Created
October 1, 2011 16:14
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MySQL WTF?
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Dvě tabulky – tabulka 'a' obsahuje pouze primární klíč 'id', tabulka 'b' obsahuje dva cizí klíče 'a1' a 'a2' – oba odkazují na 'a.id' – s vlastnostní ON DELETE CASCADE | |
CREATE TABLE `a` ( | |
`id` int(11) NOT NULL, | |
PRIMARY KEY (`id`) | |
) ENGINE=InnoDB DEFAULT CHARSET=utf8; | |
CREATE TABLE `b` ( | |
`a1` int(11) NOT NULL, | |
`a2` int(11) NOT NULL, | |
KEY `a1` (`a1`), | |
KEY `a2` (`a2`), | |
CONSTRAINT `b_ibfk_2` FOREIGN KEY (`a2`) REFERENCES `a` (`id`) ON DELETE CASCADE, | |
CONSTRAINT `b_ibfk_1` FOREIGN KEY (`a1`) REFERENCES `a` (`id`) ON DELETE CASCADE | |
) ENGINE=InnoDB DEFAULT CHARSET=utf8; | |
Naplníme je daty... | |
INSERT INTO `a` (`id`) VALUES (1), (2), (3), (4), (5); | |
INSERT INTO `b` (`a1`, `a2`) VALUES (1,1), (1,2), (1,3); | |
Tento select: | |
SELECT a2 FROM b WHERE a1 = 1; | |
Očekávaně vrátí: 1, 2, 3 | |
Ale pokud ho použijeme jako vnořený v DELETE: | |
DELETE FROM a WHERE id IN (SELECT a2 FROM b WHERE a1 = 1); | |
Smaže se pouze jeden řádek – ten s id 1. Po jeho smazání zafunguje kaskádové smazání řádků z tabulky 'b' a záhadně se vytratí i z výsledku toho vnořeného selectu. | |
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