BEFORE trigger execution in SQL - Mini Project: Build & Apply

Create a BEFORE Trigger to Validate Data

📖 Scenario: You are managing a small online store database. You want to make sure that no product can be added with a negative price. To do this, you will create a BEFORE trigger that checks the price before inserting a new product.

🎯 Goal: Build a BEFORE trigger on the products table that prevents inserting a product with a negative price by raising an error.

📋 What You'll Learn

Create a products table with columns id (integer), name (text), and price (numeric).

Create a BEFORE INSERT trigger named check_price_before_insert on the products table.

The trigger should check if the new price is less than 0.

If the price is negative, the trigger should raise an error to prevent the insert.

💡 Why This Matters

🌍 Real World

Triggers help enforce business rules automatically in databases, such as preventing invalid data entry.

💼 Career

Database developers and administrators use triggers to maintain data integrity and automate checks.

1

Create the products table

Create a table called products with columns id as integer, name as text, and price as numeric.

SQL

-- Create the products table with id, name, and price columns
-- Your code here

Hint

Use CREATE TABLE statement with the specified columns and types.

2

Create the BEFORE INSERT trigger function

Create a trigger function called check_price_before_insert_func that checks if NEW.price is less than 0 and raises an exception with the message 'Price cannot be negative' if true.

SQL

CREATE TABLE products (
    id INTEGER,
    name TEXT,
    price NUMERIC
);

-- Create the trigger function to check price before insert
-- Your code here

Hint

Use CREATE FUNCTION with RETURNS trigger and write the logic inside BEGIN ... END.

3

Create the BEFORE INSERT trigger

Create a BEFORE INSERT trigger named check_price_before_insert on the products table that calls the function check_price_before_insert_func.

SQL

CREATE TABLE products (
    id INTEGER,
    name TEXT,
    price NUMERIC
);

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

-- Create the BEFORE INSERT trigger on products
-- Your code here

Hint

Use CREATE TRIGGER with BEFORE INSERT ON products and EXECUTE FUNCTION.

4

Test the trigger by inserting data

Insert a product with id 1, name 'Pen', and price 2.5. Then try to insert a product with id 2, name 'Notebook', and price -5 to see the trigger prevent the insert.

SQL

CREATE TABLE products (
    id INTEGER,
    name TEXT,
    price NUMERIC
);

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

CREATE TRIGGER check_price_before_insert
BEFORE INSERT ON products
FOR EACH ROW
EXECUTE FUNCTION check_price_before_insert_func();

-- Insert a valid product and an invalid product
-- Your code here

Hint

Use INSERT INTO products with the exact values given.

Practice

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

easy

A. To execute code after data is inserted or updated

B. To delete data automatically

C. To execute code before a data modification happens

D. To create a backup of the database

Solution

1. Step 1: Understand trigger timing

   A BEFORE trigger runs before the data change happens, allowing checks or modifications.

2. Step 2: Compare options

   To execute code before a data modification happens correctly states the trigger runs before data modification. Options A, C, and D describe other actions not related to BEFORE triggers.

3. Final Answer:

   To execute code before a data modification happens -> Option C

4. Quick Check:

   BEFORE trigger = runs before data change [OK]

Hint: BEFORE triggers run before data changes happen [OK]

Common Mistakes:

• Confusing BEFORE with AFTER triggers
• Thinking triggers delete data automatically
• Assuming triggers create backups

2. Which of the following is the correct syntax to create a BEFORE INSERT trigger in SQL?

easy

A. CREATE TRIGGER trg BEFORE DELETE ON table_name FOR EACH ROW BEGIN ... END;

B. CREATE TRIGGER trg AFTER INSERT ON table_name BEGIN ... END;

C. CREATE TRIGGER trg BEFORE UPDATE ON table_name FOR EACH ROW BEGIN ... END;

D. CREATE TRIGGER trg BEFORE INSERT ON table_name FOR EACH ROW BEGIN ... END;

Solution

1. Step 1: Identify correct trigger timing and event

   The question asks for a BEFORE INSERT trigger, so timing is BEFORE and event is INSERT.

2. Step 2: Match syntax

   CREATE TRIGGER trg BEFORE INSERT ON table_name FOR EACH ROW BEGIN ... END; matches CREATE TRIGGER trg BEFORE INSERT ON table_name FOR EACH ROW BEGIN ... END;. Other options have wrong timing or event.

3. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON table_name FOR EACH ROW BEGIN ... END; -> Option D

4. Quick Check:

   BEFORE INSERT syntax = CREATE TRIGGER trg BEFORE INSERT ON table_name FOR EACH ROW BEGIN ... END; [OK]

Hint: BEFORE triggers use 'BEFORE event ON table' syntax [OK]

Common Mistakes:

• Using AFTER instead of BEFORE
• Mixing INSERT with UPDATE or DELETE
• Omitting FOR EACH ROW clause

3. Given this trigger code:

CREATE TRIGGER check_age BEFORE INSERT ON users FOR EACH ROW BEGIN IF NEW.age < 18 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Age must be 18 or older'; END IF; END;

What happens if you try to insert a user with age 16?

medium

A. The insert fails with an error message 'Age must be 18 or older'

B. The age is automatically set to 18

C. The insert is ignored silently

D. The insert succeeds and user is added

Solution

1. Step 1: Understand trigger logic

   The trigger checks if NEW.age is less than 18 and raises an error if true.

2. Step 2: Apply to age 16

   Since 16 < 18, the trigger raises an error with message 'Age must be 18 or older', stopping the insert.

3. Final Answer:

   The insert fails with an error message 'Age must be 18 or older' -> Option A

4. Quick Check:

   BEFORE trigger can stop invalid inserts [OK]

Hint: BEFORE triggers can block invalid data with SIGNAL [OK]

Common Mistakes:

• Thinking insert succeeds despite error
• Assuming silent ignore instead of error
• Believing trigger changes data automatically

4. Consider this trigger code:

CREATE TRIGGER trg BEFORE UPDATE ON products FOR EACH ROW BEGIN SET NEW.price = NEW.price * 0.9; END;

When trying to create this trigger, you get an error. What is the likely cause?

medium

A. The syntax is missing a delimiter or semicolon inside the body

B. The trigger body must be enclosed in BEGIN and END

C. The trigger must specify FOR EACH ROW for row-level triggers

D. You cannot modify NEW values in a BEFORE UPDATE trigger

Solution

1. Step 1: Check trigger syntax

   The trigger body uses multiple statements and must handle delimiters properly.

2. Step 2: Identify missing delimiter

   Without changing the delimiter, the semicolon inside the body ends the statement early, causing syntax error.

3. Final Answer:

   The syntax is missing a delimiter or semicolon inside the body -> Option A

4. Quick Check:

   Use DELIMITER to define trigger body [OK]

Hint: Use DELIMITER to avoid semicolon conflicts in triggers [OK]

Common Mistakes:

• Thinking NEW values can't be changed in BEFORE triggers
• Ignoring delimiter changes for multi-statement bodies
• Missing FOR EACH ROW clause

5. You want to ensure that before inserting a new order, the quantity is never negative. Which trigger code correctly enforces this using a BEFORE INSERT trigger?

hard

A. CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SET NEW.quantity = 0; END IF; END;

B. CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END;

C. CREATE TRIGGER trg AFTER INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END;

D. CREATE TRIGGER trg BEFORE UPDATE ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END;

Solution

1. Step 1: Identify correct timing and event

   We want to check before inserting, so BEFORE INSERT is correct.

2. Step 2: Check enforcement method

   To stop negative quantity, raising an error with SIGNAL is best. CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END; uses BEFORE INSERT and SIGNAL correctly.

3. Step 3: Eliminate wrong options

   CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SET NEW.quantity = 0; END IF; END; silently changes quantity, not enforcing. CREATE TRIGGER trg AFTER INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END; uses AFTER INSERT, too late to stop insert. CREATE TRIGGER trg BEFORE UPDATE ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END; uses BEFORE UPDATE, not insert.

4. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Negative quantity not allowed'; END IF; END; -> Option B

5. Quick Check:

   Use BEFORE INSERT + SIGNAL to block invalid data [OK]

Hint: Use BEFORE INSERT with SIGNAL to block bad data [OK]

Common Mistakes:

• Using AFTER INSERT instead of BEFORE INSERT
• Silently fixing data instead of raising error
• Checking on UPDATE instead of INSERT