Trigger function creation in PostgreSQL - Mini Project: Build & Apply

Create a Trigger Function in PostgreSQL

📖 Scenario: You are managing a simple employee database. You want to keep track of when employee records are updated by automatically recording the update time.

🎯 Goal: Build a trigger function in PostgreSQL that updates a timestamp column whenever an employee record is modified.

📋 What You'll Learn

Create a table named employees with columns id, name, and last_modified

Create a trigger function named update_last_modified that sets last_modified to the current timestamp

Create a trigger named set_last_modified that calls the trigger function before any update on employees

💡 Why This Matters

🌍 Real World

Automatically tracking when records are updated is common in business databases to maintain data accuracy and audit trails.

💼 Career

Database developers and administrators often create triggers to enforce business rules and automate data management tasks.

1

Create the employees table

Create a table called employees with columns: id as serial primary key, name as varchar(100), and last_modified as timestamp without time zone.

PostgreSQL

-- Create the employees table with id, name, and last_modified columns
-- Your code here

Hint

Use CREATE TABLE with the specified columns and types.

2

Create the trigger function update_last_modified

Create a trigger function named update_last_modified that returns trigger and sets NEW.last_modified to NOW() before returning NEW.

PostgreSQL

CREATE TABLE employees (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    last_modified TIMESTAMP WITHOUT TIME ZONE
);

-- Create the trigger function update_last_modified
-- Your code here

Hint

Use CREATE OR REPLACE FUNCTION with RETURNS TRIGGER and set NEW.last_modified = NOW().

3

Create the trigger set_last_modified

Create a trigger named set_last_modified on the employees table that fires BEFORE UPDATE and calls the trigger function update_last_modified.

PostgreSQL

CREATE TABLE employees (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    last_modified TIMESTAMP WITHOUT TIME ZONE
);

CREATE OR REPLACE FUNCTION update_last_modified()
RETURNS TRIGGER AS $$
BEGIN
    NEW.last_modified = NOW();
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Create the trigger set_last_modified on employees BEFORE UPDATE
-- Your code here

Hint

Use CREATE TRIGGER with BEFORE UPDATE ON employees and EXECUTE FUNCTION update_last_modified().

4

Verify the trigger function setup

Add a comment at the end of your code confirming the trigger set_last_modified is ready to update last_modified on employee updates.

PostgreSQL

CREATE TABLE employees (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    last_modified TIMESTAMP WITHOUT TIME ZONE
);

CREATE OR REPLACE FUNCTION update_last_modified()
RETURNS TRIGGER AS $$
BEGIN
    NEW.last_modified = NOW();
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER set_last_modified
BEFORE UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION update_last_modified();

-- Add a comment confirming the trigger is set up
-- Your code here

Hint

Add a simple comment line confirming the trigger setup.

Practice

1. What is the correct return type for a trigger function in PostgreSQL?

easy

A. void

B. trigger

C. integer

D. boolean

Solution

1. Step 1: Understand trigger function requirements

   Trigger functions must return a special type that PostgreSQL recognizes for triggers.

2. Step 2: Identify the correct return type

   The return type for trigger functions is always trigger, not standard types like void or integer.

3. Final Answer:

   trigger -> Option B

4. Quick Check:

   Trigger functions return type = trigger [OK]

Hint: Trigger functions always return type 'trigger' [OK]

Common Mistakes:

• Using void or integer as return type
• Forgetting to specify return type
• Confusing trigger function with normal function

2. Which of the following is the correct way to start a trigger function in PostgreSQL?

easy

A. CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION my_trigger() RETURNS boolean AS $$ BEGIN RETURN TRUE; END; $$ LANGUAGE plpgsql;

C. CREATE TRIGGER my_trigger() RETURNS trigger AS $$ BEGIN RETURN OLD; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION my_trigger() RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Check syntax for trigger function creation

   Trigger functions must be created with CREATE FUNCTION, return type trigger, and use plpgsql language.

2. Step 2: Identify correct function body and return statement

   Trigger functions must return NEW or OLD row, so RETURN NEW; is correct here.

3. Final Answer:

   CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   Trigger function syntax = CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql; [OK]

Hint: Trigger functions use RETURNS trigger and RETURN NEW or OLD [OK]

Common Mistakes:

• Using RETURNS void instead of trigger
• Using CREATE TRIGGER instead of CREATE FUNCTION
• Returning boolean or void instead of NEW or OLD

3. Given this trigger function, what will be the result when a new row is inserted?

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;

medium

A. The row is inserted regardless of age

B. The function returns NULL causing an error

C. The row is always rejected

D. The row is inserted only if age is 18 or older

Solution

1. Step 1: Analyze the condition in the trigger function

   The function checks if NEW.age < 18 and raises an exception if true.

2. Step 2: Understand effect of exception and return

   If age is less than 18, insertion stops with error. Otherwise, RETURN NEW; allows insertion.

3. Final Answer:

   The row is inserted only if age is 18 or older -> Option D

4. Quick Check:

   Exception blocks underage inserts = The row is inserted only if age is 18 or older [OK]

Hint: Exception stops insert; RETURN NEW allows it [OK]

Common Mistakes:

• Thinking all rows insert regardless
• Assuming RETURN NEW inserts row without checks
• Confusing RAISE EXCEPTION with warnings

4. Identify the error in this trigger function code:

CREATE FUNCTION update_timestamp() RETURNS trigger AS $$
BEGIN
  NEW.updated_at = NOW();
  RETURN NEW
END;
$$ LANGUAGE plpgsql;

medium

A. Cannot assign to NEW in trigger functions

B. Incorrect return type, should be void

C. Missing semicolon after RETURN NEW

D. Missing BEGIN keyword

Solution

1. Step 1: Check syntax for statements inside function

   Each statement must end with a semicolon in PL/pgSQL.

2. Step 2: Identify missing semicolon

   The line RETURN NEW lacks a semicolon at the end, causing syntax error.

3. Final Answer:

   Missing semicolon after RETURN NEW -> Option C

4. Quick Check:

   PL/pgSQL statements end with semicolon [OK]

Hint: Check semicolons after each statement [OK]

Common Mistakes:

• Forgetting semicolon after RETURN NEW
• Thinking NEW cannot be assigned
• Confusing return type with void

5. You want to create a trigger function that automatically sets a column last_modified to the current timestamp whenever a row is updated. Which of the following trigger function definitions correctly achieves this?

hard

A. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN OLD.last_modified := NOW(); RETURN OLD; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION set_last_modified() RETURNS void AS $$ BEGIN NEW.last_modified := NOW(); END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified = NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Identify correct assignment operator in PL/pgSQL

   PL/pgSQL uses := for assignment, not =.

2. Step 2: Check return type and returned row

   Trigger functions must return type trigger and return NEW for BEFORE UPDATE triggers.

3. Step 3: Evaluate each option

   CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; uses correct assignment :=, returns NEW, and has correct return type. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified = NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; uses = which is invalid for assignment. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN OLD.last_modified := NOW(); RETURN OLD; END; $$ LANGUAGE plpgsql; assigns to OLD which is read-only. CREATE FUNCTION set_last_modified() RETURNS void AS $$ BEGIN NEW.last_modified := NOW(); END; $$ LANGUAGE plpgsql; returns void which is invalid.

4. Final Answer:

   CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option A

5. Quick Check:

   Use := for assignment and return NEW [OK]

Hint: Use := for assignment and return NEW in trigger [OK]

Common Mistakes:

• Using = instead of := for assignment
• Returning OLD instead of NEW
• Using void return type for trigger function