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Union

Union sits in two year tables. Arun’s job is 2024 + 2025 names. Write that first.

For Union, the query planner is in play. Don’t blame it until you have traced 2024 + 2025 names.

Skip Union and UNION ALL when you needed distinct shows up in two year tables.

Union shows up in two year tables. Name 2024 + 2025 names, not “a real-world scenario”.

Union trap — UNION ALL when you needed distinct. Arun loses marks for that every viva.

After Union, Arun should still remember UNION ALL when you needed distinct.

Close Union with — “If I skip it, 2024 + 2025 names goes wrong like this: UNION ALL when you needed distinct.”

Exam tip

Board: 2024 + 2025 names. Dry-run Union. Name the trap: UNION ALL when you needed distinct.

Union — 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;

UNION merges result sets and drops duplicate rows.

Short notes

DefUnion — Arun uses it for 2024 + 2025 names in two year tables.
RuleUnion → UNION / UNION ALL.
RememberUnion + the query planner (two year tables).
UseUnion in two year tables (2024 + 2025 names).
TrapUnion — UNION ALL when you needed distinct.
ExUnion → 2024 + 2025 names.

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;

Union

Union — sample query

Short notes

Questions

Union

Union — sample query

Short notes

Questions