Databases for Beginners
by Sergey Okinchuk
Many-to-Many and the Junction TableTopic 10

Chapter Three · Asking Questions

Reading Data — SELECT

The Marquee's four tables are built; this chapter is where you finally start asking them things. Every page turns a plain-English question Lora actually has — what's on tonight, which films run under 100 minutes, how many bookings does each screening have — into a query you can read out loud, and shows the answer that comes back. By the end you can filter, sort, compute, and count, which covers most of the SQL most people ever write.

6 topics

So far the database has mostly listened: Chapter 2 declared the tables, the types, and the keys, and the data settled into its shape. Now the direction reverses. A query is a question written in SQL, and SELECT is the word that starts every question. This chapter has one promise, and it is the best promise in the book: every question Lora can say in English about her cinema can be said in SQL too, usually in one short sentence that reads almost the same way.

The six topics build one skill in layers. First the bare SELECT — ask for columns, get a table back. Then WHERE narrows the rows, ORDER BY and LIMIT put the answer in order and cut it to size, and expressions let a query compute things that were never stored. The last two topics change what a row of the answer even means: GROUP BY collapses many rows into counts and sums, and HAVING filters those groups the way WHERE filters rows. Nothing in this chapter ever changes the data — reading is perfectly safe, so you can be as curious as you like.

The chapter's whole promise — a question becomes a query becomes an answer
The question, in English"What's on tonight after seven?"
→
The question, in SQLSELECT screen, starts_at FROM screenings WHERE …
→
The answer — a tablethree rows: 19:30, 20:00, 21:45

Topics in This Chapter

Topic 10
Your First SELECT
SELECT which columns, FROM which table — the sentence shape behind every query you'll ever read. The poster wall as your first result, and the quiet fact that powers everything later: every query returns a table.
SQL
Topic 11
Filtering with WHERE
One true-or-false test per row: true stays, false goes. Comparisons, AND and OR with the parentheses trap that catches everyone, and the strange way NULL slips past every ordinary test.
SQL
Topic 12
Sorting and Limiting
Without ORDER BY there is no order — only luck. Sorting ascending and descending, tie-breaking with a second column, and why LIMIT only means "top 5" after the sorting is done.
SQL
Topic 13
Expressions, Aliases, and Functions
Queries can compute while they read: matinee prices derived on the fly, functions like ROUND and UPPER, and AS to give the new column a readable name. Store facts, derive conclusions.
SQL
Topic 14
Counting and Summing: GROUP BY
"How many bookings per screening?" changes what a row of the answer means. Aggregates collapse many rows into one number, GROUP BY decides what "many" is, and the iron rule keeps it honest.
SQL
Topic 15
HAVING: Filtering the Groups Themselves
WHERE runs before the groups exist, so it can't see their counts — HAVING runs after, and can. Plus the logical pipeline every query follows, drawn once and used forever.
SQL