Beginner with triggers - sql

Question

Im a beginner in database and i got this difficult auction database project.
Im using SQL Server Management Studio also.
create table user(
name char(10) not null,
lastname char(10) not null
)
create table item(
buyer varchar(10) null,
seller varchar(10) not null,
startprice numeric(5) not null,
description char(22) not null,
start_date datetime not null,
end_date datetime not null,
seller char(10) not null,
item_nummer numeric(9) not null,
constraint fk_user foreign key (buyer) references user (name)
)
Basically what the rule im trying to make here is:
Column buyer has NULL unless the time (start_date and end_date) is over and startprice didnt go up or increased. Then column buyer will get the name from table user who bidded on the item.
The rule is a bid too difficult for me to make, i was thinking to make a trigger, but im not sure..

Answer 1

Your model is incorrect. First you need a table to store the bids. Then when the auction is over, you update the highest one as the winning bid. Proably the best way is to have a job that runs once a minute and finds the winners of any newly closed auctions.
A trigger will not work on the two tables you have because triggers only fire on insert/update or delete. It would not fire because the time is past. Further triggers are an advanced technique and a db beginner should avoid them as you can do horrendous damage with a badly written trigger.
You could have a trigger that works on insert to the bids table, that updates the bid to be the winner and takes that status away from the previous winner. Then you simply stop accepting new bids at the time the auction is over. Your application could show the bidder who is marked as the winner as the elader if the auction is till open and teh winner if it is closed.

Answer 2

There are some initial problems with your schema that need addressed before tackling your question. Here are changes I would make to significantly ease the implementation of the answer:
-- Added brackets around User b/c "user" is a reserved keyword
-- Added INT Identity PK to [User]
CREATE TABLE [user]
(
UserId INT NOT NULL
IDENTITY
PRIMARY KEY
, name CHAR(10) NOT NULL
, lastname CHAR(10) NOT NULL
)
/* changed item_nummer (I'm not sure what a nummer is...) to ItemId int not null identity primary key
Removed duplicate Seller columns and buyer column
Replaced buyer/seller columns with FK references to [User].UserId
Add currentBid to capture current bid
Added CurrentHighBidderId
Added WinningBidderId as computed column
*/
CREATE TABLE item
(
ItemId INT NOT NULL
IDENTITY
PRIMARY KEY
, SellerId INT NOT NULL
FOREIGN KEY REFERENCES [User] ( UserId )
, CurrentHighBidderId INT NULL
FOREIGN KEY REFERENCES [User] ( UserId )
, CurrentBid MONEY NOT NULL
, StartPrice NUMERIC(5) NOT NULL
, Description CHAR(22) NOT NULL
, StartDate DATETIME NOT NULL
, EndDate DATETIME NOT NULL
)
go
ALTER TABLE dbo.item ADD
WinningBidderId AS CASE WHEN EndDate < CURRENT_TIMESTAMP
AND currentBid > StartPrice THEN CurrentHighBidderId ELSE NULL END
GO
With the additional columns a computed column can return the correct information. If you must return the winner's name instead of id, then you could keep the schema above the same, add an additional column to store the user's name, populate it with a trigger and keep the computed column to conditionally show/not show the winner..