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2. How SQL runs a query

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You write a query top to bottom, starting with SELECT. But the database doesn't run it in that order. Knowing the real order clears up most early confusion — especially "why can't I use my alias here?"

The order you write vs. the order it runs

A fuller query has more parts:

SELECT   title, rental_rate          -- 5. pick columns to show
FROM     film                        -- 1. start with this table
WHERE    rental_rate > 2.99          -- 2. keep only rows that match
ORDER BY rental_rate DESC            -- 6. sort the result
LIMIT    5;                          -- 7. keep only the first few

The database actually works through it like this:

  1. FROM — get the rows from the table.
  2. WHERE — throw away rows that don't match the condition.
  3. GROUP BY — bundle rows into groups (later module).
  4. HAVING — filter those groups (later module).
  5. SELECT — pick or compute the columns to return.
  6. ORDER BY — sort what's left.
  7. LIMIT — take just the top N rows.

So the machine grabs the data first and decides what columns to show almost last. That single fact explains a lot.

Why the order matters

Because WHERE runs before SELECT, a name you invent in SELECT (an alias) doesn't exist yet when WHERE runs:

SELECT title, rental_rate * 100 AS price_cents
FROM film
WHERE rental_rate > 2.99   -- fine: rental_rate is a real column
ORDER BY price_cents;      -- fine: ORDER BY runs AFTER SELECT, so the alias exists

ORDER BY can use price_cents because sorting happens after the columns are computed. Trying to write WHERE price_cents > 299 would fail — that alias isn't born yet. (You'd repeat the expression, or wrap the query — both covered later.)

Try it

List the title and length of every film rated 'NC-17', sorted by title in alphabetical order.

SELECT title, length
FROM film
WHERE rating = 'NC-17'
-- add sorting by title

SELECT title, length
FROM film
WHERE rating = 'NC-17'
ORDER BY title;

Keep the 7-step order in the back of your mind. Every clause you learn from here slots into one of those steps.

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