Trigger for data validation in PostgreSQL - Mini Project: Build & Apply

Trigger for Data Validation in PostgreSQL

📖 Scenario: You are managing a customer database for an online store. To keep data clean, you want to make sure that no customer can be added with an invalid email address.

🎯 Goal: Create a PostgreSQL trigger that validates the email format before inserting a new customer record. If the email is invalid, the insertion should be stopped.

📋 What You'll Learn

Create a table called customers with columns id (integer, primary key), name (text), and email (text).

Create a trigger function called validate_email that checks if the email contains an '@' character.

Create a trigger called email_check_trigger that calls validate_email before inserting a new row into customers.

If the email is invalid, the trigger function should raise an exception to prevent the insert.

💡 Why This Matters

🌍 Real World

Triggers help keep data clean automatically by validating data before it enters the database, reducing errors and bad data.

💼 Career

Database developers and administrators use triggers to enforce business rules and data integrity without relying on application code.

1

Create the customers table

Create a table called customers with columns: id as an integer primary key, name as text, and email as text.

PostgreSQL

-- Your code here to create the customers table

Hint

Use SERIAL PRIMARY KEY for the id column to auto-increment.

2

Create the trigger function validate_email

Create a trigger function called validate_email in PL/pgSQL that checks if NEW.email contains the '@' character. If not, raise an exception with the message 'Invalid email address'.

PostgreSQL

CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name TEXT,
    email TEXT
);

-- Your code here to create the validate_email trigger function

Hint

Use POSITION to check if '@' is in the email string.

3

Create the trigger email_check_trigger

Create a trigger called email_check_trigger that calls the validate_email function before inserting a new row into the customers table.

PostgreSQL

CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name TEXT,
    email TEXT
);

CREATE OR REPLACE FUNCTION validate_email() RETURNS TRIGGER AS $$
BEGIN
    IF POSITION('@' IN NEW.email) = 0 THEN
        RAISE EXCEPTION 'Invalid email address';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Your code here to create the email_check_trigger trigger

Hint

Use BEFORE INSERT ON customers FOR EACH ROW EXECUTE FUNCTION validate_email() to create the trigger.

4

Test the trigger by inserting valid and invalid emails

Insert a valid customer with email '[e-mail]' and an invalid customer with email 'bobexample.com' to test the trigger. Use the exact insert statements shown.

PostgreSQL

CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name TEXT,
    email TEXT
);

CREATE OR REPLACE FUNCTION validate_email() RETURNS TRIGGER AS $$
BEGIN
    IF POSITION('@' IN NEW.email) = 0 THEN
        RAISE EXCEPTION 'Invalid email address';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER email_check_trigger
BEFORE INSERT ON customers
FOR EACH ROW
EXECUTE FUNCTION validate_email();

-- Your code here to insert test customers

Hint

Use two INSERT INTO customers (name, email) VALUES (...) statements with the exact emails given.

Practice

1. What is the main purpose of a trigger in PostgreSQL for data validation?

easy

A. To create new tables based on existing ones

B. To speed up query execution by indexing data

C. To automatically check and enforce rules on data before it is saved

D. To backup the database automatically

Solution

1. Step 1: Understand trigger role

   Triggers run automatically when data changes, allowing checks on data.

2. Step 2: Identify validation purpose

   Data validation means checking data correctness before saving it.

3. Final Answer:

   To automatically check and enforce rules on data before it is saved -> Option C

4. Quick Check:

   Trigger = automatic data check [OK]

Hint: Triggers run automatically to check data before saving [OK]

Common Mistakes:

• Thinking triggers speed up queries
• Confusing triggers with backups
• Assuming triggers create tables

2. Which of the following is the correct way to declare a BEFORE INSERT trigger in PostgreSQL?

easy

A. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name;

B. CREATE TRIGGER trg AFTER INSERT ON table_name EXECUTE PROCEDURE func_name();

C. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE PROCEDURE func_name();

D. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name();

Solution

1. Step 1: Recall PostgreSQL trigger syntax

   PostgreSQL uses EXECUTE FUNCTION for triggers since version 11.

2. Step 2: Identify correct timing and syntax

   BEFORE INSERT triggers run before inserting data; syntax must match.

3. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name(); -> Option D

4. Quick Check:

   BEFORE INSERT + EXECUTE FUNCTION = CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name(); [OK]

Hint: Use EXECUTE FUNCTION for triggers in PostgreSQL 11+ [OK]

Common Mistakes:

• Using EXECUTE PROCEDURE instead of EXECUTE FUNCTION
• Confusing BEFORE and AFTER timing
• Missing parentheses after function name

3. Given this trigger function to prevent negative prices:

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

What happens if you try to insert a row with price = -5?

medium

A. An error is raised and the insert is stopped

B. The row is inserted with price -5

C. The price is automatically set to 0

D. The trigger is ignored and insert proceeds

Solution

1. Step 1: Analyze trigger function logic

   The function checks if NEW.price is less than 0 and raises an exception if true.

2. Step 2: Understand RAISE EXCEPTION effect

   RAISE EXCEPTION stops the operation and returns an error to the user.

3. Final Answer:

   An error is raised and the insert is stopped -> Option A

4. Quick Check:

   Negative price triggers error [OK]

Hint: RAISE EXCEPTION stops insert on invalid data [OK]

Common Mistakes:

• Assuming data is inserted anyway
• Thinking price auto-corrects
• Ignoring trigger effects

4. You wrote this trigger function:

CREATE FUNCTION validate_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;

But when inserting age = 15, no error occurs. What is the likely mistake?

medium

A. RAISE EXCEPTION syntax is incorrect

B. The trigger is not attached to the table

C. The function does not return NEW

D. The trigger function is missing LANGUAGE plpgsql

Solution

1. Step 1: Check function correctness

   The function correctly raises exception and returns NEW, syntax is fine.

2. Step 2: Consider trigger attachment

   If no error occurs, likely the trigger is not linked to the table to run the function.

3. Final Answer:

   The trigger is not attached to the table -> Option B

4. Quick Check:

   Trigger must be attached to run function [OK]

Hint: Attach trigger to table to activate validation [OK]

Common Mistakes:

• Forgetting to create the trigger after function
• Assuming function runs without trigger
• Misreading RAISE EXCEPTION syntax

5. You want to ensure that a user's email is unique and not empty using a trigger. Which approach correctly combines data validation and uniqueness check in PostgreSQL?

hard

A. Create a BEFORE INSERT OR UPDATE trigger that raises exception if NEW.email is empty or exists in the table

B. Use a UNIQUE constraint on email column only, no trigger needed

C. Create an AFTER INSERT trigger that deletes duplicates after insertion

D. Use a trigger that sets empty emails to a default value

Solution

1. Step 1: Understand validation needs

   Email must be non-empty and unique before saving data.

2. Step 2: Choose trigger timing and logic

   BEFORE INSERT OR UPDATE trigger can check NEW.email and query table for duplicates, raising exception if invalid.

3. Final Answer:

   Create a BEFORE INSERT OR UPDATE trigger that raises exception if NEW.email is empty or exists in the table -> Option A

4. Quick Check:

   Validate and check uniqueness before insert/update [OK]

Hint: Use BEFORE trigger to check and stop invalid data [OK]

Common Mistakes:

• Relying only on UNIQUE constraint without empty check
• Using AFTER trigger to fix duplicates (too late)
• Setting default instead of raising error