SQL · Theory

Constraints

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Constraints

Constraints is a SQL idea you prove with lookup by roll, not with a slogan.

Constraints order — meaning → PRIMARY KEY(roll) + INDEX → one limit. Dumping ten features helps nobody.

Without Constraints, a big table gets messy and lookup by roll is hard to trust.

Constraints shows up in a big table. Name lookup by roll, not “a real-world scenario”.

Constraints trap — indexing a low-cardinality flag only. Arun loses marks for that every viva.

After Constraints, Arun should still remember indexing a low-cardinality flag only.

Close Constraints with — “If I skip it, lookup by roll goes wrong like this: indexing a low-cardinality flag only.”

Exam tip

Say Constraints in one breath, then PRIMARY KEY(roll) + INDEX, then indexing a low-cardinality flag only.

Constraints — sample query

-- Constraints
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;

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

Short notes

DefConstraints — Arun uses it for lookup by roll in a big table.
RuleConstraints → PRIMARY KEY(roll) + INDEX.
RememberConstraints + InnoDB (a big table).
UseConstraints in a big table (lookup by roll).
TrapConstraints — indexing a low-cardinality flag only.
ExConstraints → lookup by roll.

-- Constraints 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;

Constraints

Constraints — sample query

Short notes

Questions

Constraints

Constraints — sample query

Short notes

Questions