How to fix Undefined object in PostgreSQL

-- WRONG: misspelled table name
SELECT * FROM user_table;  -- Error: relation "user_table" does not exist

-- CORRECT: proper table name
SELECT * FROM users;

-- Created as uppercase
CREATE TABLE "Users" (id SERIAL);

-- WRONG: fails because it looks for "users" (lowercase)
SELECT * FROM users;  -- Error: relation "users" does not exist

-- CORRECT: quote the identifier to preserve case
SELECT * FROM "Users";

-- Check if a table exists
SELECT EXISTS (
  SELECT 1 FROM information_schema.tables 
  WHERE table_name = 'users'
);

-- Find which schema contains the object
SELECT table_schema FROM information_schema.tables 
WHERE table_name = 'users';

-- List all tables in current schema
\dt

-- List all tables across all schemas
\dt *.*

-- Object is in "public" schema
SELECT * FROM public.users;

-- Object is in "archive" schema
SELECT * FROM archive.users;

SET search_path TO public, archive;
SELECT * FROM users;  -- Will find users in public or archive

-- Migration 001: Create base table
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  email VARCHAR(255) UNIQUE
);

-- Migration 002: Create dependent table
CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES users(id)  -- users must exist first
);

CREATE TABLE IF NOT EXISTS users (
  id SERIAL PRIMARY KEY
);

CREATE TABLE IF NOT EXISTS posts (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES users(id)
);