table with records that can be arranged - mysql

Question

How can I make a table that has rows with order to one another and can be rearranged:
Example:
Rows:idappearance name
Records
(1,"john"),(2,"mike")
Now, I want to insert "Avi" between them:
not having to worry about rearranging them
(1,"john"),(2,"Avi"),(3,"mike")
This table can have a foreign key in
another table (like departments..).
idappearance is the order of appearance I want
to set, doesn't need to be PK.
It needs to handle about 50K of names so O(n) isn't best solution.

Answer 1

Simple solution would be having reasonable numerical gaps between records. In other words;
(10000,"John"),(20000,"mike")
(10000,"John"),(15000,"Avi"),(20000,"mike")
(10000,"John"),(12500,"tom"),(15000,"avi"),(20000,"mike")
etc..
Gap between records should be determined based on your data domain

Answer 2

You could have a trigger on inserts. I don't use MySQL, but here's the code for sql-server...
Basically, on an insert, the trigger increments the appearanceId of all rows with appearanceId which are equal to or greater than the new appearance id.
CREATE Table OrderedTable
(
id int IDENTITY,
name varchar(50),
appearanceOrder int
)
GO
CREATE TRIGGER dbo.MyTrigger
ON dbo.OrderedTable
AFTER INSERT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
UPDATE OrderedTable SET
AppearanceOrder = AppearanceOrder + 1
WHERE AppearanceOrder >= (
SELECT TOP 1 AppearanceOrder
FROM inserted )
AND id NOT IN (
SELECT id
FROM inserted )
END
GO
INSERT INTO OrderedTable VALUES ('Alice', 1)
INSERT INTO OrderedTable VALUES ('Bob', 1)
INSERT INTO OrderedTable VALUES ('Charlie', 1)
INSERT INTO OrderedTable VALUES ('David', 1)
This returns David, Charlie, Bob, Alice as expected.
SELECT *
FROM OrderedTable
ORDER BY AppearanceOrder
Note that I haven't fully tested this implementation. One issue is that it will leave holes in the AppearanceOrder if items are deleted, or the inserts deliberately insert outside the current range. If these matter, they are left as an exercise to the reader ;-)

Answer 3

If appearance order were a double-precision floating point number, you could insert any name between any two adjacent names with a single insert. If you start with a table like this:
create table test (
person_id integer primary key,
person_name varchar(10) not null,
appearance_order double precision not null unique
);
insert into test values (100, 'John', 11);
insert into test values (23, 'Mike', 12);
Insert Avi between them by simply
insert into test values (3, 'Avi', 11.5);
Sort by the column 'appearance_order'.
select * from test order by appearance_order
100 John 11
3 Avi 11.5
23 Mike 12
Insert Eva between John and Avi by
insert into test values (31, 'Eva', 11.25);
select * from test order by appearance_order
100 John 11
31 Eva 11.25
3 Avi 11.5
23 Mike 12
You do need to separate identification from sort order. That means using one column for the id number (and as the target for foreign key references) and another for the appearance order. But, depending on your application, you might not need a unique constraint on appearance_order.