INSTEAD OF trigger for views in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a view named 'employee_view' selecting all columns from 'employees'.

PostgreSQL

CREATE VIEW employee_view AS SELECT [1] FROM employees;

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

A*

Bdepartment

Csalary

Did, name

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting only one column when all are needed.

Forgetting to specify the table name.

The * selects all columns from the table.

2fill in blank

medium

Complete the code to create an INSTEAD OF trigger function named 'employee_view_insert' for the view.

PostgreSQL

CREATE FUNCTION employee_view_insert() RETURNS trigger AS $$ BEGIN INSERT INTO employees VALUES ([1]); RETURN NEW; END; $$ LANGUAGE plpgsql;

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

ANEW.id, NEW.name, NEW.salary, NEW.department

Bid, NEW.name, NEW.salary, NEW.department

Cid, name, salary, department

DNEW.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using column names without NEW. prefix.

Missing some columns in the insert statement.

The NEW keyword references the new row's columns in the trigger function.

3fill in blank

hard

Fix the error in the trigger creation by completing the code to attach the INSTEAD OF INSERT trigger to 'employee_view'.

PostgreSQL

CREATE TRIGGER employee_view_insert_trigger INSTEAD OF INSERT ON employee_view FOR EACH ROW EXECUTE FUNCTION [1]();

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

Ainsert_trigger

Bemployee_insert_trigger

Cinsert_employee

Demployee_view_insert

Attempts:

3 left

💡 Hint

Common Mistakes

Using a trigger name instead of the function name.

Misspelling the function name.

The trigger must call the function employee_view_insert defined earlier.

4fill in blank

hard

Fill both blanks to complete the INSTEAD OF UPDATE trigger function that updates the 'employees' table when the view is updated.

PostgreSQL

CREATE FUNCTION employee_view_update() RETURNS trigger AS $$ BEGIN UPDATE employees SET [1] = NEW.[2], salary = NEW.salary, department = NEW.department WHERE id = OLD.id; RETURN NEW; END; $$ LANGUAGE plpgsql;

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

Aname

Bid

Csalary

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Using different column names for the SET and NEW parts.

Trying to update the primary key column.

The name column is updated by setting it to NEW.name.

5fill in blank

hard

Fill all three blanks to complete the INSTEAD OF DELETE trigger function that deletes from 'employees' when a row is deleted from the view.

PostgreSQL

CREATE FUNCTION employee_view_delete() RETURNS trigger AS $$ BEGIN DELETE FROM employees WHERE [1] = OLD.[2]; RETURN [3]; END; $$ LANGUAGE plpgsql;

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

Aid

COLD

DNEW

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW instead of OLD in a DELETE trigger.

Mismatching column names in WHERE clause.

The delete matches on id column and returns OLD row after deletion.

Practice

1. What is the main purpose of an INSTEAD OF trigger on a view in PostgreSQL?

easy

A. To speed up queries on the view

B. To run custom code instead of the default insert, update, or delete on the view

C. To automatically create indexes on the view

D. To prevent any changes to the view data

Solution

1. Step 1: Understand what views normally do

   Views in PostgreSQL are virtual tables that do not store data themselves and usually cannot be directly updated.

2. Step 2: Role of INSTEAD OF triggers

   INSTEAD OF triggers let you define custom actions that run instead of the usual insert, update, or delete on the view, making it updatable.

3. Final Answer:

   To run custom code instead of the default insert, update, or delete on the view -> Option B

4. Quick Check:

   INSTEAD OF triggers = custom code on views [OK]

Hint: INSTEAD OF triggers replace default actions on views [OK]

Common Mistakes:

• Thinking INSTEAD OF triggers speed up queries
• Confusing INSTEAD OF triggers with indexes
• Believing INSTEAD OF triggers block all changes

2. Which of the following is the correct syntax to create an INSTEAD OF trigger on a view named my_view for INSERT operations?

easy

A. CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func();

B. CREATE TRIGGER trg_instead_of_insert INSTEAD OF INSERT ON my_view EXECUTE FUNCTION trg_func();

C. CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT EXECUTE FUNCTION trg_func();

D. CREATE TRIGGER trg_instead_of_insert ON my_view BEFORE INSERT EXECUTE FUNCTION trg_func();

Solution

1. Step 1: Recall correct trigger syntax in PostgreSQL

   The syntax requires specifying the trigger name, the event timing (INSTEAD OF), the event type (INSERT), the target (ON my_view), and the function to execute.

2. Step 2: Identify the full correct syntax

   CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func(); correctly includes "ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func();" which is the proper syntax for INSTEAD OF triggers on views.

3. Final Answer:

   CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func(); -> Option A

4. Quick Check:

   INSTEAD OF triggers need FOR EACH ROW and correct order [OK]

Hint: INSTEAD OF triggers use 'FOR EACH ROW' and 'EXECUTE FUNCTION' [OK]

Common Mistakes:

• Placing INSTEAD OF before ON
• Omitting FOR EACH ROW
• Using BEFORE instead of INSTEAD OF

3. Given the following setup:

CREATE VIEW emp_view AS SELECT id, name FROM employees;
CREATE FUNCTION emp_view_insert() RETURNS trigger AS $$ BEGIN INSERT INTO employees(id, name) VALUES (NEW.id, NEW.name); RETURN NEW; END; $$ LANGUAGE plpgsql;
CREATE TRIGGER trg_emp_insert INSTEAD OF INSERT ON emp_view FOR EACH ROW EXECUTE FUNCTION emp_view_insert();
INSERT INTO emp_view (id, name) VALUES (1, 'Alice');
SELECT * FROM employees WHERE id = 1;

What will the SELECT query return?

medium

A. Empty result because views cannot be inserted

B. No rows

C. Syntax error on INSERT

D. (1, 'Alice')

Solution

1. Step 1: Understand the INSTEAD OF trigger effect

   The trigger function inserts the new row into the base table employees when an insert is done on the view emp_view.

2. Step 2: Trace the INSERT and SELECT

   The INSERT INTO emp_view triggers emp_view_insert(), which inserts (1, 'Alice') into employees. The SELECT then finds this row.

3. Final Answer:

   (1, 'Alice') -> Option D

4. Quick Check:

   INSTEAD OF trigger inserts row into employees [OK]

Hint: INSTEAD OF triggers redirect inserts to base tables [OK]

Common Mistakes:

• Assuming views cannot be inserted into
• Expecting syntax error on insert
• Forgetting trigger function inserts data

4. You wrote this trigger function for an INSTEAD OF UPDATE trigger on a view:

CREATE FUNCTION trg_update() RETURNS trigger AS $$ BEGIN UPDATE employees SET name = NEW.name WHERE id = NEW.id; END; $$ LANGUAGE plpgsql;

When you try to update the view, you get an error. What is the problem?

medium

A. The function does not return a value

B. The trigger should be BEFORE, not INSTEAD OF

C. The UPDATE statement syntax is incorrect

D. The trigger function must be written in SQL, not plpgsql

Solution

1. Step 1: Check trigger function requirements

   Trigger functions must return a value, usually NEW or OLD, depending on the trigger type.

2. Step 2: Identify missing RETURN statement

   The function lacks a RETURN statement, causing an error when the trigger tries to get a result.

3. Final Answer:

   The function does not return a value -> Option A

4. Quick Check:

   Trigger functions must return NEW or OLD [OK]

Hint: Always RETURN NEW or OLD in trigger functions [OK]

Common Mistakes:

• Omitting RETURN in trigger functions
• Confusing INSTEAD OF with BEFORE triggers
• Assuming plpgsql is not allowed

5. You want to create an updatable view for a table products(id, name, price) that only allows updating the price through the view. Which approach using an INSTEAD OF trigger is correct?

hard

A. Create an INSTEAD OF UPDATE trigger that updates all columns in products regardless of input

B. Create a BEFORE UPDATE trigger on the view that blocks changes to name and id

C. Create an INSTEAD OF UPDATE trigger that updates only the price column in products and returns NEW

D. Create a rule instead of a trigger to handle updates on the view

Solution

1. Step 1: Understand the goal

   The view should allow updating only the price column, so the trigger must update only that column in the base table.

2. Step 2: Choose the correct trigger approach

   INSTEAD OF UPDATE triggers on views let you control exactly what updates happen. Updating only price and returning NEW is correct.

3. Final Answer:

   Create an INSTEAD OF UPDATE trigger that updates only the price column in products and returns NEW -> Option C

4. Quick Check:

   INSTEAD OF triggers control updates precisely [OK]

Hint: INSTEAD OF UPDATE triggers can limit which columns update [OK]

Common Mistakes:

• Updating all columns ignoring restrictions
• Using BEFORE triggers on views (not supported)
• Using rules instead of triggers for complex logic