高级使用自动增量？

Question

I'll try and make this question as brief as possible. I was wondering if there is a way to do the following via PHP & MySQL coding:

1. Delete an auto-incremented id entry (say entry 12 of 20), and at the same time lower the rest of the entries by 1. (Example: Delete entry 12 == Entry 13 becomes 12, 14 becomes 13, etc. etc.)

2. Manually (ie via form) insert an entry with an ID so that the rest are pushed up if specified by the user (and defaults to normal behaviour if nothing is specified. (Example: User specifies that he wants his entry to be ID 14, but there is already an ID 14 in the database. What should happen would be that the user input pushes the already existing entry up to 15, & the rest of the existing entries follow a similar fashion. If the user does not specify an ID, default to normal behaviour.)

I'm still learning MySQL & PHP and the code I need is for an image gallery system that I've been building for a while now.

Is it at all possible?

Answer 1

That is really not an efficient way to use auto-increment columns (databases are not designed to do that). You'd wind up doing effectively a table scan any time you inserted or removed an item.

If you want to have an ordering that you can insert at will, it'd be better to maintain a separate mapping (eg have a separate entry that stores a list of IDs that you can manipulate at will).

Answer 2

For task 1, you would need to manually set the AUTO_INCREMENT value for that table after modifying all of the IDs so there's no gap when the next insert happens. A far better way is to not use an auto-increment ID at all - have an ID column, and use MAX(id)+1 to find the required ID at the time of record insertion.

here's my psuedocode:

for a deletion:

$n = 12;
DELETE from items where id='$n';
UPDATE items SET id=id-1 WHERE id > '$n'

for a specified insertion:

$n = 15;
(if there is a record with id = 15)
   UPDATE items SET id=id+1 WHERE id >= '$n'
INSERT into items (id,..) values ('$n',...)

for an unspecified insertion

$n = SELECT MAX(id)+1 FROM items
INSERT into items (id,..) values ('$n',...)

Answer 3

For #1 try this SQL commands:

DELETE FROM table WHERE id=12;
UPDATE table set id=id-1 WHERE id > 12;
ALTER TABLE table AUTO_INCREMENT=AUTO_INCREMENT-1;

But it is a bad idea to use ID field like this. You shouldn't modify it's value after row was inserted into table.

Answer 4

You can use this for your 1) question. Run the whole block together. @id should be set to the row you want to delete.

SET @id = 12;
DELETE FROM tbl WHERE id = @id;
UPDATE  tbl set id = id-1 WHERE id > @id;
ALTER TABLE tbl AUTO_INCREMENT = 0;

MySQL disclaimer: You cannot reset the counter to a value less than or equal to any that have already been used. For MyISAM, if the value is less than or equal to the maximum value currently in the AUTO_INCREMENT column, the value is reset to the current maximum plus one. For InnoDB, you can use ALTER TABLE ... AUTO_INCREMENT = value as of MySQL 5.0.3, but if the value is less than the current maximum value in the column, no error occurs and the current sequence value is not changed.