Database triggers for automation in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a database trigger?

A database trigger is a special rule that runs automatically when certain changes happen in the database, like adding or changing data.

beginner

How do triggers help automate tasks in Supabase?

Triggers in Supabase run code automatically when data changes, so you don't have to do tasks manually. For example, sending notifications or updating related data.

beginner

When can a trigger be set to run?

A trigger can run before or after data is inserted, updated, or deleted in a table.

intermediate

What language is used to write trigger functions in Supabase?

Trigger functions in Supabase are written in SQL or PL/pgSQL, which is a special language for PostgreSQL databases.

intermediate

Why should you be careful with triggers?

Triggers run automatically and can affect many rows quickly. If not written well, they can slow down the database or cause unexpected results.

What event can a database trigger NOT respond to?

A User login

B Data insertion

C Data deletion

D Data update

Database triggers respond to data changes like insert, update, or delete, but not to user login events.

In Supabase, which language is commonly used to write trigger functions?

A PL/pgSQL

B Ruby

C Python

D JavaScript

Supabase uses PostgreSQL, so triggers are written in PL/pgSQL or SQL.

When does an AFTER trigger run?

A Before data changes

B After data changes

C Only on data deletion

D Only on data insertion

An AFTER trigger runs after the data change has happened.

Which is a good use of a database trigger?

A Change the database schema

B Send emails directly from the user's browser

C Automatically update a timestamp when a row changes

D Run a web server

Triggers can update data automatically, like timestamps, but cannot run web servers or change schema.

What is a risk of using triggers without care?

A They delete the database

B They make the database faster

C They prevent data changes

D They can slow down database operations

Poorly designed triggers can slow down the database because they run automatically on data changes.

Explain how database triggers automate tasks in Supabase and give an example.

Think about what happens when data is added or changed.

Describe the difference between BEFORE and AFTER triggers and when you might use each.

Consider if you want to check or modify data before saving or react after saving.

Practice

1. What is the main purpose of a database trigger in Supabase?

easy

A. To manually update data in the database

B. To automatically run a function when data changes in a table

C. To create a backup of the database

D. To increase the database storage size

Solution

1. Step 1: Understand what triggers do

   Triggers run automatically when data changes like insert, update, or delete.

2. Step 2: Identify the purpose in Supabase

   Supabase triggers automate running functions on data changes without manual intervention.

3. Final Answer:

   To automatically run a function when data changes in a table -> Option B

4. Quick Check:

   Trigger = auto-run function on data change [OK]

Hint: Triggers run code automatically on data changes [OK]

Common Mistakes:

• Thinking triggers require manual activation
• Confusing triggers with backups
• Assuming triggers increase storage

2. Which of the following is the correct syntax to create a trigger in Supabase SQL?

easy

A. CREATE TRIGGER trigger_name WHEN INSERT ON table_name EXECUTE function_name();

B. CREATE TRIGGER trigger_name BEFORE UPDATE table_name CALL function_name();

C. CREATE TRIGGER trigger_name ON table_name AFTER DELETE RUN function_name();

D. CREATE TRIGGER trigger_name AFTER INSERT ON table_name EXECUTE FUNCTION function_name();

Solution

1. Step 1: Recall correct trigger syntax in PostgreSQL (used by Supabase)

   The correct syntax uses CREATE TRIGGER, timing (AFTER/BEFORE), event (INSERT/UPDATE/DELETE), ON table, and EXECUTE FUNCTION.

2. Step 2: Match options to syntax

   CREATE TRIGGER trigger_name AFTER INSERT ON table_name EXECUTE FUNCTION function_name(); matches the correct syntax: CREATE TRIGGER name AFTER INSERT ON table EXECUTE FUNCTION func(); others use wrong keywords like CALL, RUN, or WHEN.

3. Final Answer:

   CREATE TRIGGER trigger_name AFTER INSERT ON table_name EXECUTE FUNCTION function_name(); -> Option D

4. Quick Check:

   CREATE TRIGGER ... EXECUTE FUNCTION ... [OK]

Hint: Use EXECUTE FUNCTION, not CALL or RUN in triggers [OK]

Common Mistakes:

• Using CALL or RUN instead of EXECUTE FUNCTION
• Missing ON keyword before table name
• Using WHEN instead of BEFORE or AFTER

3. Given this trigger function in Supabase SQL:

CREATE FUNCTION log_insert() RETURNS trigger AS $$ BEGIN INSERT INTO logs(table_name, action) VALUES (TG_TABLE_NAME, 'insert'); RETURN NEW; END; $$ LANGUAGE plpgsql;

And this trigger:

CREATE TRIGGER after_insert_log AFTER INSERT ON users EXECUTE FUNCTION log_insert();

What happens when a new row is inserted into the users table?

medium

A. A new row is added to the logs table recording the insert action on users

B. The insert into users is blocked and logs table is updated

C. Nothing happens because the trigger is AFTER INSERT

D. The users table row is deleted after insert

Solution

1. Step 1: Understand the trigger function behavior

   The function inserts a row into logs with the table name and action 'insert' whenever triggered.

2. Step 2: Understand the trigger timing and event

   The trigger runs AFTER INSERT on users, so after a new user row is added, the function runs and logs the action.

3. Final Answer:

   A new row is added to the logs table recording the insert action on users -> Option A

4. Quick Check:

   AFTER INSERT triggers log insert action [OK]

Hint: AFTER INSERT triggers run after data is added [OK]

Common Mistakes:

• Thinking AFTER INSERT blocks the insert
• Assuming trigger does nothing
• Confusing trigger timing with BEFORE

4. You wrote this trigger function:

CREATE FUNCTION update_timestamp() RETURNS trigger AS $$ BEGIN NEW.updated_at = NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql;

And this trigger:

CREATE TRIGGER update_time BEFORE UPDATE ON orders EXECUTE FUNCTION update_timestamp();

But the updated_at column is not changing on updates. What is the likely problem?

medium

A. The trigger is AFTER UPDATE instead of BEFORE UPDATE

B. The trigger function is missing RETURN OLD

C. The trigger function does not modify NEW row correctly

D. The trigger function is not marked as VOLATILE

Solution

1. Step 1: Check how NEW row is modified in trigger

   The function sets NEW.updated_at = NOW() but does not assign it back properly; in PL/pgSQL, you must assign NEW.updated_at and then RETURN NEW.

2. Step 2: Confirm trigger timing and return

   The trigger is BEFORE UPDATE and returns NEW, which is correct. The problem is likely the function does not properly update the NEW record.

3. Final Answer:

   The trigger function does not modify NEW row correctly -> Option C

4. Quick Check:

   Modify NEW row correctly in BEFORE UPDATE triggers [OK]

Hint: Always RETURN NEW after modifying it in BEFORE UPDATE triggers [OK]

Common Mistakes:

• Not returning NEW after changes
• Using AFTER UPDATE instead of BEFORE UPDATE
• Returning OLD instead of NEW

5. You want to automatically archive deleted rows from the products table into products_archive using a trigger. Which approach correctly achieves this in Supabase?

hard

A. Create an AFTER DELETE trigger on products that inserts OLD row into products_archive

B. Create a BEFORE DELETE trigger on products that deletes from products_archive

C. Create an AFTER INSERT trigger on products_archive that copies rows to products

D. Create a BEFORE UPDATE trigger on products that moves rows to products_archive

Solution

1. Step 1: Understand the goal

   You want to save deleted rows from products into products_archive automatically.

2. Step 2: Choose correct trigger timing and action

   AFTER DELETE trigger on products can access OLD row data and insert it into products_archive. BEFORE DELETE is too early to archive after deletion. Other options do not archive deleted rows.

3. Final Answer:

   Create an AFTER DELETE trigger on products that inserts OLD row into products_archive -> Option A

4. Quick Check:

   AFTER DELETE triggers archive OLD row [OK]

Hint: Use AFTER DELETE triggers to archive deleted rows [OK]

Common Mistakes:

• Using BEFORE DELETE to archive after deletion
• Confusing insert and delete triggers
• Trying to archive on update instead of delete