问题
When designing a database, what decisions do you consider when deciding how big your nvarchar should be.
If i was to make an address table my gut reaction would be for address line 1 to be nvarchar(255) like an old access database.
I have found using this has got me in bother with the old 'The string would be truncated'. I know that this can be prevented by limiting the input box but if a user really has a address line one that is over 255 this should be allowed.
How big should I make my nvarchar(????)
回答1:
My recommendation: make them just as big as you REALLY need them.
E.g. for a zip code column, 10-20 chars are definitely enough. Ditto for a phone number. E-Mails might be longer, 50-100 chars. Names - well, I usually get by with 50 chars, ditto for first names. You can always and easily extend fields if you really need to - that's no a big undertaking at all.
There's really no point in making all varchar/nvarchar fields as big as they can be. After all, a SQL Server page is fixed and limited to 8060 bytes per row. Having 10 fields of NVARCHAR(4000) is just asking for trouble.... (since if you actually try to fill them with too much data, SQL Server will barf at you).
If you REALLY need a really big field, use NVARCHAR/VARCHAR(MAX) - those are stored in your page, as long as they fit, and will be sent to "overflow" storage if they get too big.
NVARCHAR vs. VARCHAR: this really boils down to do you really need "exotic" characters, such as Japanese, Chinese, or other non-ASCII style characters? In Europe, even some of the eastern European characters cannot be represented by VARCHAR fields anymore (they will be stripped of their hachek (? spelling ?). Western European languages (English, German, French, etc.) are all very well served by VARCHAR.
BUT: NVARCHAR does use twice as much space - on disk and in your SQL Server memory - at all times. If you really need it, you need it - but do you REALLY ? :-) It's up to you.
Marc
回答2:
I don't use nvarchar personally :-) I always use varchar.
However, I tend to use 100 for name and 1000 for comments. Trapping and dealing with longer strings is something the client can do, say via regex, so SQL only gets the data it expects.
You can avoid truncation errors be parameterising the calls, for example via stored procs. If the parameter is defined as varchar(200), say, then truncation happens silently if you send > 200. The truncation error is thrown only for an INSERT or UPDATE statement: with parameters it won't happen.
The 255 "limit" for SQL Server goes back to 6.5 because vachar was limited to 255. SQL Server 7.0 + changed to 8000 and added support for unicode
Edit:
Why I don't use nvarchar: Double memory footprint, double index size, double disk size, simply don't need it. I work for a big Swiss company with offices globally so I'm not being parochial.
Also discussed here: varchar vs nvarchar performance
On further reflection, I'd suggest unicode appeals to client developers but as a developer DBA I focus on performance and efficiency...
回答3:
It depends on what the field represents. If I'm doing a quick prototype I leave the defaults of 255. For anything like comments etc I'd probably put it to 1000.
The only way I'd make it smaller really is on things I definately know the siez of, zip codes or NI numbers etc.
回答4:
For columns that you need to have certain constraints on - like names, emails, addresses, etc - you should put a reasonably high max length. For instance a first name of more than 50 characters seems a bit suspicious and an input above that size will probably contain more that just a first name. But for the initial design of a database, take that reasonable size and double it. So for first names, set it to 100 (or 200 if 100 is your 'reasonable size'). Then put the app in production, let the users play around for a sufficiently long time to gather data and then check the actual max(len(FirstName)). Are there any suspicious values there? Anything above 50 chars? Find out what's in there and see if it's actually a first name or not. If it's not, the input form probably needs better explanations/validations.
Do the same for comments; Set them to nvharchar(max) initially. Then come back when your database has grown enough for you to start optimizing performance. Take the max length of the comments, double it and you have a good max length for your column.
来源：https://stackoverflow.com/questions/919871/how-big-do-you-make-your-nvarchar