SQL · Theory

Union

Theory

296/439

Union

UNION stacks results. SELECT name FROM y2024 UNION SELECT name FROM y2025; Unique names from both years. Column count must match. UNION ALL keeps duplicates and is cheaper.

ORDER BY after the last SELECT, in brackets if needed. Types should be compatible (both names, not name vs marks).

Trap — UNION when you needed JOIN. Trap: UNION ALL vs UNION mixed up in a viva.

On the example next to this theory — UNION merges result sets and drops duplicate rows.

Exam tip

SELECT name FROM y2024 UNION SELECT name FROM y2025;

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 stacks SELECTs. UNION removes duplicates.
RuleSame column count. UNION ALL keeps dupes.
TrapUNION vs JOIN.

SQL · Theory

Union

← All stacks

Theory

296/439

Union

UNION stacks results. SELECT name FROM y2024 UNION SELECT name FROM y2025; Unique names from both years. Column count must match. UNION ALL keeps duplicates and is cheaper.

ORDER BY after the last SELECT, in brackets if needed. Types should be compatible (both names, not name vs marks).

Trap — UNION when you needed JOIN. Trap: UNION ALL vs UNION mixed up in a viva.

On the example next to this theory — UNION merges result sets and drops duplicate rows.

Exam tip

SELECT name FROM y2024 UNION SELECT name FROM y2025;

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 stacks SELECTs. UNION removes duplicates.
RuleSame column count. UNION ALL keeps dupes.
TrapUNION vs JOIN.

Union

Union — sample query

Short notes

Questions

Union

Union — sample query

Short notes

Questions