问题
Say you have a table:
`item`
With fields:
`id` VARCHAR( 36 ) NOT NULL
,`order` BIGINT UNSIGNED NOT NULL
And:
Unique(`id`)
And you call:
INSERT INTO `item` (
`item`.`id`,`item`.`order`
) SELECT uuid(), `item`.`order`+1
MySql will insert the same uuid into all of the newly created rows.
So if you start with:
aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa, 0
bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb, 1
You'll end up with:
aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa, 0
bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb, 1
cccccccc-cccc-cccc-cccc-cccccccccccc, 1
cccccccc-cccc-cccc-cccc-cccccccccccc, 2
How do I command MySql to create a different uuid for each row?
I know that the following works as expected in MSSQL:
INSERT INTO item (
id,[order]
) SELECT newid(), [order]+1
n.b. I know I could SELECT the results, loop through them and issue a separate INSERT command for each row from my PHP code but I don't want to do that. I want the work to be done on the database server where it's supposed to be done.
回答1:
Turns out uuid() is generating a different uuid per row.
But instead of generating all the chunks randomly, as I would normally expect, MySql appears to only be generating the 2nd chunk randomly. Presumably to be more efficient.
So at a glance the uuids appear identical when in fact MySql has altered the 2nd chunk. e.g.
cccccccc-cccc-cccc-cccc-cccccccccccc
ccccdddd-cccc-cccc-cccc-cccccccccccc
cccceeee-cccc-cccc-cccc-cccccccccccc
ccccffff-cccc-cccc-cccc-cccccccccccc
I assume if there is a collision it would try again.
My bad.
回答2:
How do I command MySql to create a different uuid foreach row?
MySQL won't allow expressions as a default value. You can work around this by allowing the field to be null. Then add insert/update triggers which, when null, set the field to uuid().
回答3:
Please try with MID(UUID(),1,36) instead of uuid().
回答4:
MySQL's UUID() function generates V1 UUIDs, which are split into time, sequence and node fields. If you call it on a single node, only a few bits in the time field will be different; this is referred to as temporal uniqueness. If you call it on different nodes at the exact same time, the node fields will be different; this is referred to as spatial uniqueness. Combining the two is very powerful and gives a guarantee of universal uniqueness, but it also leaks information about the when and where each V1 UUID was created, which can be a security issue. Oops.
V4 UUIDs are generally more popular now because they hash that data (and more) together and thus don't leak anything, but you'll need a different function to get them--and beware what they'll do to performance if you have high INSERT volume; MySQL (at least for now) isn't very good at indexing (pseudo)random values, which is why V1 is what they give you.
回答5:
First generate an uniq string using the php uniqid() function and insert to the ID field.
来源：https://stackoverflow.com/questions/6300849/mysql-insert-select-uuid