INSERT trigger in SQL - Mini Project: Build & Apply

Create an INSERT Trigger to Log New Orders

📖 Scenario: You manage an online store database. You want to keep track of every new order placed by customers by automatically recording details into a separate log table.

🎯 Goal: Build an INSERT trigger on the orders table that automatically inserts a record into the order_log table whenever a new order is added.

📋 What You'll Learn

Create a table called orders with columns order_id (integer), customer_name (varchar), and order_total (decimal).

Create a table called order_log with columns log_id (integer, auto-increment), order_id (integer), and log_message (varchar).

Create an INSERT trigger named log_new_order on the orders table.

The trigger should insert a log entry into order_log with the order_id and a message like 'New order added'.

💡 Why This Matters

🌍 Real World

Automatically logging changes in a database helps keep track of important events without manual effort.

💼 Career

Database triggers are commonly used in real jobs to enforce business rules and audit data changes.

1

Create the orders table

Create a table called orders with columns order_id as INTEGER, customer_name as VARCHAR(100), and order_total as DECIMAL(10,2).

SQL

-- Create the orders table
-- Your code here

Hint

Use CREATE TABLE orders and define the columns with their types exactly as specified.

2

Create the order_log table

Create a table called order_log with columns log_id as INTEGER AUTO_INCREMENT PRIMARY KEY, order_id as INTEGER, and log_message as VARCHAR(255).

SQL

CREATE TABLE orders (
    order_id INTEGER,
    customer_name VARCHAR(100),
    order_total DECIMAL(10,2)
);
-- Create the order_log table
-- Your code here

Hint

Remember to set log_id as the primary key and auto-increment it.

3

Create the INSERT trigger log_new_order

Create an INSERT trigger named log_new_order on the orders table that runs AFTER insert.

SQL

CREATE TABLE orders (
    order_id INTEGER,
    customer_name VARCHAR(100),
    order_total DECIMAL(10,2)
);

CREATE TABLE order_log (
    log_id INTEGER AUTO_INCREMENT PRIMARY KEY,
    order_id INTEGER,
    log_message VARCHAR(255)
);
-- Create the INSERT trigger log_new_order
-- Your code here

Hint

Use NEW.order_id to get the inserted order's ID inside the trigger.

4

Complete the trigger with proper delimiter handling

Add the delimiter commands before and after the trigger creation to allow the trigger body to be parsed correctly.

SQL

CREATE TABLE orders (
    order_id INTEGER,
    customer_name VARCHAR(100),
    order_total DECIMAL(10,2)
);

CREATE TABLE order_log (
    log_id INTEGER AUTO_INCREMENT PRIMARY KEY,
    order_id INTEGER,
    log_message VARCHAR(255)
);

-- Add delimiter commands and create the trigger
-- Your code here

Hint

Use DELIMITER $$ before and DELIMITER ; after the trigger to handle the semicolons inside the trigger body.

Practice

1. What is the main purpose of an INSERT trigger in a database?

easy

A. To update existing rows in a table

B. To automatically run code when a new row is added to a table

C. To delete rows from a table

D. To create a new table

Solution

1. Step 1: Understand what triggers do

   Triggers are special procedures that run automatically in response to certain events in the database.

2. Step 2: Focus on INSERT triggers

   An INSERT trigger specifically runs when a new row is added to a table.

3. Final Answer:

   To automatically run code when a new row is added to a table -> Option B

4. Quick Check:

   INSERT trigger = runs on new row insert [OK]

Hint: INSERT triggers run on new rows added [OK]

Common Mistakes:

• Confusing INSERT triggers with UPDATE or DELETE triggers
• Thinking triggers create or drop tables
• Believing triggers run manually

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

easy

A. CREATE TRIGGER trg AFTER INSERT ON table_name FOR EACH ROW BEGIN ... END;

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

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

D. CREATE TRIGGER trg ON table_name INSERT BEGIN ... END;

Solution

1. Step 1: Identify correct event and timing

   For an INSERT trigger, the event is INSERT and timing can be BEFORE or AFTER. CREATE TRIGGER trg AFTER INSERT ON table_name FOR EACH ROW BEGIN ... END; uses AFTER INSERT which is valid.

2. Step 2: Check syntax structure

   CREATE TRIGGER trg AFTER INSERT ON table_name FOR EACH ROW BEGIN ... END; uses correct syntax: CREATE TRIGGER name AFTER INSERT ON table FOR EACH ROW BEGIN ... END;

3. Final Answer:

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

4. Quick Check:

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

Hint: INSERT triggers use AFTER or BEFORE INSERT ON table [OK]

Common Mistakes:

• Using DELETE or UPDATE instead of INSERT
• Wrong order of keywords
• Missing FOR EACH ROW clause

3. Given this trigger code:

CREATE TRIGGER trg_log AFTER INSERT ON employees FOR EACH ROW BEGIN INSERT INTO log_table VALUES (NEW.id, NEW.name); END;

What happens when a new employee row is inserted?

medium

A. A new row is added to log_table with the inserted employee's id and name

B. The employees table is deleted

C. The trigger causes a syntax error

D. Nothing happens because NEW is invalid

Solution

1. Step 1: Understand the trigger action

   The trigger runs AFTER a new row is inserted into employees. It inserts a row into log_table using NEW.id and NEW.name.

2. Step 2: Confirm NEW usage

   NEW refers to the newly inserted row's values, so the insert into log_table uses correct values.

3. Final Answer:

   A new row is added to log_table with the inserted employee's id and name -> Option A

4. Quick Check:

   NEW accesses inserted row data [OK]

Hint: NEW accesses inserted row values inside INSERT triggers [OK]

Common Mistakes:

• Thinking NEW is invalid outside triggers
• Assuming trigger deletes data
• Confusing AFTER and BEFORE timing

4. Identify the error in this trigger:

CREATE TRIGGER trg_check BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Quantity cannot be negative'; END IF; END;

medium

A. Trigger must be AFTER INSERT, not BEFORE INSERT

B. NEW cannot be used in BEFORE INSERT triggers

C. SIGNAL SQLSTATE is invalid syntax

D. Missing delimiter to separate statements inside BEGIN...END

Solution

1. Step 1: Check trigger syntax for multiple statements

   Inside BEGIN...END, multiple statements need delimiters (like semicolons) or the delimiter must be changed before creating the trigger.

2. Step 2: Confirm other parts are valid

   NEW can be used in BEFORE INSERT triggers, SIGNAL SQLSTATE is valid in many SQL dialects, and BEFORE INSERT is allowed.

3. Final Answer:

   Missing delimiter to separate statements inside BEGIN...END -> Option D

4. Quick Check:

   Multiple statements need delimiters [OK]

Hint: Use delimiters inside BEGIN...END blocks [OK]

Common Mistakes:

• Forgetting to change delimiter before trigger creation
• Misusing NEW in triggers
• Confusing BEFORE and AFTER timing rules

5. You want to create an INSERT trigger that prevents inserting duplicate email addresses into a users table. Which approach is best?

hard

A. Use a trigger to insert the duplicate email into a log table

B. Use an AFTER INSERT trigger to delete the row if email is duplicate

C. Use a BEFORE INSERT trigger to check if NEW.email exists and raise an error if yes

D. Use a trigger to update the existing row with the new email

Solution

1. Step 1: Understand when to prevent duplicates

   To stop duplicates before they happen, use a BEFORE INSERT trigger to check if the email already exists.

2. Step 2: Why other options fail

   AFTER INSERT triggers run after insertion, so deleting duplicates after insert is inefficient and can cause issues. Updating existing rows or logging duplicates does not prevent duplicates.

3. Final Answer:

   Use a BEFORE INSERT trigger to check if NEW.email exists and raise an error if yes -> Option C

4. Quick Check:

   Prevent duplicates before insert with BEFORE INSERT trigger [OK]

Hint: Prevent duplicates before insert with BEFORE INSERT trigger [OK]

Common Mistakes:

• Trying to fix duplicates after insertion
• Using triggers to update instead of prevent
• Ignoring unique constraints and relying only on triggers