Hiep Nguyen schrieb:
hi list,
reading manual on mysql regarding auto_increment with multiple-column
index:
CREATE TABLE animals (
grp ENUM('fish','mammal','bird') NOT NULL,
id MEDIUMINT NOT NULL AUTO_INCREMENT,
name CHAR(30) NOT NULL,
PRIMARY KEY (grp,id)
);
INSERT INTO animals (grp,name) VALUES
('mammal','dog'),('mammal','cat'),
('bird','penguin'),('fish','lax'),('mammal','whale'),
('bird','ostrich');
SELECT * FROM animals ORDER BY grp,id;
+--------+----+---------+
| grp | id | name |
+--------+----+---------+
| fish | 1 | lax |
| mammal | 1 | dog |
| mammal | 2 | cat |
| mammal | 3 | whale |
| bird | 1 | penguin |
| bird | 2 | ostrich |
+--------+----+---------+
my question is what id would be if i:
UPDATE `animals` SET `grp`='mammal' WHERE `grp`='bird' AND `id`='2' AND
`name`='ostrich' LIMIT 1;
you do not need `name`='ostrich' in WHERE, cause grp,id are already UNIQUE
your key is grp,id (bird,2)
but your query will fail, because there is already grp,id (mammal,2) and
therre can not be two identical UNIQUE (PRIMARY) keys
--
Sebastian
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