SQL · Theory

EquiJoin

Theory

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EquiJoin

EquiJoin is a SQL idea you prove with EquiJoin on Riya's board, not with a slogan.

EquiJoin on the board: one EquiJoin step Riya can write from memory. Then say what EquiJoin on Riya's board looks like after.

Without EquiJoin, a small live site gets messy and EquiJoin on Riya's board is hard to trust.

Use EquiJoin when EquiJoin on Riya's board must stay clear. If a simpler SQL step works, use that instead.

If EquiJoin on Riya's board breaks under EquiJoin, check copy-pasting EquiJoin without a dry-run first.

After EquiJoin, Riya should still remember copy-pasting EquiJoin without a dry-run.

Close EquiJoin with: “If I skip it, EquiJoin on Riya's board goes wrong like this: copy-pasting EquiJoin without a dry-run.”

Diagram

  students.id
       =
  marks.student_id
        │
        ▼
     one result row

Exam tip

For EquiJoin: definition + a small live site + one failure.

EquiJoin — sample query

-- EquiJoin
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

EquiJoin: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefEquiJoin — Riya uses it for EquiJoin on Riya's board in a small live site.
RuleEquiJoin → one EquiJoin step Riya can write from memory.
RememberEquiJoin + the query planner (a small live site).
UseEquiJoin in a small live site (EquiJoin on Riya's board).
TrapEquiJoin — copy-pasting EquiJoin without a dry-run.
ExEquiJoin → EquiJoin on Riya's board.

-- EquiJoin CREATE TABLE demo ( id INTEGER PRIMARY KEY, label TEXT NOT NULL, qty INTEGER DEFAULT 0 ); INSERT INTO demo (id, label, qty) VALUES (1, 'alpha', 2), (2, 'beta', 5); SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

EquiJoin

EquiJoin — sample query

Short notes

Questions

EquiJoin

EquiJoin — sample query

Short notes

Questions