Trigger for data validation in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a trigger in PostgreSQL?

A trigger is a special procedure that automatically runs when certain events happen in the database, like inserting or updating data.

beginner

How can triggers help with data validation?

Triggers can check data before it is saved and stop the action if the data does not meet rules, ensuring only valid data is stored.

intermediate

What is the difference between BEFORE and AFTER triggers?

BEFORE triggers run before the data change happens, so they can prevent invalid data. AFTER triggers run after the change, useful for logging or actions that don't block data.

intermediate

Write a simple trigger function in PostgreSQL that checks if a new user's age is at least 18.

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

intermediate

How do you attach a trigger function to a table for INSERT events?

Use CREATE TRIGGER with the table name, specify BEFORE INSERT, and link the trigger function. Example: CREATE TRIGGER validate_age BEFORE INSERT ON users FOR EACH ROW EXECUTE FUNCTION check_age();

What event would you use a BEFORE trigger for?

A To create a new table

B To log data after it is inserted

C To delete data automatically

D To check data before it is inserted or updated

BEFORE triggers run before data changes, so they are perfect for checking or validating data.

Which language is commonly used to write PostgreSQL trigger functions?

A PL/pgSQL

B Python

C JavaScript

D HTML

PL/pgSQL is the built-in procedural language for writing trigger functions in PostgreSQL.

What happens if a trigger raises an exception during data validation?

A The data change continues anyway

B The data change is stopped and an error is shown

C The database restarts

D The trigger is ignored

Raising an exception in a trigger stops the data change and returns an error to the user.

Which statement correctly creates a trigger for validating data on INSERT?

A CREATE TRIGGER validate BEFORE UPDATE ON table EXECUTE FUNCTION validate_func();

B CREATE TRIGGER validate AFTER DELETE ON table EXECUTE FUNCTION validate_func();

C CREATE TRIGGER validate BEFORE INSERT ON table FOR EACH ROW EXECUTE FUNCTION validate_func();

D CREATE TRIGGER validate ON table FOR EACH STATEMENT EXECUTE FUNCTION validate_func();

The correct syntax includes BEFORE INSERT, FOR EACH ROW, and EXECUTE FUNCTION.

Triggers are useful for:

A Automatically enforcing rules on data changes

B Manually editing data in tables

C Backing up the database

D Creating user accounts

Triggers automatically run code to enforce rules or actions when data changes.

Explain how a trigger can be used to validate data before inserting it into a table.

Think about the timing of the trigger and what happens if data is wrong.

Describe the steps to create and attach a trigger function for data validation in PostgreSQL.

Consider both the function and the trigger creation commands.

Practice

1. What is the main purpose of a trigger in PostgreSQL for data validation?

easy

A. To create new tables based on existing ones

B. To speed up query execution by indexing data

C. To automatically check and enforce rules on data before it is saved

D. To backup the database automatically

Solution

1. Step 1: Understand trigger role

   Triggers run automatically when data changes, allowing checks on data.

2. Step 2: Identify validation purpose

   Data validation means checking data correctness before saving it.

3. Final Answer:

   To automatically check and enforce rules on data before it is saved -> Option C

4. Quick Check:

   Trigger = automatic data check [OK]

Hint: Triggers run automatically to check data before saving [OK]

Common Mistakes:

• Thinking triggers speed up queries
• Confusing triggers with backups
• Assuming triggers create tables

2. Which of the following is the correct way to declare a BEFORE INSERT trigger in PostgreSQL?

easy

A. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name;

B. CREATE TRIGGER trg AFTER INSERT ON table_name EXECUTE PROCEDURE func_name();

C. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE PROCEDURE func_name();

D. CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name();

Solution

1. Step 1: Recall PostgreSQL trigger syntax

   PostgreSQL uses EXECUTE FUNCTION for triggers since version 11.

2. Step 2: Identify correct timing and syntax

   BEFORE INSERT triggers run before inserting data; syntax must match.

3. Final Answer:

   CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name(); -> Option D

4. Quick Check:

   BEFORE INSERT + EXECUTE FUNCTION = CREATE TRIGGER trg BEFORE INSERT ON table_name EXECUTE FUNCTION func_name(); [OK]

Hint: Use EXECUTE FUNCTION for triggers in PostgreSQL 11+ [OK]

Common Mistakes:

• Using EXECUTE PROCEDURE instead of EXECUTE FUNCTION
• Confusing BEFORE and AFTER timing
• Missing parentheses after function name

3. Given this trigger function to prevent negative prices:

CREATE FUNCTION check_price() RETURNS trigger AS $$ BEGIN IF NEW.price < 0 THEN RAISE EXCEPTION 'Price cannot be negative'; END IF; RETURN NEW; END; $$ LANGUAGE plpgsql;

What happens if you try to insert a row with price = -5?

medium

A. An error is raised and the insert is stopped

B. The row is inserted with price -5

C. The price is automatically set to 0

D. The trigger is ignored and insert proceeds

Solution

1. Step 1: Analyze trigger function logic

   The function checks if NEW.price is less than 0 and raises an exception if true.

2. Step 2: Understand RAISE EXCEPTION effect

   RAISE EXCEPTION stops the operation and returns an error to the user.

3. Final Answer:

   An error is raised and the insert is stopped -> Option A

4. Quick Check:

   Negative price triggers error [OK]

Hint: RAISE EXCEPTION stops insert on invalid data [OK]

Common Mistakes:

• Assuming data is inserted anyway
• Thinking price auto-corrects
• Ignoring trigger effects

4. You wrote this trigger function:

CREATE FUNCTION validate_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;

But when inserting age = 15, no error occurs. What is the likely mistake?

medium

A. RAISE EXCEPTION syntax is incorrect

B. The trigger is not attached to the table

C. The function does not return NEW

D. The trigger function is missing LANGUAGE plpgsql

Solution

1. Step 1: Check function correctness

   The function correctly raises exception and returns NEW, syntax is fine.

2. Step 2: Consider trigger attachment

   If no error occurs, likely the trigger is not linked to the table to run the function.

3. Final Answer:

   The trigger is not attached to the table -> Option B

4. Quick Check:

   Trigger must be attached to run function [OK]

Hint: Attach trigger to table to activate validation [OK]

Common Mistakes:

• Forgetting to create the trigger after function
• Assuming function runs without trigger
• Misreading RAISE EXCEPTION syntax

5. You want to ensure that a user's email is unique and not empty using a trigger. Which approach correctly combines data validation and uniqueness check in PostgreSQL?

hard

A. Create a BEFORE INSERT OR UPDATE trigger that raises exception if NEW.email is empty or exists in the table

B. Use a UNIQUE constraint on email column only, no trigger needed

C. Create an AFTER INSERT trigger that deletes duplicates after insertion

D. Use a trigger that sets empty emails to a default value

Solution

1. Step 1: Understand validation needs

   Email must be non-empty and unique before saving data.

2. Step 2: Choose trigger timing and logic

   BEFORE INSERT OR UPDATE trigger can check NEW.email and query table for duplicates, raising exception if invalid.

3. Final Answer:

   Create a BEFORE INSERT OR UPDATE trigger that raises exception if NEW.email is empty or exists in the table -> Option A

4. Quick Check:

   Validate and check uniqueness before insert/update [OK]

Hint: Use BEFORE trigger to check and stop invalid data [OK]

Common Mistakes:

• Relying only on UNIQUE constraint without empty check
• Using AFTER trigger to fix duplicates (too late)
• Setting default instead of raising error