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By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0. 3601504 fiddles created (47950 in the last week).
select version();
version()
5.6.47
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 hidden batch(es)
CREATE TABLE x1 (y INTEGER NOT NULL);
✓
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CREATE TABLE a1 (b INTEGER NOT NULL, INDEX(b));
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INSERT INTO x1 VALUES (1), (1), (2), (2), (3), (3), (4), (5);
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INSERT INTO a1 VALUES (3), (3), (4), (5), (6), (6), (7), (7), (8);
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 hidden batch(es)
-- SELECT * FROM x1; SELECT * FROM a1;
✓
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-- Query 1 - same result as for Query 2 - DISTINCT is redundant SELECT y FROM x1 UNION SELECT b FROM a1;
y
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-- Query 2 - same result as for Query 1 - DISTINCT is redundant SELECT y FROM x1 UNION SELECT DISTINCT b FROM a1;
y
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-- Query 3 - UNION ALL - all dupes are included SELECT y FROM x1 UNION ALL SELECT b FROM a1;
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 hidden batch(es)
SELECT y FROM x1 UNION DISTINCT SELECT b FROM a1;
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