errors in database design

stored procedures can be satisfied with non-store procedure solutions. Harmful design flaws often go unnoticed. In short, whenever we talk about the relational database model, we’re talking about the normalized database. So let’s insert significantly more records into the book_comment table. second, sorting letter-by-letter is sometimes wrong when accents come into play. Database design is a combination of knowledge and experience; the software industry has evolved a lot since its early days. In my opinion, we have a serious problem. Suppose that we want to store some additional customer attributes. For example, a table you query might have columns added or deleted, or their types might have changed. You’ve probably made some of these mistakes when you were starting your database design career. The first attempt to solve it should be to find long running queries, ask your database to explain them, and look for sequential scans. Almost too good to be true. The users must see the promotion date in their own time zone. Good planning is not specific to data modeling; it’s applicable to almost any IT (and non-IT) project. For example, at the end of the day, we’ll store the number of calls we made that day, the number of successful sales, etc. We may also store a small set of reporting data inside the database. Separating frequently and infrequently used data into multiple tables is not the only way of dealing with high volume data. How to Avoid 8 Common Database Development Mistakes Common Mistake 1. Consider using application-level caching for heavyweight, infrequently updated data. Notice that: Now imagine the mess would we create if our model contained hundreds of tables. If it is in English it cannot. An Unemotional Logical Look at SQL Server Naming Conventions, All About Indexes Part 2: MySQL Index Structure and Performance. There’s a point when you’re really close to the data model, but you haven’t started actually drawing it yet. If you face a major change in your design and you already have a lot of code written, you shouldn’t try for a quick fix. If we have content in one language only, it is clear, but if we have content in 15 or 30 languages, which alphabet should determine the order? The Database Is Corrupt . What happens if someone deletes or modifies some important data in our bookstore and we notice it after three months? IT Solutions Zambia can design database solutions in both access and SQL or a combination of both with an Access front end and SQL back end on a SQL server. Once again, we’ll lose a little hard drive space, but we’ll avoid recalculating data or connecting to the reporting database (if we have one). Microsoft Access Tips to Avoid 17 Common Form Design Mistakes Provided by Luke Chung, President of FMS, Inc.. Access forms are extremely powerful. We have populated the database with some manually created test data. Then there are also other information in the user table, for example their basic info like login, password and full name. While they hold UNIQUE values, they don’t make good primary keys. Changes in the model were as follows (on the example of the purchase table): The last error is a tricky one as it appears only in some systems, mostly in multi-lingual ones. However, if you've never done this, contact your web host for help. If you wrap something in double quotes in SQL, it becomes a delimited identifier and most databases will interpret it in a case-sensitive way. I’ve split the list of errors into two main groups: those that are non-technical in nature and those that are strictly technical. Here is the model with order table renamed to purchase: Let’s inspect our model further. When you design a database, you’re designing it to ensure it meets the needs of the business and the system that uses it. It seems PostgreSQL is not wise enough to select 30 newest records without sorting all 600,000 of them. 4 – Not Considering Possible Volume or Traffic, different data types to store date and time. Book description is likely to remain unchanged, so it is a good candidate to be cached. comment on books and rate them after reading. The “customer” table is our entity, the “attribute” table is obviously our attribute, and the “attribute_value” table contains the value of that attribute for that customer. For example, special offers’ expiry times (the most important feature in any store) must be understood by all users in the same way. However, when setting text field limits, you should always remember about text encoding. On the other hand, some tables in your model will be quite specific. Thus, we would avoid making changes in the database. We all get excited when a new project starts and, going into it, everything looks great. So if you try to issue a SQL query: the database management system will complain. Skipping is only an option if 1) you have a really small project; 2) the tasks and goals are clear, and 3) you’re in a real hurry. It will cost some time, but you will deliver a better product and sleep much better. One additional note: I prefer to use single-column auto-generated integer attributes as the primary key. It happens. Any criticism is welcome. Data backup If the system you’re building is another iteration of an existing project, you can estimate the expected size of the data in your system by looking at the data volume in the old system. The more you sell the more rows there will be in the purchase table. For example, if you expect the book description to be very long, you can use application-level caching so that you don’t have to retrieve this heavyweight data often. What you decide to use is closely related to the problem you’ll solve, but it’s important to include the client’s preferences and their current IT infrastructure. Ask them to explain what you don’t understand. If it exceeds the limit, Oracle will throw an error when attempting to save the data to the database. You’ll probably have a few of them, while the other tables will be some of the usual ones (e.g. This is important, both for your schedule and for the client’s timeline. It is relatively easy to start and difficult to master. This is when it’s wise to decide how you will name objects in your model, in the database, and in the general application. You know which technologies you’ll use, from the database engine and programming languages to other tools. Which tables will be the central tables in your model? While we could store some aggregated numbers in our operational database, we should do this only when we truly need to. So if the bookstore customer can format the comment using some kind of WYSIWYG editor then limiting the size of the “comment” field can be potentially harmful because when the customer exceeds the maximum comment length (1000 characters in raw HTML) then the number of characters they see in the GUI can still be much below 1000. In some cases, the limitations of the … Poor Naming Standards. Instead of explaining it in general, let’s see an example. It’s definitely the best practice. In such cases, it would be wise to perform these calculations during off hours (thus avoiding performance issues during working hours). And while it is importing, I will check time of execution of the previous query. No list of mistakes is ever going to be exhaustive. Here is our bookstore model after the changes: What if the bookstore runs internationally? You should find balance between security of data and simplicity of the model. Some are very common regardless of the business, e.g. To embrace the digitization efforts, AHIMA has adopted an initiati… our. our. So, let’s start with the non-technical issues first, then move to the technical ones. Outline 1. Databases have different data types to store date and time. Maybe you’re still making them, or you’ll make some in the future. A quick fix could save some time now and would probably work fine for a while, but it can turn into a real nightmare later. remember they will not always be used; the database may decide not to use an index if it estimates the cost of using it will be bigger that doing a sequential scan or some other operation, remember that using indexes comes at a cost –, consider non-default types of indexes if needed; consult your database manual if your index does not seem to be working well. Then we will have a good chance to restore it and avert the loss. Type varchar(100) means 100 characters in PostgreSQL but 100 bytes in Oracle! Imagine finding a bug a few months after you’ve closed the project. Non-Technical Database Design Errors #1 Poor Planning . So – language of content can affect ordering of records, and ignoring the language can lead to unexpected results when sorting data. While the model will store the data we need, working with such data is much more complicated. scheduling bulk deletes at night, to avoid unnecessary table locks. sometimes you may want to use language specific to data being browsed. In this article we will try to make learning database design a little simpler, by showing some common errors people make when designing their databases. Keeping versions and logging events makes your database more complex. Right Way To Fix Common Errors In Database Design. If you store a client’s name in two different places, you should make any changes (insert/update/delete) to both places at the same time. Some examples: BMP (Basic Multilingual Plane, Unicode Plane 0) is a set of characters that can be encoded using 2 bytes per character in UTF-16. Programmers should develop standardized components in the system to handle time zone issues automatically. Database Design ... values will reduce the possible errors in data entry. It is a feature of this particular language. Database management system manages the data accordingly. Managing time zones in date and datetime fields can be a serious issue in a multinational system. People (myself included) do a lot of really stupid things, at times, in the name of “getting it done.”. All of these cause changes in the model. Let’s see the query plan: The query plan tells us how the database is going to process the query and what the possible time cost of computing its results will be. What rules will apply when naming tables and other objects? Don’t forget about dictionaries and relations between tables. Have you experienced any of the issues mentioned in this article? Here you can see that we named a table with a word “order”. They are rarely updated or retrieved, so you can deal with longer access time to this table. Remember to keep the terminology similar to whatever the client currently uses. The simplest solution is to split data into two tables: one for basic info (often read), the other for bonus points info (frequently updated). Separate frequently updated data from frequently read data. These calculations could use many tables and consume a lot of resources. You have a performance problem? If you find them, probably adding some indexes will speed things up a lot. If you want to learn to design databases, you should for sure have some theoretic background, like knowledge about database normal forms and transaction isolation levels. user_account, role). consider the balance between risk and costs; remember the Pareto principle stating that roughly 80% of the effects come from 20% of the causes; don’t protect your data from unlikely accidents, focus on the likely ones. They add value to your code and they relate the technology to the real-world problem you need to solve. As database technology moves from the task of supporting paper systems to actually becoming the central digitized health information system, a "basic understanding" becomes inadequate. In our example let’s assume that the GUI designer of our bookstore decided that 30 newest comments will be shown in the home screen. And here PostgreSQL tells us it is going to do a “Seq Scan on book_comment” which means that it will check all records of book_comment table, one by one, to sort them by value of send_ts column. Also, stay in contact with your client and the developers throughout the project. And when you’re explaining technical details to the client, use language and terminology they understand. Let’s take a look at one example. The price paid for a poorly-documented project can be pretty high, a few times higher than the price we pay to properly document everything. The database has eight tables and no data in it. For some users, it will be “December 24, 19.59”, for others it will be “December 25, 4.49”. Ever. Using identity/guid columns as your only key First Normal Form dig this my work I haven't noticed any performance decrease. Why does it take longer? The benefits are: Efficient data analysis with reports. Redundant data should generally be avoided in any model. (See Point 4 about naming conventions.). Instead of a single table called purchase, you can have two tables: purchase, for current purchases, and archived_purchase, for completed orders. Both models “work”, so there are no problems on the technical side. For example, if you try to save the word “mother” in Chinese – 母親 – and your database encoding is UTF-8, then such a string will have 2 characters but 6 bytes on disk. The art of designing a good database is like swimming. If you know this in advance, you can separate current, processed purchases from completed purchases. So, before you start creating any names, make a simple document (maybe just a few pages long) that describes the naming convention you have used. It could contain properties like “customer value”, “contact details”, “additional info” etc. As an example learners could discuss the impact of errors such as the accidental deletion of a field in a query or report the renaming of a field changing data types etc. That solution doesn’t need to be a complete application – maybe a short document or even a few sentences in the language of the client’s business. This way update operations don’t slow down read operations. To do so, let’s create an index on this column: Now our query to select the newest 30 out of 600,000 records takes 67 milliseconds again. We all get excited when a … It takes less than 70 milliseconds on my laptop. You can use this information when you design a model for a new system. Note , the genius of a database is in its design . People may forget about them, but these skills are also a very important part of the design process. consider which data is important to be tracked for changes/versioned. their business plans related to this project and also their overall picture) and what they want this project to achieve now and in the future. Explain everything needing additional explaining, and basically write down everything you think will be useful one day. Allow creation of multiple indexes on a table, as well as unique indexes within a table. You understand the data flow in the client’s company. A historical example is the Sputnik 1 launching engineers giving verbal instructions to the technicians who were assembling it. You can read more about naming conventions in these two articles: Normalization is an essential part of database design. For most of us, documentation comes at the end of the project. Now, if I wanted to complicate things even more, I can create another table, this time named ORDER (in uppercase), and PostgreSQL will not detect a naming conflict: The magic behind that is that if an identifier is not wrapped in double quotes, it is called “ordinary identifier” and automatically converted to upper case before use – it is required by SQL 92 standard. To reduce the time spent on unexpected changes, you should: If you try to avoid making changes in your data model when you see a potential problem — or if you opt for a quick fix instead of doing it properly — you’ll pay for that sooner or later. Obviously, if you don’t have technical skills, you won’t know how to do something. avoid using SQL and database engine-specific keywords as names; Oracle has the aforementioned limit of 4000 bytes for, Oracle will store CLOBs of size below 4 KB directly in the table, and such data will be accessible as quickly as any. (E.g. keep access logs for our system for three months at least – and this is unlikely, they are probably already rotated. Will we group tables using names? It would allow us to add new properties easily (because we add them as values in the “customer_attribute” table). be able to associate the fact of deleting the data with some URL in our access log. And it is a hard limit – there is no way you can exceed that. Of course, the value is redundant; however, what we gain in performance is significantly more than what we lose (some hard drive space). Accidental deletion of data. The bottom line is – don’t use keywords as object names. More Database Design Errors – Confusion with Many-to-Many Relationships by Michael Blaha My last blog addressed primary key and foreign key errors. Database development and design are at the core of any data-intensive project including nearly all business applications. I personally think it has helped me a lot when it comes to DB designing. Customers come from all over the world and use different time zones. If you have to use it, only use it when you’re 100% sure that it is really needed. The system should allow customers to perform the following activity: So the initial database model could look like this: To test the model, we will generate SQL for the model using Vertabelo and create a new database in PostgreSQL RDBMS. First, we’ll add a dictionary with a list of all the possible properties we could assign to a customer. adjusting your database transaction isolation level. The art of designing a good database is like swimming. In such a situation just change the type to text and don’t bother limiting length in the database. How do we check that? An operational database is not meant to store reporting data, and mixing these two is generally a bad practice. Database designing is crucial to high performance database system. If you know how to restore your database from a backup, by all means, go ahead and do it. What name will we use for the ID columns? limiting length of text columns in the database is good in general, for security and performance reasons. Database design best practices call for drift prevention in the design stage, not as an after-thought when it's too late. You can read more about normalization in this article. This is definitely a non-technical problem, but it is a major and common issue. The short version is that I recommend you add an index wherever you expect it’ll be needed. Let’s assume we want to design a database for an online bookstore. The database development life cycle has a number of stages that are followed when developing database … Handling time zones properly requires cooperation between database and code of the application. Data operations using SQL is relatively simple Database development life cycle . we must be able to restore the data without too much work. if applicable, apply collation to columns and tables – see. ), How will we name foreign keys? And why could a 3000-character text exceed 4000 bytes on disk? Topic: Database Design Errors and How to Avoid them The following Outline must be followed when you write your paper. Use them as alternate keys instead. ), there comes the second question – who did it? Therefore, GUIDs seem like a great candidate for the primary key column. You can avoid problems by declaring scalar variables with %TYPE qualifiers and record variables to hold query results with %ROWTYPE qualifiers. Adam Evanovich lives in Iowa in the United States and frequently works on contract in various industries. The design mistakes listed in this article may seem small and insignificant at the start. But there are two traps here: We will illustrate this with a simple SQL query on French words: This is a result of sorting words letter-by-letter, left to right. If they enter a simple comment, like this: then it will take only 17 characters. And if it is so – there is no error here. But that’s not the case. Remember that you name not only tables and their columns, but also indexes, constraints and foreign keys. The design process should therefore always be viewed in this context. Note that we will not talk about database normalization – we assume the reader knows database normal forms and has a basic knowledge of relational databases. There are good database design guidelines all over the internet as well as bad practices and things to avoid in This website uses cookies to improve your experience while you navigate through the website. However, the developer can easily make mistakes to cause a form to behave incorrectly or poorly. And that includes almost everything, from writing simple SELECT queries to getting all customer-related values to inserting, updating, or deleting values. In a multi-time zone system date column type efficiently does not exist. Multiple indexes on a table allow faster sorts and queries based on various parameters. You never know if or when you’ll need that extra info. Most often we assume that sorting words in a language is as simple as sorting them letter by letter, according to the order of letters in the alphabet. The Author. Talk with developers and clients and don’t be afraid to ask vital business questions. But honestly, that’s usually not the case. You can also add them after the database is in production if you see that adding index in a certain spot will improve performance. For example, if you’re building a model for a cab company, you’ll have tables for vehicles, drivers, clients etc. If a database is not normalized, we’ll run into a bunch of issues related to data integrity. In case of handling time zones the database must cooperate with application code. Also must be able to create one index on multiple columns. I recommend that you avoid using composite primary keys. In short, we should avoid the EAV structure. My company (GenieDB) has developed a replicated database engine that provides "AP" semantics, then developed a "consistency buffer" that provides a consistent view of the database - as long as there are no server or network failures; then providing a degraded service, with some fraction of the records in the database becoming "eventually consistent" while the rest remain "immediately … In Oracle, the varchar column type is limited to 4000 bytes. Every database should be normalized to at least 3NF (primary keys are defined, columns are atomic, and there are no repeating groups, partial dependencies, or transitive dependencies). Of course, if you keep your database backed up regularly, you're going to be all right. We add it here because we encounter it quite often, and it does not seem to be widely known. They are sometimes also known as UUIDs (Universally Unique Identifiers). This is the part of the project where we can make crucial mistakes. The database design provider or advisor, which you pick up, should be neglect these rules. Online database design and modeling tool used and loved by more than 315,395 users including 50,000+ organizations, from leading government agencies to enterprise-class firms to smaller-sized companies and more than 150,000 freelancers, database admins, developers and engineers. Here is the final version of our bookstore model: If you have any questions or you need our help, you can contact us through But in other languages it may be possible. So to select these comments, we will use the following query: How fast does this query run? Criterion: Discuss how potential errors in the design and construction of a database can be avoided. tuning your database connection pool size and/or thread pool size. Always check and see if any changes have been made since your last discussion. Database design is the organization of data according to a database model.The designer determines what data must be stored and how the data elements interrelate. Design your programs to work when the database is not in the state you expect. Luckily, we can help it by telling PostgreSQL to sort this table by send_ts, and save the results. Smashing is proudly running on Netlify.. Fonts by Latinotype. At the start, the project is still a blank page and you and your client are happy to begin working on something that will create a better future for both of you. Yet we still have issues with poor datatype choices, the use… 4 n00b MySQL Mistakes Every Programmer Makes - DevOps.com - […] of the common database errors identified by Thomas Larock on the SQL Rockstar site is playing it safe by… Now let’s look at errors with many-to-many relationships. We’ll start with the simple and obvious – naming standards. If we’re well-organized, we’ve probably documented things along the way and we’ll only need to wrap everything up. Everyone agrees that great database performance starts with great database design. 2006–2020.. How long will the whole project take? How do . Multi user access. 2. Miscellaneous Database Design Errors

by Michael Blaha This is the third and final blog in a series about database design errors. A similar approach should be taken when logging events in a multi-time zone system. Here we will focus on the model design, of course. There are multiple ways of achieving this goal: As usual, it is best to keep the proverbial golden mean. But still, we need to stay focused. browse and search books by book title, description and author information. Imagine a system where parts of user info are frequently updated by an external system (for example, the external system computes bonus points of a kind). With a commitment to quality content for the design community. (Most likely “id_” and the name of the referenced table.). He started using Access in 1997 to record notes in a small database … If you mean Christmas Eve midnight in your own time zone, you must say “December 24, 23.59 UTC” (or whatever your time zone is). What does this mean? This is all great, and a great future will probably be the final result. Analyze that area and implement changes if they will improve the system’s quality and performance. Note that different databases can have different limitations for varying character and text fields. We can’t go back in time and help you undo your errors, but we can save you from some future (or present) headaches. Join our weekly newsletter to be notified about the latest posts. This will add data in sequential order to the primary key and provide optimal performance. a VAT ID). Time of events should always be logged in a standardized way, in one selected time zone, for example UTC, so that you could be able to order events from oldest to newest with no doubt. When it comes to organizing data, I see the same mistakes in database design as I see in object design: Some developers like to turn everything into a … Will we use uppercase and lowercase letters, or just lowercase? In fact, there are some situations where redundant data is desirable: In most cases, we shouldn’t use redundant data because: I hope that reading this article has given you some new insights and will encourage you to follow data modeling best practices. full audit logging can be implemented using triggers or other mechanisms available to the database management system being used; such audit logs can be stored in separate schemas to make altering and deleting impossible. It is relatively easy to start and difficult to master. In this article, I’ve listed 24 different database design mistakes that you should try to avoid. all client-related tables contain “client_”, all task-related tables contain “task_”, etc.). In a small database, like ours, it is not a very important matter indeed. Before you sit down to draw a data model, you need to be sure that: During the planning phase, you should get answers to these questions: Only when you have all these answers are you ready to share an initial solution to the problem. Fortunately, there is enough knowledge available to help database designers achieve the best results. To determine this, we need to: This will for sure take a lot of time and it does not have a big chance of success. Rarely updated or retrieved, so some examples may be web application-specific deletes or modifies important... Be wise to perform these calculations during off hours ( thus avoiding performance during... Decided that 30 newest comments will be “December 25, 4.49” some in the United and... Is generally a bad practice also add them after the changes: what the... Has to estimate the expected volume of data in it some time, but is... Created test data to a customer on multiple columns efficiently does not seem be. Similar approach should be taken when logging events makes your database design errors – confusion with many-to-many relationships and reporting. By: not deleting it, everything looks great lowercase letters, or deleting values it contain! Varchar column type is limited to 4000 errors in database design now but could become an issue later, don’t use keywords object!, ask yourself database for an online bookstore generated using Vertabelo and practical.... That there are no problems on the data flow in the database should establish naming to... Code which uses these tables, but also indexes, constraints and foreign key errors is. Is really unlikely find them, while the other tables will be used for in... Constraints and foreign keys, everything looks great still, not given” a! Long a project will last and if it exceeds the scope of this article may seem small and insignificant the. Personally think it has helped me a lot blamed the database is not a very long word list, update. The non-technical issues first, then move to the data to the data and for simplest. Very basic audit trail working with such data is important, both for your schedule and for the they! That different databases can have different limitations for varying character and text.. During working hours ) data being browsed users who created / modified.... Bigger data, as you probably remember, “order” is a reserved word in Server. The final result limit, Oracle will throw an error when attempting to save results. Means, go ahead and do it as it should have been done, no matter the... Type to text and don’t be afraid to ask vital business questions avoid future problems with names, don’t it! You need to later without having to recalculate it the varchar column type is limited to 4000 bytes all world. Type efficiently does not seem to be widely known will check time of execution of the usual (. Many tables and consume a lot them, probably adding some indexes will speed things up a lot can... Be able to create one index on multiple columns hours ( thus avoiding performance issues working! Estimate the expected volume of data and the name of the previous query one is proper update timestamps, with. That the original data and the “copy” errors in database design always in consistent States bigger data difficult!, with values, they can begin to fit the data flow in United! All task-related tables contain “task_”, etc ) cause a Form to behave incorrectly or poorly remember about encoding. Here because we add them as values in the purchase table can be satisfied non-store! And mixing these two is generally a bad practice your web host for help others it will cost time. To inserting, updating, or deleting values have creation, errors in database design write! Remember that you name not only tables and other objects: there is some kind of audit trail start! Frequently and infrequently used errors in database design into multiple tables is not specific to data being.. Access time to users, preferably in their own time zone of achieving goal! In double quotes – so called “delimited identifiers” – are required to stay unchanged whole model and simplify work... Different database design provider or advisor, which you pick up, should be neglect these rules separation we! A proper locale Purpose or Frequency of the main requirements it has helped me a lot of resources and... Simple database development life cycle we’ll store the “attribute_value” for that attribute series about database design

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