger on the products table that prevents the price from being set below zero. Which trigger function code correctly enforces this rule?

hard

A. BEGIN
IF NEW.price < 0 THEN
NEW.price := 0;
END IF;
RETURN NEW;
END;

B. BEGIN
IF NEW.price < 0 THEN
RAISE EXCEPTION 'Price cannot be negative';
END IF;
RETURN NEW;
END;

C. BEGIN
IF OLD.price < 0 THEN
RAISE EXCEPTION 'Price cannot be negative';
END IF;
RETURN NEW;
END;

D. BEGIN
IF NEW.price < 0 THEN
DELETE FROM products WHERE id = NEW.id;
END IF;
RETURN NEW;
END;

Solution

1. Step 1: Identify correct condition check

   The trigger must check NEW.price to prevent negative values before update.

2. Step 2: Choose proper action on invalid data

   Raising an exception stops the update and prevents invalid price.

3. Step 3: Eliminate incorrect options

   BEGIN
     IF NEW.price < 0 THEN
       NEW.price := 0;
     END IF;
     RETURN NEW;
   END;

   silently changes price to 0 (may hide errors), C checks OLD.price (wrong), D deletes row (not appropriate).

4. Final Answer:

   BEGIN IF NEW.price < 0 THEN RAISE EXCEPTION 'Price cannot be negative'; END IF; RETURN NEW; END; -> Option B

5. Quick Check:

   Use RAISE EXCEPTION on NEW.price < 0 to stop update [OK]

Hint: Raise error on NEW.price < 0 to block update [OK]

Common Mistakes:

• Checking OLD.price instead of NEW.price
• Silently fixing invalid data instead of error
• Deleting rows inside BEFORE trigger