Trigger execution order in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BEFORE INSERT trigger on table 'orders'.

PostgreSQL

CREATE TRIGGER before_insert_order
BEFORE [1] ON orders
FOR EACH ROW EXECUTE FUNCTION check_order();

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

AINSERT

BDELETE

CUPDATE

DTRUNCATE

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing UPDATE or DELETE instead of INSERT.

Using TRUNCATE which is not supported for triggers.

The trigger must fire before an INSERT operation, so the correct event is INSERT.

2fill in blank

medium

Complete the code to specify the trigger timing as AFTER.

PostgreSQL

CREATE TRIGGER after_update_customer
[1] UPDATE ON customers
FOR EACH ROW EXECUTE FUNCTION log_update();

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

ADURING

BBEFORE

CAFTER

DINSTEAD OF

Attempts:

3 left

💡 Hint

Common Mistakes

Using BEFORE when the trigger should run after.

Choosing INSTEAD OF which is for views, not tables.

The trigger should execute after the UPDATE operation, so the timing is AFTER.

3fill in blank

hard

Fix the error in the trigger creation by choosing the correct trigger event.

PostgreSQL

CREATE TRIGGER trg_delete_product
BEFORE [1] ON products
FOR EACH ROW EXECUTE FUNCTION archive_product();

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

AINSERT

BDELETE

CUPDATE

DSELECT

Attempts:

3 left

💡 Hint

Common Mistakes

Using INSERT or UPDATE instead of DELETE.

Using SELECT which is not a valid trigger event.

The trigger is named 'trg_delete_product' and should fire before a DELETE operation, so the event must be DELETE.

4fill in blank

hard

Fill both blanks to create a trigger that fires BEFORE UPDATE or DELETE on the 'employees' table.

PostgreSQL

CREATE TRIGGER trg_before_change
BEFORE [1] OR [2] ON employees
FOR EACH ROW EXECUTE FUNCTION validate_change();

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

AUPDATE

BINSERT

CDELETE

DTRUNCATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using INSERT or TRUNCATE instead of UPDATE or DELETE.

Mixing up the order of events.

The trigger fires before UPDATE or DELETE events, so the blanks must be UPDATE and DELETE.

5fill in blank

hard

Fill all three blanks to create a trigger that fires AFTER INSERT, UPDATE, or DELETE on 'sales'.

PostgreSQL

CREATE TRIGGER trg_after_change
AFTER [1] OR [2] OR [3] ON sales
FOR EACH ROW EXECUTE FUNCTION audit_changes();

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

AINSERT

BUPDATE

CDELETE

DSELECT

Attempts:

3 left

💡 Hint

Common Mistakes

Including SELECT which is invalid.

Missing one of the three events.

The trigger fires after INSERT, UPDATE, or DELETE events, so the blanks must be INSERT, UPDATE, and DELETE.

Practice

1. In PostgreSQL, when multiple triggers are set for the same event and timing on a table, how does the database decide the order in which to execute them?

easy

A. It executes triggers based on the size of the table.

B. It executes triggers randomly.

C. It executes triggers in the order they were created.

D. It executes triggers in alphabetical order by their trigger names.

Solution

1. Step 1: Understand trigger execution timing

   PostgreSQL runs triggers automatically on table events like INSERT, UPDATE, or DELETE.

2. Step 2: Identify execution order rule

   When multiple triggers exist for the same event and timing, PostgreSQL executes them in the order they were created.

3. Final Answer:

   It executes triggers in the order they were created. -> Option C

4. Quick Check:

   Trigger order = creation order [OK]

Hint: Remember: PostgreSQL orders triggers by creation order [OK]

Common Mistakes:

• Assuming triggers run in alphabetical order
• Thinking triggers run randomly
• Believing trigger order depends on table size

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

easy

A. CREATE TRIGGER alpha_trigger BEFORE INSERT ON users FOR EACH ROW EXECUTE FUNCTION func_name();

B. CREATE TRIGGER alpha_trigger ON users BEFORE INSERT EXECUTE FUNCTION func_name();

C. CREATE TRIGGER alpha_trigger BEFORE INSERT EXECUTE FUNCTION func_name() ON users;

D. CREATE TRIGGER alpha_trigger BEFORE INSERT ON users EXECUTE PROCEDURE func_name();

Solution

1. Step 1: Recall correct CREATE TRIGGER syntax

   The correct syntax is: CREATE TRIGGER name BEFORE event ON table FOR EACH ROW EXECUTE FUNCTION function_name();

2. Step 2: Check each option

   CREATE TRIGGER alpha_trigger BEFORE INSERT ON users FOR EACH ROW EXECUTE FUNCTION func_name(); matches the correct syntax exactly. CREATE TRIGGER alpha_trigger BEFORE INSERT ON users EXECUTE PROCEDURE func_name(); uses EXECUTE PROCEDURE which is deprecated in modern PostgreSQL versions.

3. Final Answer:

   CREATE TRIGGER alpha_trigger BEFORE INSERT ON users FOR EACH ROW EXECUTE FUNCTION func_name(); -> Option A

4. Quick Check:

   Correct syntax uses EXECUTE FUNCTION and ON table [OK]

Hint: Use EXECUTE FUNCTION and ON table in CREATE TRIGGER [OK]

Common Mistakes:

• Using EXECUTE PROCEDURE instead of EXECUTE FUNCTION
• Placing ON table after EXECUTE FUNCTION
• Omitting FOR EACH ROW clause

3. Given two triggers on table orders: alpha_trigger and beta_trigger, both BEFORE INSERT triggers. Which trigger runs first when a new row is inserted?

medium

A. alpha_trigger runs first because 'a' comes before 'b' alphabetically.

B. beta_trigger runs first because 'b' comes before 'a' alphabetically.

C. Both triggers run simultaneously.

D. The trigger created last runs first.

Solution

1. Step 1: Identify trigger names and order

   Triggers are named alpha_trigger and beta_trigger. Alphabetically, 'alpha' comes before 'beta'.

2. Step 2: Apply PostgreSQL trigger execution order

   PostgreSQL executes triggers in the order they were created for the same event and timing, not alphabetical order.

3. Final Answer:

   beta_trigger runs first because it was created before alpha_trigger. -> Option B

4. Quick Check:

   Trigger order = creation order [OK]

Hint: Trigger execution order depends on creation order, not alphabetical [OK]

Common Mistakes:

• Assuming alphabetical order matters
• Thinking triggers run simultaneously
• Confusing creation order

4. You have two AFTER UPDATE triggers on table products: zeta_trigger and alpha_trigger. You want zeta_trigger to run before alpha_trigger. What is the problem with this setup?

medium

A. Triggers run in reverse alphabetical order, so zeta_trigger runs first as desired.

B. PostgreSQL does not support multiple triggers on the same event.

C. You must rename triggers to numbers to control order.

D. PostgreSQL runs triggers alphabetically, so alpha_trigger runs before zeta_trigger regardless of creation order.

Solution

1. Step 1: Understand trigger execution order

   PostgreSQL executes multiple triggers for the same event and timing in the order they were created.

2. Step 2: Analyze the trigger names

   Trigger names do not affect execution order; creation order does. So the trigger created first runs first.

3. Final Answer:

   PostgreSQL runs triggers in creation order, so alpha_trigger runs before zeta_trigger if created first. -> Option D

4. Quick Check:

   Trigger order = creation order [OK]

Hint: Creation order controls trigger execution, not alphabetical order [OK]

Common Mistakes:

• Assuming creation order controls execution
• Believing reverse alphabetical order applies
• Thinking trigger renaming is unnecessary

5. You have three BEFORE DELETE triggers on table customers: clean_up, archive, and notify. You want archive to run first, then clean_up, then notify. How should you rename the triggers to ensure this execution order?

hard

A. No renaming needed; PostgreSQL runs triggers in creation order.

B. Rename triggers to notify, clean_up, archive.

C. Rename triggers to a_archive, b_clean_up, c_notify.

D. Rename triggers to 1_archive, 2_clean_up, 3_notify.

Solution

1. Step 1: Understand trigger execution order

   PostgreSQL executes triggers in the order they were created for the same event and timing.

2. Step 2: Renaming triggers does not affect execution order

   Renaming triggers does not change execution order; only creation order matters.

3. Final Answer:

   No renaming needed; PostgreSQL runs triggers in creation order. -> Option A

4. Quick Check:

   Trigger order = creation order [OK]

Hint: Trigger execution order depends on creation order, not name [OK]

Common Mistakes:

• Assuming alphabetical order controls execution
• Believing numeric or alphabetical prefixes affect order
• Not understanding creation order importance