BEFORE trigger behavior in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a BEFORE trigger in PostgreSQL?

A BEFORE trigger is a special function that runs automatically before an INSERT, UPDATE, or DELETE operation on a table. It can modify or check data before the change happens.

beginner

When does a BEFORE trigger execute?

It executes just before the data modification statement (INSERT, UPDATE, DELETE) is applied to the table, allowing changes or checks before the data is saved.

intermediate

Can a BEFORE trigger modify the data being inserted or updated?

Yes, a BEFORE trigger can change the data values before they are saved to the table. For example, it can adjust a column value or reject the operation.

intermediate

What happens if a BEFORE trigger raises an exception?

If a BEFORE trigger raises an exception, the data modification is canceled and the error is returned to the user. This can be used to enforce rules or validations.

beginner

How does a BEFORE trigger differ from an AFTER trigger?

A BEFORE trigger runs before the data change happens and can modify or stop it. An AFTER trigger runs after the change is done and cannot modify the data being changed.

When does a BEFORE trigger run in PostgreSQL?

A After the data modification statement executes

B Before the data modification statement executes

C Only during SELECT queries

D Only when the database starts

A BEFORE trigger runs before the INSERT, UPDATE, or DELETE operation modifies the table.

Can a BEFORE trigger change the data being inserted?

A Yes, it can modify the data before saving

B Only after the data is saved

C Only if the data is numeric

D No, it can only read data

BEFORE triggers can modify the data before it is saved to the table.

What happens if a BEFORE trigger raises an error?

A The data change proceeds anyway

B The error is ignored

C The database restarts

D The data change is canceled

Raising an error in a BEFORE trigger stops the data modification and returns the error.

Which SQL operations can a BEFORE trigger be attached to?

A INSERT, UPDATE, DELETE

B SELECT only

C CREATE and DROP

D ALTER only

BEFORE triggers are used with INSERT, UPDATE, and DELETE operations.

How does a BEFORE trigger differ from an AFTER trigger?

A Both run at the same time

B BEFORE triggers run after data changes; AFTER triggers run before

C BEFORE triggers run before data changes; AFTER triggers run after

D BEFORE triggers only run on SELECT

BEFORE triggers run before the data change, allowing modification; AFTER triggers run after the change.

Explain what a BEFORE trigger does and when it runs in PostgreSQL.

Think about what happens before saving data to a table.

Describe how a BEFORE trigger can affect data integrity or validation.

Consider how you might stop bad data from entering your table.

Practice

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

easy

A. To run code after data is inserted or updated

B. To delete rows automatically

C. To run custom code before data is inserted or updated

D. To create new tables dynamically

Solution

1. Step 1: Understand trigger timing

   BEFORE triggers execute before the actual data change happens in the table.

2. Step 2: Identify trigger purpose

   They allow checking or modifying data before it is saved, preventing bad data if needed.

3. Final Answer:

   To run custom code before data is inserted or updated -> Option C

4. Quick Check:

   BEFORE trigger = runs before data change [OK]

Hint: BEFORE triggers run before saving data [OK]

Common Mistakes:

• Confusing BEFORE with AFTER triggers
• Thinking triggers create or delete tables
• Assuming triggers run only after data changes

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

easy

A. CREATE TRIGGER trg BEFORE INSERT ON users CALL func();

B. CREATE TRIGGER trg AFTER INSERT ON users EXECUTE FUNCTION func();

C. CREATE TRIGGER trg BEFORE INSERT INTO users EXECUTE FUNCTION func();

D. CREATE TRIGGER trg BEFORE INSERT ON users EXECUTE FUNCTION func();

Solution

1. Step 1: Check trigger timing and event

   The trigger must be BEFORE INSERT on the table users.

2. Step 2: Verify syntax for calling function

   PostgreSQL uses EXECUTE FUNCTION to call the trigger function.

3. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON users EXECUTE FUNCTION func(); -> Option D

4. Quick Check:

   Correct syntax uses BEFORE INSERT ON and EXECUTE FUNCTION [OK]

Hint: Use BEFORE INSERT ON table EXECUTE FUNCTION func() [OK]

Common Mistakes:

• Using AFTER instead of BEFORE
• Writing INTO instead of ON
• Using CALL instead of EXECUTE FUNCTION

3. Consider this BEFORE INSERT trigger function that changes the new row's status to 'active':

CREATE FUNCTION set_status() RETURNS trigger AS $$
BEGIN
  NEW.status := 'active';
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_set_status BEFORE INSERT ON accounts
FOR EACH ROW EXECUTE FUNCTION set_status();

INSERT INTO accounts (id, status) VALUES (1, 'pending');
SELECT status FROM accounts WHERE id = 1;

What will be the output of the SELECT query?

medium

A. active

B. NULL

C. pending

D. Error: cannot insert

Solution

1. Step 1: Understand BEFORE INSERT trigger effect

   The trigger sets NEW.status to 'active' before the row is inserted.

2. Step 2: Check inserted data

   Even though 'pending' was given, the trigger changes it to 'active' before saving.

3. Final Answer:

   active -> Option A

4. Quick Check:

   BEFORE trigger modifies data before insert [OK]

Hint: BEFORE triggers can modify NEW row data before insert [OK]

Common Mistakes:

• Assuming original value 'pending' is saved
• Thinking trigger runs after insert
• Expecting NULL or error without reason

4. Given this trigger function:

CREATE FUNCTION check_age() RETURNS trigger AS $$
BEGIN
  IF NEW.age < 18 THEN
    RAISE EXCEPTION 'Age must be 18 or older';
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Which problem will occur if you create a BEFORE INSERT trigger using this function and try to insert a row with age = 16?

medium

A. An error will be raised and insertion will stop

B. The trigger will silently ignore the age check

C. The row will be inserted with age 16

D. The age will be automatically set to 18

Solution

1. Step 1: Analyze trigger logic

   If NEW.age is less than 18, the trigger raises an exception.

2. Step 2: Understand effect of RAISE EXCEPTION

   Raising an exception stops the insert and returns an error to the client.

3. Final Answer:

   An error will be raised and insertion will stop -> Option A

4. Quick Check:

   RAISE EXCEPTION stops insert with error [OK]

Hint: RAISE EXCEPTION in BEFORE trigger stops insert with error [OK]

Common Mistakes:

• Thinking the row inserts anyway
• Assuming age auto-corrects
• Ignoring that exceptions stop execution

5. You want to create a BEFORE UPDATE trigger on the products table that prevents the price from being set below zero. Which trigger function code correctly enforces this rule?

hard

A. BEGIN
IF NEW.price < 0 THEN
NEW.price := 0;
END IF;
RETURN NEW;
END;

B. BEGIN
IF NEW.price < 0 THEN
RAISE EXCEPTION 'Price cannot be negative';
END IF;
RETURN NEW;
END;

C. BEGIN
IF OLD.price < 0 THEN
RAISE EXCEPTION 'Price cannot be negative';
END IF;
RETURN NEW;
END;

D. BEGIN
IF NEW.price < 0 THEN
DELETE FROM products WHERE id = NEW.id;
END IF;
RETURN NEW;
END;

Solution

1. Step 1: Identify correct condition check

   The trigger must check NEW.price to prevent negative values before update.

2. Step 2: Choose proper action on invalid data

   Raising an exception stops the update and prevents invalid price.

3. Step 3: Eliminate incorrect options

   BEGIN
     IF NEW.price < 0 THEN
       NEW.price := 0;
     END IF;
     RETURN NEW;
   END;

   silently changes price to 0 (may hide errors), C checks OLD.price (wrong), D deletes row (not appropriate).

4. Final Answer:

   BEGIN IF NEW.price < 0 THEN RAISE EXCEPTION 'Price cannot be negative'; END IF; RETURN NEW; END; -> Option B

5. Quick Check:

   Use RAISE EXCEPTION on NEW.price < 0 to stop update [OK]

Hint: Raise error on NEW.price < 0 to block update [OK]

Common Mistakes:

• Checking OLD.price instead of NEW.price
• Silently fixing invalid data instead of error
• Deleting rows inside BEFORE trigger