as-statement-39.psql

/**
The AS claue allows us to create alias on columns.

Example syntax:
SELECT column AS new_name
FROM table;
*/

-- Let's get the SUM of the amount in the payment table but output it to the customer as a more readable alias.

SELECT SUM(amount) AS net_revenue
FROM payment; -- Will output net_revenue: 61312.04

/**
The AS operator gets executed at the very end of a query,
meaning that we can not use the alias inside a WHERE opeator.

Let's walk through some examples.
*/

SELECT COUNT(*) AS num_transactions
FROM payment;

-- An example where you can not use an alias to filter by.
-- Example:

SELECT customer_id, SUM(amount)
FROM payment
GROUP BY customer_id
HAVING SUM(amount) > 100;


-- Same query as above except with an alias on sum.
SELECT customer_id, SUM(amount) AS total_spent
FROM payment
GROUP BY customer_id
hAVING SUM(amount) < 100; 
-- HAVING SUM(total_spent) 
-- These AS statements are executed at the very end of the query, that's why this won't work.