BEFORE INSERT triggers in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BEFORE INSERT trigger that sets the 'created_at' column to the current timestamp.

MySQL

CREATE TRIGGER set_created_at BEFORE INSERT ON users FOR EACH ROW SET NEW.created_at = [1];

Drag options to blanks, or click blank then click option'

ANOW()

BCURRENT_DATE

CSYSDATE()

DGETDATE()

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that returns only the date, which misses the time part.

Using a function from another SQL dialect that MySQL does not support.

The NOW() function returns the current date and time in MySQL, which is suitable for a timestamp column.

2fill in blank

medium

Complete the code to create a BEFORE INSERT trigger that prevents inserting a user with an empty username.

MySQL

CREATE TRIGGER check_username BEFORE INSERT ON users FOR EACH ROW BEGIN IF NEW.username = [1] THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Username cannot be empty'; END IF; END;

Drag options to blanks, or click blank then click option'

ANULL

B0

C' '

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for NULL instead of an empty string.

Using a space character instead of an empty string.

Checking for an empty string '' ensures the username is not blank.

3fill in blank

hard

Fix the error in the trigger code that tries to set NEW.status to 'active' before insert.

MySQL

CREATE TRIGGER activate_user BEFORE INSERT ON users FOR EACH ROW SET NEW.status = [1];

Drag options to blanks, or click blank then click option'

A"active"

Bactive

C'active'

DACTIVE

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted strings which cause syntax errors.

Using double quotes which MySQL treats differently.

String literals in MySQL must be enclosed in single quotes, so 'active' is correct.

4fill in blank

hard

Fill both blanks to create a BEFORE INSERT trigger that sets NEW.created_at to current timestamp and NEW.updated_at to NULL.

MySQL

CREATE TRIGGER set_timestamps BEFORE INSERT ON users FOR EACH ROW BEGIN SET NEW.created_at = [1]; SET NEW.updated_at = [2]; END;

Drag options to blanks, or click blank then click option'

ANOW()

BNULL

CCURRENT_TIMESTAMP

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of NULL for updated_at.

Using CURRENT_TIMESTAMP for updated_at when it should be NULL.

NOW() sets the current timestamp, and NULL sets the updated_at column to no value initially.

5fill in blank

hard

Fill all three blanks to create a BEFORE INSERT trigger that sets NEW.created_at to current timestamp, NEW.updated_at to NULL, and prevents inserting if NEW.email is NULL.

MySQL

CREATE TRIGGER validate_user BEFORE INSERT ON users FOR EACH ROW BEGIN IF NEW.email IS [1] THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Email cannot be NULL'; END IF; SET NEW.created_at = [2]; SET NEW.updated_at = [3]; END;

Drag options to blanks, or click blank then click option'

ANULL

BNOW()

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty string instead of NULL to check email.

Using wrong functions for timestamps.

The trigger checks if NEW.email IS NULL to prevent missing emails, sets created_at to current time with NOW(), and sets updated_at to NULL initially.

Practice

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

easy

A. To select data from another table before insertion

B. To delete rows after they are inserted

C. To modify or validate data before it is inserted into a table

D. To create a new table before inserting data

Solution

1. Step 1: Understand the trigger timing

   A BEFORE INSERT trigger runs before the new row is saved to the table.

2. Step 2: Identify the trigger's purpose

   It allows you to check or change the data before it is inserted, ensuring data integrity or applying rules.

3. Final Answer:

   To modify or validate data before it is inserted into a table -> Option C

4. Quick Check:

   BEFORE INSERT triggers run before insert = A [OK]

Hint: BEFORE INSERT triggers act before saving data [OK]

Common Mistakes:

• Confusing BEFORE INSERT with AFTER INSERT triggers
• Thinking triggers create or delete tables
• Assuming triggers run after data is saved

2. Which of the following is the correct syntax to create a BEFORE INSERT trigger on a table named users?

easy

A. CREATE TRIGGER before_insert_users AFTER INSERT ON users FOR EACH ROW SET NEW.name = UPPER(NEW.name);

B. CREATE TRIGGER before_insert_users BEFORE INSERT ON users FOR EACH ROW SET NEW.name = UPPER(NEW.name);

C. CREATE TRIGGER before_insert_users BEFORE UPDATE ON users FOR EACH ROW SET NEW.name = UPPER(NEW.name);

D. CREATE TRIGGER before_insert_users BEFORE INSERT ON users FOR EACH STATEMENT SET NEW.name = UPPER(NEW.name);

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: Check syntax details

   Triggers must be defined FOR EACH ROW to access NEW values. CREATE TRIGGER before_insert_users BEFORE INSERT ON users FOR EACH ROW SET NEW.name = UPPER(NEW.name); matches this exactly.

3. Final Answer:

   CREATE TRIGGER before_insert_users BEFORE INSERT ON users FOR EACH ROW SET NEW.name = UPPER(NEW.name); -> Option B

4. Quick Check:

   BEFORE INSERT + FOR EACH ROW = D [OK]

Hint: BEFORE INSERT triggers use FOR EACH ROW, not STATEMENT [OK]

Common Mistakes:

• Using AFTER instead of BEFORE
• Using BEFORE UPDATE instead of BEFORE INSERT
• Using FOR EACH STATEMENT instead of FOR EACH ROW

3. Given this trigger and insert statement:

CREATE TRIGGER check_age BEFORE INSERT ON employees FOR EACH ROW BEGIN IF NEW.age < 18 THEN SET NEW.age = 18; END IF; END;

What will be the age value after running:

INSERT INTO employees (name, age) VALUES ('Alice', 16);

medium

A. 18

B. NULL

C. 16

D. Error: age cannot be less than 18

Solution

1. Step 1: Understand trigger logic

   The trigger checks if NEW.age is less than 18, and if so, sets NEW.age to 18 before insert.

2. Step 2: Apply trigger to inserted data

   The insert tries to add age 16, which is less than 18, so trigger changes it to 18.

3. Final Answer:

   18 -> Option A

4. Quick Check:

   Trigger sets age to 18 if less than 18 = 18 [OK]

Hint: BEFORE INSERT can modify NEW values before saving [OK]

Common Mistakes:

• Assuming original value 16 is saved
• Expecting an error instead of modification
• Confusing NEW with OLD values

4. Consider this trigger code:

CREATE TRIGGER trg BEFORE INSERT ON orders FOR EACH ROW BEGIN IF NEW.quantity < 1 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Quantity must be positive'; END IF; END;

What error will occur if you run:

INSERT INTO orders (product_id, quantity) VALUES (101, 0);

medium

A. Runtime error with message 'Quantity must be positive'

B. No error, row inserted with quantity 0

C. Syntax error in trigger definition

D. Trigger ignored, default quantity used

Solution

1. Step 1: Analyze trigger condition

   The trigger checks if NEW.quantity is less than 1, which is true for 0.

2. Step 2: Understand SIGNAL statement effect

   The SIGNAL statement raises a runtime error with the given message, stopping the insert.

3. Final Answer:

   Runtime error with message 'Quantity must be positive' -> Option A

4. Quick Check:

   SIGNAL raises error if condition met = B [OK]

Hint: SIGNAL in BEFORE INSERT stops invalid inserts [OK]

Common Mistakes:

• Thinking insert succeeds with invalid data
• Confusing SIGNAL with warning
• Assuming syntax error instead of runtime error

5. You want to ensure that every new row inserted into the products table has a unique SKU code in uppercase. Which BEFORE INSERT trigger code correctly enforces this?

hard

A. CREATE TRIGGER sku_uppercase BEFORE UPDATE ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); END;

B. CREATE TRIGGER sku_uppercase AFTER INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); END;

C. CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = LOWER(NEW.sku); END;

D. CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); IF EXISTS (SELECT 1 FROM products WHERE sku = NEW.sku) THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate SKU'; END IF; END;

Solution

1. Step 1: Ensure SKU is uppercase before insert

   CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); IF EXISTS (SELECT 1 FROM products WHERE sku = NEW.sku) THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate SKU'; END IF; END; uses SET NEW.sku = UPPER(NEW.sku) in a BEFORE INSERT trigger, correctly modifying the new row.

2. Step 2: Check for duplicate SKU before insert

   CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); IF EXISTS (SELECT 1 FROM products WHERE sku = NEW.sku) THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate SKU'; END IF; END; checks if the SKU already exists and raises an error to prevent duplicates.

3. Step 3: Verify other options

   CREATE TRIGGER sku_uppercase AFTER INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); END; uses AFTER INSERT, too late to modify NEW. CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = LOWER(NEW.sku); END; uses LOWER instead of UPPER. CREATE TRIGGER sku_uppercase BEFORE UPDATE ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); END; triggers on UPDATE, not INSERT.

4. Final Answer:

   CREATE TRIGGER sku_uppercase BEFORE INSERT ON products FOR EACH ROW BEGIN SET NEW.sku = UPPER(NEW.sku); IF EXISTS (SELECT 1 FROM products WHERE sku = NEW.sku) THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate SKU'; END IF; END; -> Option D

5. Quick Check:

   Modify NEW and check duplicates BEFORE INSERT = A [OK]

Hint: Use BEFORE INSERT to modify and validate NEW row [OK]

Common Mistakes:

• Using AFTER INSERT to modify NEW values
• Not checking duplicates before insert
• Modifying SKU to lowercase instead of uppercase