Trigger function creation in PostgreSQL

Introduction

A trigger function runs automatically when certain changes happen in a database table. It helps keep data correct or do extra work without you typing commands every time.

You want to check or change data right after someone adds or updates a row.

You need to keep track of changes by saving history automatically.

You want to prevent wrong data from being saved.

You want to update related tables when data changes.

You want to send notifications or logs when data changes.

Syntax

PostgreSQL

CREATE FUNCTION function_name()
RETURNS trigger AS $$
BEGIN
    -- your code here
    RETURN NEW; -- or RETURN OLD;
END;
$$ LANGUAGE plpgsql;

The function must return type 'trigger'.

Use RETURN NEW to keep the new row or RETURN OLD to keep the old row in UPDATE/DELETE triggers.

Examples

This function prints a message when a row is updated.

PostgreSQL

CREATE FUNCTION log_update()
RETURNS trigger AS $$
BEGIN
    RAISE NOTICE 'Row updated';
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

This function stops saving rows with negative amounts.

PostgreSQL

CREATE FUNCTION prevent_negative()
RETURNS trigger AS $$
BEGIN
    IF NEW.amount < 0 THEN
        RAISE EXCEPTION 'Negative amount not allowed';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Sample Program

This example creates a table and a trigger function that stops negative balances. The last insert will cause an error.

PostgreSQL

CREATE TABLE accounts (
    id SERIAL PRIMARY KEY,
    balance INT
);

CREATE FUNCTION check_balance()
RETURNS trigger AS $$
BEGIN
    IF NEW.balance < 0 THEN
        RAISE EXCEPTION 'Balance cannot be negative';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER balance_check
BEFORE INSERT OR UPDATE ON accounts
FOR EACH ROW EXECUTE FUNCTION check_balance();

-- Try to insert a negative balance (this will fail)
INSERT INTO accounts (balance) VALUES (-100);

Important Notes

Trigger functions must be written in a procedural language like plpgsql.

You can use NEW to access the new row and OLD for the old row inside the function.

Remember to attach the trigger function to a table with CREATE TRIGGER.

Summary

Trigger functions run automatically on table changes.

They must return type 'trigger' and use RETURN NEW or RETURN OLD.

Use triggers to enforce rules or automate tasks in the database.

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