我正在编写一些软件,需要将项目存储在数据库中,这些项目需要具有"优先级",因此我们最终使用
ID | Name | Priority
--------+--------------+----------
1 | Pear | 4
2 | Apple | 2
3 | Orange | 1
4 | Banana | 3
所以现在,最重要的水果是橘子,然后是苹果,然后是香蕉,然后是梨。
现在,我想把梨放在第一位,所以梨,桔子,苹果,香蕉。表格将看起来像:
ID | Name | Priority
--------+--------------+----------
1 | Pear | 1
2 | Apple | 3
3 | Orange | 2
4 | Banana | 4
用PHP和Postgres实现这一点的最佳方法是什么。考虑到表格不会超过12-13个项目,我考虑过在更新所有内容之前选择整个表格并重写优先级。
*重要*
优先级可以按任何顺序更改,因此优先级7可以设置为优先级3(从而将优先级3以下的所有项目都下移一级),我们需要缩小优先级7的项目在优先级列表中留下的差距。
好的,这是我尝试保持优先级的唯一性和连续性。通过触发器+函数实现。最困难的部分是避免触发器内的更新可能导致的无限递归。这可以通过一个脏/彩色标志来解决,这个标志必须放在桌子里面。它的价值并不重要;只是它的变化。
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE fruits
( id INTEGER NOT NULL PRIMARY KEY
, zname varchar NOT NULL
, priority INTEGER NOT NULL
, flipflag boolean NOT NULL default false
, CONSTRAINT unique_priority UNIQUE (priority) DEFERRABLE INITIALLY DEFERRED
);
INSERT INTO fruits(id,zname,priority) VALUES
(1 , 'Pear' ,4)
,(2 , 'Apple' ,2)
,(3 , 'Orange' ,1)
,(4 , 'Banana' ,3)
;
CREATE function shift_priority()
RETURNS TRIGGER AS $body$
BEGIN
UPDATE fruits fr
SET priority = priority +1
, flipflag = NOT flipflag -- alternating bit protocol ;-)
WHERE NEW.priority < OLD.priority
AND OLD.flipflag = NEW.flipflag -- redundant condition
AND fr.priority >= NEW.priority
AND fr.priority < OLD.priority
AND fr.id <> NEW.id -- exlude the initiating row
;
UPDATE fruits fr
SET priority = priority -1
, flipflag = NOT flipflag
WHERE NEW.priority > OLD.priority
AND OLD.flipflag = NEW.flipflag
AND fr.priority <= NEW.priority
AND fr.priority > OLD.priority
AND fr.id <> NEW.id
;
RETURN NEW;
END;
$body$
language plpgsql;
CREATE TRIGGER shift_priority
AFTER UPDATE OF priority ON fruits
FOR EACH ROW
WHEN (OLD.flipflag = NEW.flipflag AND OLD.priority <> NEW.priority)
EXECUTE PROCEDURE shift_priority()
;
UPDATE fruits
SET priority = 1
WHERE id=1;
结果:
SELECT * FROM fruits ORDER BY id;
NOTICE: drop cascades to 2 other objects
DETAIL: drop cascades to table tmp.fruits
drop cascades to function tmp.shift_priority()
DROP SCHEMA
CREATE SCHEMA
SET
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "fruits_pkey" for table "fruits"
NOTICE: CREATE TABLE / UNIQUE will create implicit index "unique_priority" for table "fruits"
CREATE TABLE
INSERT 0 4
CREATE FUNCTION
CREATE TRIGGER
UPDATE 1
id | zname | priority | flipflag
----+--------+----------+----------
1 | Pear | 1 | f
2 | Apple | 3 | t
3 | Orange | 2 | t
4 | Banana | 4 | t
(4 rows)
一个事务中的两个更新应该可以在这么小的表上正常工作。
create temp table priorities (
id integer primary key,
name varchar(15) not null,
priority integer not null check (priority > 0 and priority < 100)
);
insert into priorities values
(1,'Pear',4),
(2,'Apple',2),
(3,'Orange',1),
(4,'Banana',3);
-- Make Pear priority 1.
begin;
update priorities
set priority = priority + 1
-- The value below is the priority you're aiming for. You want
-- Pear to be #1, so you use ">= 1".
where priority >= 1;
update priorities
set priority = 1 where name = 'Pear';
commit;
为了方便起见,您可以将其封装在存储过程中。