Click here to Skip to main content
11,923,676 members (68,198 online)
Click here to Skip to main content
Add your own
alternative version


748 bookmarked

SQL Server DO's and DONT's

, 8 Nov 2002 CPOL
Rate this:
Please Sign up or sign in to vote.
SQL Server database best practices

SQL Server DO's and DON'Ts

So, you are now the leader of a SQL Server based project and this is your first one, perhaps migrating from Access. Or maybe you have performance problems with your SQL Server and don't know what to do next. Or maybe you simply want to know of some design guidelines for solutions using SQL Server and designing Database Access Layers (DAL): this article is for you.

Even if you are not using SQL Server, most of these design guidelines apply to other DBMS, too: Sybase is a very similar environment for the programmer, and Oracle designs may benefit from this too. I won't show here how to use specific T-SQL tricks, nor won't give you miracle solutions for your SQL Server problem. This is by no means a complete, closed issue. What I intend to do is give you some advices for a sound design, with lessons learned through the last years of my life, seeing the same design errors being done again and again.

DO know your tools.

Please, don't underestimate this tip. This is the best of all of those you'll see in this article. You'd be surprised of how many SQL Server programmers don't even know all T-SQL commands and all of those effective tools SQL Server has.

"What? I need to spend a month learning all those SQL commands I'll never use???" you might say. No, you don't need to. But spend a weekend at MSDN and browse through all T-SQL commands: the mission here is to learn a lot of what can and what can't be done. And, in the future, when designing a query, you'll remember "Hey, there's this command that does exactly what I need", and then you'll refer again to MSDN to see its exact syntax.

In this article I'll assume that you already know the T-SQL syntax or can find about it on MSDN.

DON'T use cursors

Let me say it again: DON'T use cursors. They should be your preferred way of killing the performance of an entire system. Most beginners use cursors and don't realize the performance hit they have. They use memory; they lock tables in weird ways, and they are slow. Worst of all, they defeat most of the performance optimization your DBA can do. Did you know that every FETCH being executed has about the same performance of executing a SELECT? This means that if your cursor has 10,000 records, it will execute about 10,000 SELECTs! If you can do this in a couple of SELECT, UPDATE or DELETE, it will be much faster.

Beginner SQL programmers find in cursors a comfortable and familiar way of coding. Well, unfortunately this lead to bad performance. The whole purpose of SQL is specifying what you want, not how it should be done.

I've once rewritten a cursor-based stored procedure and substituted some code for a pair of traditional SQL queries. The table had only 100,000 records and the stored procedure used to take 40 minutes to process. You should see the face of the poor programmer when the new stored procedure took 10 seconds to run!

Sometimes it's even faster to create a small application that gets all the data, proccess it and update the server. T-SQL was not done with loop performance in mind.

If you are reading this article, I need to mention: there is no good use for cursors; I have never seen cursors being well used, except for DBA work. And good DBAs, most of the time, know what they are doing. But, if you are reading this, you are not a DBA, right?

DO normalize your tables

There are two common excuses for not normalizing databases: performance and pure laziness. You'll pay for the second one sooner or later; and, about performance, don't optimize what's not slow. Often I see programmers de-normalizing databases because "this will be slow". And, more frequent than the inverse, the resulting design is slower. DBMSs were designed to be used with normalized databases, so design with normalization in mind.


This is hard to get used, I know. And I confess: often I use it; but try to specify only the columns you'll need. This will:

  1. Reduce memory consumption and network bandwidth
  2. Ease security design
  3. Gives the query optimizer a chance to read all the needed columns from the indexes

DO know how your data will be/is being acessed

A robust index design is one of the good things you can do for your database. And doing this is almost an art form. Everytime you add an index to a table, things get faster on SELECT, but INSERT and DELETE will be much slower. There's a lot of work in building and mantaining indexes. If you add several indexes to a table to speed your SELECT, you'll soon notice locks being held for a long time while updating indexes. So, the question is: what is being done with this table? Reading or Updating data? This question is tricky, specially with the DELETE and UPDATE, because they often involve a SELECT for the WHERE part and after this they update the table.

DON'T create an index on the "Sex" column

This is useless. First, let's understand how indexes speed up table access. You can see indexes as a way of quickly partitioning a table based on a criteria. If you create an index with a column like "Sex", you'll have only two partitions: Male and Female. What optimization will you have on a table with 1,000,000 rows? Remember, mantaining an index is slow. Always design your indexes with the most sparse columns first and the least sparse columns last, e.g, Name + Province + Sex.

DO use transactions

Specially on long-running queries. This will save you when things get wrong. Working with data for some time you'll soon discover some unexpected situation which will make your stored procured crash.

DO beware of deadlocks

Always access your tables on the same order. When working with stored procedures and transactions, you may find this soon. If you lock the table A then table B, always lock them in this very same order in all stored procedures. If you, by accident, lock the table B and then table A in another procedure some day you'll have a deadlock. Deadlocks can be tricky to find if the lock sequence is not carefully designed.

DON'T open large recordsets

A common request on programming forums is: "How can I quickly fill this combo with 100,00 items?". Well, this is an error. You can't and you shouldn't. First, your user will hate browsing through 100,000 records to find the right one. A better UI is needed here, because you should ideally show no more that 100 or 200 records to your users.

DON'T use server side cursors

Unless you know what your are doing. Client side cursors often (not always) put less overhead on the network and on the server, and reduce locking time.

DO use parametrized queries

Sometimes I see in programming forums, questions like: "My queries are failing with some chars, e.g. quotes. How can I avoid it?". And a common answer is: "Replace it by double quotes". Wrong. This is only a workaround and will still fail with other chars, and will introduce serious security bugs. Besides this, it will trash the SQL Server caching system, which will cache several similar queries, instead of caching only one. Learn how to use parameterized queries (in ADO, through the use of the Command Object, or in ADO.NET the SqlCommand) and never have these problems again.

DO always test with large databases

It's a common pattern programmers developing with a small test database, and the end user using large databases. This is an error: disk is cheap, and performance problems will only be noticed when it's too late.

DON'T import bulk data with INSERT

Unless strictly necessary. Use DTS or the BCP utility and you'll have both a flexible and fast solution.

DO beware of timeouts

When querying a database, the default timeout is often low, like 15 seconds or 30 seconds. Remember that report queries may run longer than this, specially when your database grows.

DON'T ignore simultaneous editing

Sometimes two users will edit the same record at the same time. When writing, the last writer wins and some of the updates will be lost. It's easy to detect this situation: create a timestamp column and check it before you write. If possible, merge changes. If there is a conflict, prompt the user for some action.

DON'T do SELECT max(ID) from Master when inserting in a Detail table.

This is another common mistake, and will fail when two users are inserting data at the same time. Use one of SCOPE_IDENTITY, IDENT_CURRENT, and @@IDENTITY. Avoid @@IDENTITY if possible because it can introduce some nasty bugs with triggers.

DO Avoid NULLable columns

When possible. They consume an extra byte on each NULLable column in each row and have more overhead associated when querying data. The DAL will be harder to code, too, because everytime you access this column you'll need to check

I'm not saying that NULLs are the evil incarnation, like some people say. I believe they can have good uses and simplify coding when "missing data" is part of your business rules. But sometimes NULLable columns are used in situations like this:


This is horrible. Please, don't do this, normalize your table. It will be more flexible and faster, and will reduce the NULLable columns.

DON'T use the TEXT datatype

Unless you are using it for really large data. The TEXT datatype is not flexible to query, is slow and wastes a lot of space if used incorrectly. Sometimes a VARCHAR will handle your data better.

DON'T use temporary tables

Unless strictly necessary. Often a subquery can substitute a temporary table. They induce overhead and will give you a big headache when programming under COM+ because it uses a database connection pool and temporary tables will last forever. In SQL Server 2000, there are alternatives like the TABLE data type which can provide in-memory solutions for small tables inside stored procedures too.

DO learn how to read a query execution plan

The SQL Server query analyzer is your friend, and you'll learn a lot of how it works and how the query and index design can affect performance through it.

DO use referential integrity

This can be a great time saver. Define all your keys, unique constraints and foreign keys. Every validation you create on the server will save you time in the future.


As I've said before, this is by no means a complete SQL Server performance and best practices guide. This would take a complete book to cover. But I really believe that this is a good start, and if you follow these practices, surely you will have much less trouble in the future.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Daniel Turini
Brazil Brazil
I develop software since I was 11. In the past 20 years, I developed software and used very different machines and languages, since Z80 based ones (ZX81, MSX) to mainframe computers. I still have passion for ASM, though no use for it anymore.
Professionally, I developed systems for managing very large databases, mainly on Sybase and SQL Server. Most of the solutions I write are for the financial market, focused on credit systems.
To date, I learned about 20 computer languages. As the moment, I'm in love with C# and the .NET framework, although I only can say I’m very proficient at C#, VB.NET(I’m not proud of this), T/SQL, C++ and libraries like ATL and STL.
I hate doing user interfaces, whether Web based or not, and I’m quite good at doing server side work and reusable components.
I’m the technical architect and one of the authors of Crivo, the most successful automated credit and risk assessment system available in Brazil.

You may also be interested in...

Comments and Discussions

Questionimprove SP Performance Pin
Member 1171017819-Nov-15 3:08
memberMember 1171017819-Nov-15 3:08 
GeneralMy vote of 4 Pin
mahmoud mansour9-Aug-15 5:30
membermahmoud mansour9-Aug-15 5:30 
GeneralVery nice article Pin
King Fisher3-Jan-15 0:03
professionalKing Fisher3-Jan-15 0:03 
GeneralMy vote of 4 Pin
S.K.Tripathi11-Aug-13 17:39
memberS.K.Tripathi11-Aug-13 17:39 
GeneralMy vote of 5 Pin
Raja Sekhar S4-Jun-13 20:03
professionalRaja Sekhar S4-Jun-13 20:03 
GeneralMy vote of 5 Pin
asp_crazy_guy13-Nov-12 19:04
memberasp_crazy_guy13-Nov-12 19:04 
QuestionDisagreement About Temporary Tables Pin
Patrick Harris29-Aug-12 3:24
memberPatrick Harris29-Aug-12 3:24 
GeneralMy vote of 5 Pin
manoj kumar choubey15-Aug-12 20:53
membermanoj kumar choubey15-Aug-12 20:53 
QuestionSQL query Optimization Pin
samu4u8-Aug-12 3:48
membersamu4u8-Aug-12 3:48 
QuestionOut Dated Pin
spyke112-Jul-11 10:52
memberspyke112-Jul-11 10:52 
AnswerRe: Out Dated Pin
David Walker29-Jun-12 4:55
memberDavid Walker29-Jun-12 4:55 
GeneralMy vote of 4 Pin
geethamw031-Dec-10 19:57
membergeethamw031-Dec-10 19:57 
GeneralMy vote of 4 Pin
gromas14-Oct-10 5:35
membergromas14-Oct-10 5:35 
GeneralMy vote of 1 Pin
nrutter23-Feb-10 1:47
membernrutter23-Feb-10 1:47 
GeneralRe: My vote of 1 Pin
venom990913-Jan-11 4:33
membervenom990913-Jan-11 4:33 
GeneralThanks a lot Pin
Member 37934911-Feb-10 1:53
memberMember 37934911-Feb-10 1:53 
GeneralThank You Pin
Abinash Bishoyi12-Jan-10 21:43
memberAbinash Bishoyi12-Jan-10 21:43 
GeneralThanks Pin
GlimmerMan8-Oct-09 6:09
memberGlimmerMan8-Oct-09 6:09 
GeneralOne more good bit of advice Pin
Member 33621415-Sep-09 5:33
memberMember 33621415-Sep-09 5:33 
GeneralThanks for the article Pin
Miguel Barros30-Aug-09 15:04
memberMiguel Barros30-Aug-09 15:04 
GeneralMy vote of 1 Pin
b_serajuddy27-Jul-09 6:56
memberb_serajuddy27-Jul-09 6:56 
GeneralUseful info Pin
mattchang1-Jul-09 1:42
membermattchang1-Jul-09 1:42 
It is indeed useful knowledge for learning SQL for the beginner like me. Thanks dude for sharing that and wait for your next great topics.
Questiontext and image types Pin
RatmilTorres8-Nov-08 17:49
memberRatmilTorres8-Nov-08 17:49 
GeneralThanks for that clear explanation Pin
josemaocu22-Aug-08 5:30
memberjosemaocu22-Aug-08 5:30 
QuestionDO Avoid NULLable columns - I say you CAN'T Pin
mlehiste6-Mar-08 10:20
membermlehiste6-Mar-08 10:20 
Answerhmm - not really. Pin
Dankarmy2-May-08 11:48
memberDankarmy2-May-08 11:48 
GeneralRe: hmm - not really. Pin
DaProgramma31-Mar-09 5:51
memberDaProgramma31-Mar-09 5:51 
GeneralRe: hmm - not really. Pin
Dankarmy31-Mar-09 8:35
memberDankarmy31-Mar-09 8:35 
GeneralRe: hmm - not really. Pin
DaProgramma31-Mar-09 9:22
memberDaProgramma31-Mar-09 9:22 
GeneralRe: hmm - not really. Pin
Raf12316-Nov-09 14:03
memberRaf12316-Nov-09 14:03 
GeneralRe: hmm - not really. Pin
Francesco Giossi19-Feb-10 11:45
memberFrancesco Giossi19-Feb-10 11:45 
GeneralRe: hmm - not really. Pin
DaProgramma20-Feb-10 3:24
memberDaProgramma20-Feb-10 3:24 
GeneralRe: hmm - not really. Pin
Francesco Giossi20-Feb-10 3:54
memberFrancesco Giossi20-Feb-10 3:54 
GeneralRe: hmm - not really. Pin
Gus Oakes10-Mar-10 4:36
memberGus Oakes10-Mar-10 4:36 
GeneralRe: hmm - not really. Pin
Member 29138848-May-10 11:21
memberMember 29138848-May-10 11:21 
GeneralRe: hmm - not really. Pin
Uwe Neumann14-Apr-11 7:55
memberUwe Neumann14-Apr-11 7:55 
NewsAbout - DON'T use cursors [modified] PinPopular
mlehiste6-Mar-08 10:08
membermlehiste6-Mar-08 10:08 
GeneralRe: About - DON'T use cursors Pin
spoodygoon26-Dec-08 3:01
memberspoodygoon26-Dec-08 3:01 
Generalproblem with Oracle db Pin
prasadbuddhika6-Feb-08 2:00
memberprasadbuddhika6-Feb-08 2:00 
GeneralGreat Pin
karthikpa18-Nov-07 19:03
memberkarthikpa18-Nov-07 19:03 
GeneralRe: Great Pin
Daniel Nutu25-Jun-10 1:53
memberDaniel Nutu25-Jun-10 1:53 
GeneralCursors Pin
topcatalpha16-Oct-07 22:45
membertopcatalpha16-Oct-07 22:45 
GeneralRe: Cursors Pin
Peter Verijke21-Nov-07 1:11
memberPeter Verijke21-Nov-07 1:11 
GeneralRe: Cursors Pin
keso227-Jan-10 1:25
memberkeso227-Jan-10 1:25 
GeneralRe: Cursors Pin
topcatalpha27-Jan-10 5:35
membertopcatalpha27-Jan-10 5:35 
GeneralRe: Cursors Pin
keso228-Jan-10 4:13
memberkeso228-Jan-10 4:13 
Generalone doubt also.. Pin
Vipin dev18-Sep-07 4:51
memberVipin dev18-Sep-07 4:51 
GeneralRe: one doubt also.. Pin
Biff_MaGriff18-Sep-07 8:15
memberBiff_MaGriff18-Sep-07 8:15 
GeneralMy two cents addition Pin
robert w. mcbean2-Sep-07 11:18
memberrobert w. mcbean2-Sep-07 11:18 
QuestionIs SELECT * Unavoidable - sometimes? Pin
brian257-Aug-07 15:09
memberbrian257-Aug-07 15:09 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Terms of Use | Mobile
Web01 | 2.8.151125.3 | Last Updated 9 Nov 2002
Article Copyright 2002 by Daniel Turini
Everything else Copyright © CodeProject, 1999-2015
Layout: fixed | fluid