Question 1

Why can a SQL join return more rows than either table?

Accepted Answer

When a join key is duplicated, rows multiply. If key 'A' has 2 rows on the left and 3 on the right, an INNER join emits 2 x 3 = 6 rows for that key alone, so the output can exceed both inputs. This duplicate-key blowup is the fan a Venn diagram cannot draw, and joincraft draws it as a one-to-many ribbon.

Question 2

Why do NULL join keys never match, even NULL = NULL?

Accepted Answer

In SQL, NULL = NULL evaluates to UNKNOWN, not TRUE, so an ON clause never pairs two NULL keys. A NULL-key row is dropped by INNER, but survives LEFT/RIGHT/FULL with the other side padded to NULL. Confusingly, GROUP BY does the opposite: it collapses all NULLs into one bucket. joincraft shows both side by side.

Question 3

Does joincraft run a real SQL engine or send my data anywhere?

Accepted Answer

No. joincraft is a teaching model, not a SQL engine: you pick one join plus an optional GROUP BY from controls, and a hand-rolled evaluator computes the exact result. Its results were cross-checked against SQLite during authoring. Everything runs and stays in your browser — the page's Content-Security-Policy blocks all network requests, so nothing is ever sent anywhere. Export CSV or print the cheat-sheet as your handoff.

SQL joins explained: watch rows pair up—duplicates, NULLs and all

Gotcha scenarios

Two tables

The operation

The join, ribbon by ribbon

Row math

Result table

The NULL corner: three-valued logic

Honest limits