Columns और expressions चुनना

SELECT id, email, full_name
FROM users;

SELECT id,
       price,
       quantity,
       price * quantity AS subtotal,
       UPPER(email) AS email_upper
FROM order_items;

SELECT first_name || ' ' || last_name AS full_name,
       EXTRACT(YEAR FROM created_at) AS signup_year
FROM users;

SELECT first_name || ' ' || last_name FROM users;
SELECT CONCAT(first_name, ' ', last_name) FROM users;

-- ||  with NULL → NULL
-- CONCAT skips NULLs (treats them as empty strings)

SELECT DISTINCT country FROM users;
SELECT DISTINCT country, city FROM users;  -- distinct pairs

-- Most recent login per user:
SELECT DISTINCT ON (user_id) user_id, login_at, ip
FROM logins
ORDER BY user_id, login_at DESC;

SELECT id,
       total,
       CASE
         WHEN total >= 1000 THEN 'whale'
         WHEN total >=  100 THEN 'regular'
         ELSE 'small'
       END AS bucket
FROM orders;

-- Single quotes: string literal
SELECT 'hello' AS greeting;

-- Double quotes: identifier (column/table name with special chars or case)
SELECT "First Name" FROM users;

-- Mixing them up is a common error.

SELECT '2024-01-15'::DATE AS d;
SELECT CAST('42' AS INTEGER) AS n;
SELECT (price::TEXT || ' USD') AS pretty FROM products;

SELECT u.email, o.total
FROM users u
JOIN orders o ON o.user_id = u.id;