SQL · Theory

If Exists

Theory

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If Exists

If Exists is simple if you keep it on paper. Asha uses it for Asha walking through If Exists in lab hour.

If Exists order: meaning → one If Exists step Asha can write from memory → one limit. Dumping ten features helps nobody.

Skip If Exists and copy-pasting If Exists without a dry-run shows up in lab hour.

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

If Exists miss — copy-pasting If Exists without a dry-run. Fix it before you talk about advanced SQL.

If Exists is not a lonely heading. Asha ties it to Asha walking through If Exists.

Close If Exists with: “If I skip it, Asha walking through If Exists goes wrong like this: copy-pasting If Exists without a dry-run.”

Exam tip

Say If Exists in one breath, then one If Exists step Asha can write from memory, then copy-pasting If Exists without a dry-run.

If Exists — 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;

If Exists — uNION merges result sets and drops duplicate rows.

Short notes

DefIf Exists — Asha uses it for Asha walking through If Exists in lab hour.
RuleIf Exists → one If Exists step Asha can write from memory.
RememberIf Exists + the query planner (lab hour).
UseIf Exists in lab hour (Asha walking through If Exists).
TrapIf Exists — copy-pasting If Exists without a dry-run.
ExIf Exists → Asha walking through If Exists.

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;

If Exists

If Exists — sample query

Short notes

Questions

If Exists

If Exists — sample query

Short notes

Questions