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Extract

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Extract

Extract is a SQL idea you prove with Dev walking through Extract, not with a slogan.

Do Extract once by hand. one Extract step Dev can write from memory. Change one input. Say the new result out loud.

Without Extract, lab hour gets messy and Dev walking through Extract is hard to trust.

Use Extract when Dev walking through Extract must stay clear. If a simpler SQL step works, use that instead.

Extract trap — only reciting the name of Extract. Dev loses marks for that every viva.

Place Extract next to nearby SQL work — one Extract step Dev can write from memory is the link.

Close Extract with: “If I skip it, Dev walking through Extract goes wrong like this: only reciting the name of Extract.”

Exam tip

Board: Dev walking through Extract. Dry-run Extract. Name the trap: only reciting the name of Extract.

Extract — sample query

-- Extract
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;

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

Short notes

DefExtract — Dev uses it for Dev walking through Extract in lab hour.
RuleExtract → one Extract step Dev can write from memory.
RememberExtract + InnoDB (lab hour).
UseExtract in lab hour (Dev walking through Extract).
TrapExtract — only reciting the name of Extract.
ExExtract → Dev walking through Extract.

-- Extract 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;

Extract

Extract — sample query

Short notes

Questions

Extract

Extract — sample query

Short notes

Questions