BEFORE INSERT triggers in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a BEFORE INSERT trigger in MySQL?

A BEFORE INSERT trigger is a special procedure that runs automatically before a new row is added to a table. It lets you check or change data before it is saved.

beginner

When exactly does a BEFORE INSERT trigger execute?

It executes right before the new data is inserted into the table, allowing you to modify or validate the data before it is saved.

intermediate

Can a BEFORE INSERT trigger modify the data being inserted?

Yes, it can change the values of the new row before it is saved, for example, to add timestamps or enforce rules.

beginner

How do you access the new row's data inside a BEFORE INSERT trigger?

You use the keyword NEW followed by the column name, like NEW.column_name, to read or change the value.

intermediate

What happens if a BEFORE INSERT trigger signals an error?

The insert operation is stopped, and the new row is not added to the table. This helps keep data clean and valid.

When does a BEFORE INSERT trigger run in MySQL?

A Before a new row is inserted

B After a new row is inserted

C When a row is updated

D When a row is deleted

A BEFORE INSERT trigger runs right before a new row is added to the table.

How do you refer to the new row's column 'age' inside a BEFORE INSERT trigger?

A OLD.age

B NEW.age

C age

D NEW['age']

You use NEW.age to access or modify the new row's 'age' value.

What happens if a BEFORE INSERT trigger raises an error?

A The insert is canceled

B The insert continues anyway

C The trigger is ignored

D The row is updated instead

If the trigger signals an error, the insert is stopped and the row is not added.

Which of these is a valid use of a BEFORE INSERT trigger?

A Delete rows from another table

B Update existing rows after insert

C Automatically set a creation timestamp

D Drop the table

BEFORE INSERT triggers are often used to set or modify data before saving, like timestamps.

Can a BEFORE INSERT trigger modify multiple columns of the new row?

A Only if the table has a primary key

B No, it can only read data

C No, it can only modify one column

D Yes, it can modify any columns before insert

BEFORE INSERT triggers can change any columns of the new row before it is saved.

Explain what a BEFORE INSERT trigger does and give an example of when you might use it.

Think about how you can change data before saving it.

Describe how to access and change the data of the new row inside a BEFORE INSERT trigger.

Remember the special keyword for new row data.

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