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Subquery

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Subquery

Subquery sits in one exam. Dev’s job is class topper. Write that first.

Subquery order — meaning → WHERE marks = (SELECT MAX…) → one limit. Dumping ten features helps nobody.

Without Subquery, one exam gets messy and class topper is hard to trust.

Use Subquery when class topper must stay clear. If a simpler SQL step works, use that instead.

Don’t do this with Subquery — correlated subquery on every row with no need. Interviewers spot it in ten seconds.

Place Subquery next to nearby SQL work — WHERE marks = (SELECT MAX…) is the link.

One breath for Subquery, then class topper, then correlated subquery on every row with no need. Sit down.

Exam tip

Say Subquery in one breath, then WHERE marks = (SELECT MAX…), then correlated subquery on every row with no need.

Subquery — sample query

CREATE TABLE a (id INTEGER, label TEXT);
CREATE TABLE b (id INTEGER, label TEXT);
INSERT INTO a VALUES (1, 'x'), (2, 'y');
INSERT INTO b VALUES (2, 'y'), (3, 'z');
SELECT label FROM a
UNION
SELECT label FROM b;

Subquery — uNION merges result sets and drops duplicate rows.

Short notes

DefSubquery — Dev uses it for class topper in one exam.
RuleSubquery → WHERE marks = (SELECT MAX…).
RememberSubquery + the query planner (one exam).
UseSubquery in one exam (class topper).
TrapSubquery — correlated subquery on every row with no need.
ExSubquery → class topper.

CREATE TABLE a (id INTEGER, label TEXT); CREATE TABLE b (id INTEGER, label TEXT); INSERT INTO a VALUES (1, 'x'), (2, 'y'); INSERT INTO b VALUES (2, 'y'), (3, 'z'); SELECT label FROM a UNION SELECT label FROM b;

Subquery

Subquery — sample query

Short notes

Questions

Subquery

Subquery — sample query

Short notes

Questions