INSERT trigger in SQL

Introduction

An INSERT trigger runs automatically when new data is added to a table. It helps you check or change data right when it enters the database.

You want to log every new user added to your system.

You need to check that new orders have valid prices before saving.

You want to automatically set a timestamp when a new record is created.

You want to prevent adding duplicate entries in a table.

You want to update related tables when new data is inserted.

Syntax

SQL

CREATE TRIGGER trigger_name
AFTER INSERT ON table_name
FOR EACH ROW
BEGIN
  -- SQL statements here
END;

The trigger runs after a new row is inserted into the table.

Use NEW.column_name to access the inserted row's data.

Examples

This trigger adds a log entry every time a new user is inserted.

SQL

CREATE TRIGGER log_new_user
AFTER INSERT ON users
FOR EACH ROW
BEGIN
  INSERT INTO user_log(user_id, action) VALUES (NEW.id, 'added');
END;

This trigger stops inserting an order if the price is not positive.

SQL

CREATE TRIGGER check_price
BEFORE INSERT ON orders
FOR EACH ROW
BEGIN
  IF NEW.price <= 0 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Price must be positive';
  END IF;
END;

Sample Program

This example creates a products table and a log table. The trigger adds a log entry every time a new product is inserted.

SQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2)
);

CREATE TABLE product_log (
  log_id INT AUTO_INCREMENT PRIMARY KEY,
  product_id INT,
  action VARCHAR(20)
);

DELIMITER $$
CREATE TRIGGER after_product_insert
AFTER INSERT ON products
FOR EACH ROW
BEGIN
  INSERT INTO product_log(product_id, action) VALUES (NEW.id, 'inserted');
END$$
DELIMITER ;

INSERT INTO products (id, name, price) VALUES (1, 'Pen', 1.20);

SELECT * FROM product_log;

Important Notes

Triggers can slow down inserts if they do a lot of work.

Use BEFORE INSERT triggers to check or change data before saving.

Use AFTER INSERT triggers to react after data is saved.

Summary

INSERT triggers run automatically when new rows are added.

They help check, change, or log data during insertion.

Use NEW to access the inserted row's values inside the trigger.

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