Before you can read the parent note, you need to earn every shape, letter and symbol it throws at you. This page introduces them one at a time, from absolute zero — plain words first, then the picture, then why the topic can't live without it.
Why the topic needs it: your whole database is built out of these boxes. If you can't name the things, you have nothing to store. Everything else (facts, links, counts) hangs off these boxes.
A single specific thing — student "Asha", id 7 — is one instance (one filled-in copy of the box). All students together form the entity set (the whole stack of copies). Keep this three-level idea in your head: type (Student) → set (all students) → instance (Asha).
Why the topic needs it: a box named Student with nothing inside is useless. Attributes are what you actually store. The parent note spends a whole section on the different flavours of attribute, so let us define each flavour's picture now.
The key attribute deserves special attention because the whole idea of tables (Primary keys and Foreign keys) rests on it.
Why the topic needs it: boxes on their own are isolated islands. Real data is about connections — who owns what, who belongs where. The diamond captures those verbs so you can later turn them into foreign keys and joins.
Why this tool and not another: we need a compact way to say "the size of a group". Words like "the number of students" get long fast; the bars ∣⋅∣ are the standard shorthand for size of a collection. (They also mean "distance from zero" for ordinary numbers, but here every entry is a thing, so it can only mean count.)
Why these tools: cardinality is about limits, not exact totals. The pair ≤ and min are exactly the vocabulary of "no more than" and "the tighter of two limits" — precisely the questions cardinality asks.
Why two different letters: if the note wrote N:N you might think both sides have the same count. Using M and N deliberately says "these two 'many's are unrelated numbers." This distinction is what forces a junction table later.
Why the topic needs it, separate from cardinality: cardinality answers how many (the max), participation answers whether it is mandatory (the min). They are two independent knobs — the parent note's Mistake 4 is confusing them. Keep them apart: count vs must-you.
Read it top-down: boxes and their key facts define what exists; the counting symbols let you bound the links; ratios plus participation fully describe a relationship; and that determines where the foreign key lives when you finally build tables in SQL CREATE TABLE and constraints.
