[Solved] This trigger function is intended to prevent empty usernames:CREATE FUNCTION check_username() RETURNS trigger AS $ BEGIN IF NEW.username = '' THEN RAISE EXCEPTION 'Username cannot be empty'; END IF;... | PostgreSQL

PostgreSQL - Triggers in PostgreSQL

This trigger function is intended to prevent empty usernames:

CREATE FUNCTION check_username() RETURNS trigger AS $$ BEGIN IF NEW.username = '' THEN RAISE EXCEPTION 'Username cannot be empty'; END IF; RETURN NEW; END; $$ LANGUAGE plpgsql;

But empty usernames are still inserted. What is the problem?

AThe trigger function is missing a LANGUAGE declaration

BThe trigger is defined as AFTER INSERT instead of BEFORE INSERT

CThe comparison should use IS NULL instead of = ''

DThe function does not return OLD

Step-by-Step Solution

Solution:

Step 1: Understand trigger timing effect
AFTER INSERT triggers run after data is inserted, so cannot block insertion.
Step 2: Identify correct timing for validation
BEFORE INSERT triggers run before insert and can prevent invalid data.
Final Answer:
The trigger is defined as AFTER INSERT instead of BEFORE INSERT -> Option B
Quick Check:
Use BEFORE triggers to block invalid inserts [OK]

Quick Trick: Use BEFORE triggers to stop invalid data insertion [OK]

Common Mistakes:

Using AFTER triggers for validation
Confusing empty string with NULL
Omitting LANGUAGE in function

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This trigger function is intended to prevent empty usernames:

Step 1: Understand trigger timing effect

Step 2: Identify correct timing for validation

Final Answer:

Quick Check:

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