需要帮助简单的sql添加记录

We use (INSERT INTO) to insert a record in the table which creates more than one record when used again. Is there any way to add a record and alternately replacing the prevoius one without adding any new record. I know this would work:

UPDATE Customers
SET ContactName='Alfred Schmidt', City='Hamburg'
WHERE CustomerName='Alfreds Futterkiste';

But what if there is no condition ie. we don't know the record, we only know the column name. Is there any way to fill only one record and alternately replace the previous record without creating 2nd record?

I got my answer and It's working now! I used:

INSERT INTO data (a, b, c)
VALUES
('1','2','3')
ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b)

OK... updating if a record exists or creating a record if there are zero records is a pretty simple matter and you have a solution for it. That having been said, I would do something different and keep track of my message of the day by date:

-- This is REALLY BASIC, but, just to give you the idea...
CREATE TABLE [dbo].[MessageOfTheDay](
    [MessageDate] [date] not null,
    [MessageContents] [nvarchar](500) not null,
    UNIQUE (MessageDate)
)

declare @MessageContents nvarchar(500), @MessageDate date
set @MessageContents = 'This is the new MOTD!!!'
set @MessageDate = GETDATE()

-- Every day, create a new record and you can keep track of previous MOTD entries...
insert into MessageOfTheDay(MessageDate, MessageContents)
values (@MessageDate, @MessageContents)

-- Get the message for today
select MessageContents from MessageOfTheDay where MessageDate = @MessageDate

-- If you want, you can now create messages for FUTURE days as well:
set @MessageContents = 'This is tomorrow''s MOTD!!!';
set @MessageDate = dateadd(D, 1,GETDATE())

insert into MessageOfTheDay(MessageDate, MessageContents)
values (@MessageDate, @MessageContents)

-- Get tomorrow's message
select MessageContents from MessageOfTheDay where MessageDate = @MessageDate

-- If you aren't necessarily going to have one per day and want to always just show the most recent entry
select MessageContents from MessageOfTheDay order by MessageDate desc limit 1

Anyway, that's just my $.02. At some point I bet you will want to look over the history of your MOTD and when you do, you will be happy that you have that history. Plus, this more accurately models the data you are trying to represent.