Hello guys! The picture attached is a screenshot of our current database design in MySql Workbench. We presented it into our professor and said that our schema was wrong particularly in the Product Categories (Cakes, Cupcakes, Pies), because it should've been inside the products table.
Can you help me improving this kind of schema by adding more tables and not just 5?
Since your other tables (cakes, pies, etc.) don't add any new attributes, it's hard to envision why you need them. Wouldn't a row in cakes just be an exact duplicate of the row it references in products?
If you need to distinguish between product category, just add a column to your products table for the category.
P.S. Please don't get into the habit of prefixing all your table names with "tbl." This is redundant, because it's obviously a table if you can query from it. Just skip the "tbl." Likewise don't encode the data type in your column names. What if you need to change the data type someday? It would break all your queries.
P.P.S. Don't use FLOAT for currency—use NUMERIC.
See https://twitter.com/billkarwin/status/347561901460447232
Also read What Every Computer Scientist Should Know About Floating-Point Arithmetic.
Related
Might be a silly question to ask but what data type should I setup a column so I can enter multiple values?
Example: I have two tables, one called Application_users and the other Products.
Application_Users has an id column.
What I want is to have a column in Products which is called Application_Users_id and I enter 1,2,3,4
The idea is if an Application_User_id is say 3, they would only see products were the Products.Application_Users_ID contains a 3.
So what data type do I use so I can enter values such as 1,2,3,4 in a column?
I have tried NVARCHAR and INTEGER but neither work (NVARCHAR works but won't let me amend it e.g. add numbers).
Let me know what everyone thinks is the best approach here please.
Thanks
John
It might be a silly question but you would be surprised how many developers makes the very same mistake. It's so often that I have a ready-to-paste comment to address it:
Read Is storing a delimited list in a database column really that bad?, where you will see a lot of reasons why the answer to this question is Absolutely yes!
And if you actually go and read this link, you'll see that it's so wrong and so frequently used that Bill Karwin addressed it in the first chapter of his book - SQL Antipatterns: Avoiding the Pitfalls of Database Programming.
Having said that, SQL Server Does support XML columns, in which you can store multiple values, but that is not the case when you want to use them. XML columns are good for storing stuff like property bags, where you can't tell in advance the data types you'll have to deal with, for example.
tl;dr; - So what should you do?
What you want to do is probably a many to many relationship between Application_users and Products.
The way to create a many to many relationship is to add another table that will be the "bridge" between the two tables - let's call it Application_users_to_products.
This table will have only two columns - application_user_id and product_id, each of them is a foreign key to the respective table, and the combination of both columns is the primary key of the bridge table.
So because I have 4 different product types (books, magazines, gifts, food) I can't just put all products in one "products" table without having a bunch of null values. So I decided to break each product up into their own tables but I know this is just wrong (https://c1.staticflickr.com/1/742/23126857873_438655b10f_b.jpg).
I also tried creating an EAV model for this (https://c2.staticflickr.com/6/5734/23479108770_8ae693053a_b.jpg), but I got stuck as I'm not sure how to link the publishers and authors tables.
I know this question has been asked a lot but I don't understand ANY of the answer's I've seen. I think this is because I'm a very visual learner and this makes it hard to understand what's being talked about when not a lot of information is given.
Your model is on the right track, except that the product name should be sufficient you don't need Gift name, book name etc. What you put in those tables is the information that is specific to the type of product that the other products don't need. The Product table contains all the common fields. I would use productid in the child tables rather than renaming it giftID, magazineID etc. It is easier to remember what things are celled when you are consistent in nameing them.
Now to be practical, you put as much as you can into the product table especially if you are going to do calculations. I prefer the child tables in this specific case to have what is mostly display information. So product contains the product name, the cost, the type of product, the units the product is sold in etc. The stuff that generally is needed to calculate the cost of an order or to have a report of what was ordered. There may be one or two fields that can contain nulls, but it simplifies the calculation type queries so much it might be worth it.
The meat of the descriptive details though would go in the child table for the type of product. These would usually only be referenced when displaying the product in the shopping area and only one at a time, so you can use the product type to let you only join to the one child table you need for display. So while the order cares about the product number and name and cost calculations, it probably doesn't need to go line by line describing the book ISBN number or the megapixels in a camera. But the description page of the product does need those things.
This approach is not purely relational, although it mostly is, but it does group the information by the meanings of the data and how they will be used which will make the database easier to understand and query. I am a big fan of relational tables because database just work better when they hit at least the third normal form but sometimes you can go too far for practicality, so the meaning of the data and the way you are grouping to use the data (and not just for the user interface, but for later reporting as well) is almost always one of my considerations in design.
Breaking each product type into its own table is fine - let the child tables use the same id as the parent Product table, and create views for the child tables that join with Product
Your case is a classic case of types and subtypes. This is often called class/subclass in object modeling and generalization/specialization in ER modeling. It's a well understood pattern. There are known techniques for dealing with this pattern.
Visit the following tabs, and read the description under the info tab (presented as "learn more"). Also look over the questions grouped under these tags.
single-table-inheritance class-table-inheritance shared-primary-key
If you want to rean in more depth use these buzzwords to search for articles on the web.
You've already discovered and discarded single table inheritance on your own. Other answers have pointed you at shared primary key. Class table inheritance involves a single table for generalized data as well as the four specialized tables. Shared primary key is generally used in conjunction with class table inheritance.
I'm trying to design a "recipe box" database and I'm having trouble getting it right. I have no idea if I'm on the right track or not, but here's what I have.
recipes(recipeID, etc.)
ingredient(ingredientID, etc.)
recipeIngredient(recipeID, ingredientID, amount)
category(categoryID, name)
recipeCategory(recipeID, categoryID, name, etc.)
So I have a couple of questions.
How am I doing so far? Is this design okay from what you all know?
How would I implement the preparation steps? Should I create an additional many-to-many implementation (something like preparation(prepID, etc.) and recipePrep(recipeID, prepID)) or just add the directions in the recipes table? I would like this to be an ordered list in the UI (webpage).
Thank you for your help.
Have you looked at any of the existing schemas out there, such as this one at DatabaseAnswers?
some thoughts:
You might want to use the same table for Recipe and Ingredient, with a type indicator column. The reason is that Recipes can contain sub-recipes. Let's call the combined table "Item". Then your RecipeIngredient table would look like
RecipeIngredient (RecipeId, ItemId, Amount).
I'd expect that the table would also have a sequencing column.
If you want to do any calculations with these recipes (e.g., scaling, nutritional analysis, production planning) then your quantities will need to specify a unit of measure. You can do that explicitly (by having a separate column for uofm) or you can use a text field for quantity and expect the user to enter values like "1 cup", or "2 tbs". If you take that approach, you'll need to make sure that what they enter is recognizable, and parse it every time you need to use it. This can become surprising complex, especially if you want to represent recipe yields in a formalized manner.
Assuming you want 1:M from recipe to category, I'm still not sure why your RecipeCategory table would have a Name column. I'd think that the name comes from the Category definition.
I agree with Dave that it's unlikely that you'd reuse preparation steps from recipe to recipe, and so a RecipePreparationSteps table (or something like it) would be more appropriate.
However, recipes are often presented with ingredients and instructions intermixed. eg.
Intro text
some ingredients.
prep instructions
some more ingredients
baking instructions.
To accomodate that, you need to cleverly set sequencing values in the RecipeIngredient and RecipePreparation step tables so that you can combine data from both in the proper order for presentation. Another approach would be, instead of these two tables, use a "RecipeLine" table such that each row can represent either an instruction OR an ingredient. I think that may be what you were suggesting. Purists would frown on this kind of table overloading, but I'm not a purist.
This is a topic I happen to know a lot about, so ask anything.
Looks like a good start. A few thoughts:
Don't see a need for a recpieCategory table. One-to-many between recipe and category should do fine.
A PreparationSteps table should contain 1-n steps for each recipe. I wouldn't try to reuse steps between recipes.
I am currently in the process of looking at a restructure our contact management database and I wanted to hear peoples opinions on solving the problem of a number of contact types having shared attributes.
Basically we have 6 contact types which include Person, Company and Position # Company.
In the current structure all of these have an address however in the address table you must store their type in order to join to the contact.
This consistent requirement to join on contact type gets frustrating after a while.
Today I stumbled across a post discussing "Table Inheritance" (http://www.sqlteam.com/article/implementing-table-inheritance-in-sql-server).
Basically you have a parent table and a number of sub tables (in this case each contact type). From there you enforce integrity so that a sub table must have a master equivalent where it's type is defined.
The way I see it, by this method I would no longer need to store the type in tables like address, as the id is unique across all types.
I just wanted to know if anybody had any feelings on this method, whether it is a good way to go, or perhaps alternatives?
I'm using SQL Server 05 & 08 should that make any difference.
Thanks
Ed
I designed a database just like the link you provided suggests. The case was to store the data for many different technical reports. The number of report types is undefined and will probably grow to about 40 different types.
I created one master report table, that has an autoincrement primary key. That table contains all common information like customer, testsite, equipmentid, date etc.
Then I have one table for each report type that contains the spesific information relating to that report type. That table have the same primary key as the master and references the master as well.
My idea for splitting this into different tables with a 1:1 relation (which normally would be a no-no) was to avoid getting one single table with a huge number of columns, that gets very difficult to maintain as your constantly adding columns.
My design with table inheritance gave me segmented data and expandability without beeing difficult to maintain. The only thing I had to do was to write special a special save method to handle writing to two tables automatically. So far I'm very happy with the design and haven't really found any drawbacks, except for a little more complicated save method.
Google on "gen-spec relational modeling". You'll find a lot of articles discussing exactly this pattern. Some of them focus on table design, while others focus on an object oriented approach.
Table inheritance pops up in a few of them.
I know this won't help much now, but initially it may have been better to have an Entity table rather than 6 different contact types. Then each Entity could have as many addresses as necessary and there would be no need for type in the join.
You'll still have the problem that if you want the sub-type fields and you have only the master contact, you'll have to know what table to go looking at - or else join to all of them. But otherwise this is a workable solution to a common problem.
Another possibility (fairly similar in structure, but different in how you think of it) is to simply put all your contacts into one table. Then for the more specific fields (birthday say for people and department for position#company) create separate tables that are associated with that contact.
Contact Table
--------------
Name
Phone Number
Address Table
-------------
Street / state, etc
ContactId
ContactBirthday Table
--------------
Birthday
ContactId
Departments Table
-----------------
Department
ContactId
It requires a different way of thinking of things though - instead of thinking of people vs. companies, you think of the various functional requirements for the task at hand - if you want to send out birthday cards, get all the contacts that have birthdays associated with them, etc..
I'm going to go out on a limb here and suggest you should rethink your normalization strategy (as you seem to be lucky enough to be able to rethink your schema quite fundamentally). If you typically store an address for each contact, then your contact table should have the address fields in it. Alternatively if the address is stored per company then the address should be stored in the company table and your contacts linked to that company.
If your contacts only have one address, or one (or even 3, just not 'many') instance of the other fields, think about rationalizing them into a single table. In my experience having a few null fields is a far better alternative than needing left joins to data you aren't sure exists.
Fortunately for anyone who vehemently disagrees with me you did ask for opinions! :) IMHO you should only normalize when you really need to. Where you are rethinking schemas, denormalization should be considered at every opportunity.
When you have a 7th type, you'll have to create another table.
I'm going to try this approach. Yes, you have to create new tables when you have a new type, but since this table will probably have different columns, you'll end up doing this anyway if you don't use this scheme.
If the tables that inherit the master don't differentiate much from one another, I'd recommend you try another approach.
May I suggest that we just add a Type table. Ie a person has an address, name etc then the student, teacher as each use case presents its self we have a PersonType table that has an entry from the person table to n types and the subsequent new tables teacher, alien, singer as the system eveolves...
I'm quite new to database design and have some questions about best practices and would really like to learn.
I am designing a database schema, I have a good idea of the requirements and now its a matter of getting it into black and white.
In this pseudo-database-layout, I have a table of customers, table of orders and table of products.
TBL_PRODUCTS:
ID
Description
Details
TBL_CUSTOMER:
ID
Name
Address
TBL_ORDER:
ID
TBL_CUSTOMER.ID
prod1
prod2
prod3
etc
Each 'order' has only one customer, but can have any number of 'products'.
The problem is, in my case, the products for a given order can be any amount (hundreds for a single order) on top of that, each product for an order needs more than just a 'quantity' but can have values that span pages of text for a specific product for a specific order.
My question is, how can I store that information?
Assuming I can't store a variable length array as single field value, the other option is to have a string that is delimited somehow and split by code in the application.
An order could have say 100 products, each product having either only a small int, or 5000 characters or free text (or anything in between), unique only to that order.
On top of that, each order must have it's own audit trail as many things can happen to it throughout it's lifetime.
An audit trail would contain the usual information - user, time/date, action and can be any length.
Would I store an audit trail for a specific order in it's own table (as they could become quite lengthy) created as the order is created?
Are there any places where I could learn more about techniques for database design?
The most common way would be to store the order items in another table.
TBL_ORDER:
ID
TBL_CUSTOMER.ID
TBL_ORDER_ITEM:
ID
TBL_ORDER.ID
TBL_PRODUCTS.ID
Quantity
UniqueDetails
The same can apply to your Order audit trail. It can be a new table such as
TBL_ORDER_AUDIT:
ID
TBL_ORDER.ID
AuditDetails
First of all, Google Third Normal Form. In most cases, your tables should be 3NF, but there are cases where this is not the case because of performance or ease of use, and only experiance can really teach you that.
What you have is not normalized. You need a "Join table" to implement the many to many relationship.
TBL_ORDER:
ID
TBL_CUSTOMER.ID
TBL_ORDER_PRODUCT_JOIN:
ID
TBL_ORDER.ID
TBL_Product.ID
Quantity
TBL_ORDER_AUDIT:
ID
TBL_ORDER.ID
Audit_Details
The basic conventional name for the ID column in the Orders table (plural, because ORDER is a keyword in SQL) is "Order Number", with the exact spelling varying (OrderNum, OrderNumber, Order_Num, OrderNo, ...).
The TBL_ prefix is superfluous; it is doubly superfluous since it doesn't always mean table, as for example in the TBL_CUSTOMER.ID column name used in the TBL_ORDER table. Also, it is a bad idea, in general, to try using a "." in the middle of a column name; you would have to always treat that name as a delimited identifier, enclosing it in either double quotes (standard SQL and most DBMS) or square brackets (MS SQL Server; not sure about Sybase).
Joe Celko has a lot to say about things like column naming. I don't agree with all he says, but it is readily searchable. See also Fabian Pascal 'Practical Issues in Database Management'.
The other answers have suggested that you need an 'Order Items' table - they're right; you do. The answers have also talked about storing the quantity in there. Don't forget that you'll need more than just the quantity. For example, you'll need the price prevailing at the time of the order. In many systems, you might also need to deal with discounts, taxes, and other details. And if it is a complex item (like an airplane), there may be only one 'item' on the order, but there will be an enormous number of subordinate details to be recorded.
While not a reference on how to design database schemas, I often use the schema library at DatabaseAnswers.org. It is a good jumping off location if you want to have something that is already roughed in. They aren't perfect and will most likely need to be modified to fit your needs, but there are more than 500 of them in there.
Learn Entity-Relationship (ER) modeling for database requirements analysis.
Learn relational database design and some relational data modeling for the overall logical design of tables. Data normalization is an important part of this piece, but by no means all there is to learn. Relational database design is pretty much DBMS independent within the main stream DBMS products.
Learn physical database design. Learn index design as the first stage of designing for performance. Some index design is DBMS independent, but physical design becomes increasingly dependent on special features of your DBMS as you get more detailed. This can require a book that's specifically tailored to the DBMS you intend to use.
You don't have to do all the above learning before you ever design and build your first database. But what you don't know WILL hurt you. Like any other skill, the more you do it, the better you'll get. And learning what other people already know is a lot cheaper than learning by trial and error.
Take a look at Agile Web Development with Rails, it's got an excellent section on ActiveRecord (an implementation of the same-named design pattern in Rails) and does a really good job of explaining these types of relationships, even if you never use Rails. Here's a good online tutorial as well.