I'm new to both Visual Paradigm and ERD. I created an entity, but am a bit lost what some of the notation is. I'm not sure what the large 'N' is, or the number 10 associated with the ID integer (both were autogenerated when creating a column). Any help would be great, I can't seem to find this in the documentation.
The "N" indicate the column is nullable or not.
The column type (integer or number) is subject to the target database specified for your project (under Tools > DB > Database Configuration). For example, by specify project target DB as Oracle then the columns with integer(10) will automatically change to number(10).
You can mark a column as primary key by simply adding "+" sign in front of the column name while in-line editing the column in ERD.
You can find the general ERD modeling tips for Visual Paradigm at https://www.visual-paradigm.com/support/documents/vpuserguide/3563/3564/85375_drawingentit.html
Related
I am currently using Postgres, however, I would like to use standard SQL types for my columns, for portability reasons.
I am attempting to do my modelling in Visual Paradigm Version 14.2 (Build sp1_20180201), under an evaluation license.
I can create an E-R diagram, and when I generate the SQL, Visual Paradigm asks me to select a database configuration before it can do this. So I selected Postgres, but now it converts all my standard types into Postgres specific types! For example, every BIGINT becomes an int8, both in the generated SQL code and the diagram.
Now when I edit the column in the diagram, I can select BIGINT again, but it now appears in italics in the drop down box, and when I save changes to the column, it is int8 again.
How do I successfully generate standard SQL code from an E-R diagram in Visual Paradigm?
Once you specified the database type then the columns will automatically changed to the compatible types for that database. In PostgreSQL the bigint is aliased in int8 and that's why your columns are show in int8 even you manually changed back to bigint. You can override this by defining bigint in User Type field of column's specification dialog. Detailed steps can be found at https://knowhow.visual-paradigm.com/database-design/custom-column-types/
I am building a db in sql server, then I will build the web app. The data has some pretty good natural key columns that I will use as the PKs. However, several of them are composite keys, which will be a bit unwieldy in the application side.
For example, a golf course can have 1, 2, or 3 courses at one location. The location has a number (32201) and each course has a name (bayou front, bayou back, bayou exec) so I would make a composite key from the number and name.
However, in the application (asp mvc) the internal routing allows for passing a single integer id from controller to view, etc for use in identifying stuff. So I am thinking of adding an non-key identity column to the Golf Course table, and others like it, so I can us the identity filed in the app. So someone can select the bayou back course on the index page of the app, and I would pass the non-key identity id number to the controller to select the details for the course from the DB and pass them to the view.
It seems at first, like I would be bypassing the purpose of the primary key by using this non-key integer, but the PK would come into play for things like referential integrity when doing updates, inserts, etc.
Thoughts?
What you're talking about sounds like a surrogate key (as opposed to a natural key). There are pros and cons to using each, and both approaches have been discussed before, rather extensively, I think.
From the answer here: go ahead and use both. They are both tools, and each one should be used when it is the best tool for the job.
The only thing I'd add for your situation is that, since your tables would have both a surrogate and natural key on the same table, make sure that one (natural key or surrogate key) is the actual PK and that the other has a unique key or index on it. That way, either one can be used to uniquely identify rows in your tables.
How do I know if I normalized correctly to 2NF or 3NF? I am still struggling how to validate, that I followed the algorithm correctly.
Is this a normlization that would correspond to 3NF? I an a little bit lost.
According to your data schema you have these rules:
At an Incident there can be MANY Responders.
A Responder can have ONE Device.
A Responder can have ONE res_latitude and ONE res_longitude.
A Device can have ONE Dev_installation.
If the above are what you want then i think it's ok (but see again the primary keys).
Also, i forgot to mention that the reason of keeping the responder_id and device_id in a separate table is to keep historical data in case device_id change responder_id. You could also merge ResponceIncidentDevice in one table with keys incident_id, responder_id, device_id so you will be able to know in what incidents a reponder went carrying what devices.
EDIT:
According to your comment you need to make the following changes. Also note that it is better to use lower case for all your tables and columns to avoid case sensitivity problems due to various engine implemantations.
Responders
responder_id res_latitude res_longitude
Responders_Devices (pk: responder_id, device_id)
responder_id device_id
1 1
1 2
2 3
2 4
3 5
Hey there are a lot many tutorials available on the subject but they are a little complex, I can understand your problem.
First of all your project isn't even legal for First Normalization form because your second table, RespondersIncidents, is a table which has two foreign keys but you have no primary keys.
Now let me simplify the rules for you.
1NF - You must have a primary key (One sentence layman definition)
2NF - No Partial Dependency, Try not to have two entries in one column and make sure that your primary key uniquely identifies the whole row.
3NF - No Functional Dependency, Make sure that in one row only your specific primary key has the power to identify the whole row. For e.g. if in one row there is primary key (auto generated) and student id as well which is unique then we have functional dependency here that means we don't need a separate primary key, we can use student id as primary key.
I hope this was informative for you. I kept it short and simple.
I don't quite understand the need to hide the existence of id column in Rails.
It is neither reflected in migration file nor the schema.rb file.
There is no way for a newbie to know for the fact that a column named id has been created by default as a primary key.
Unless they go and check the actual schema of the table in database (rails dbconsole).
I can see the timestamps macro included by default in the migration file as well as in schema.rb as two fields created_at and updated_at. Here, a developer at least gets a clue. Rails could have done the same for id column too. But it doesn't.
Why the secrecy around id column? Is it a part of the famous convention over configuration? Or is it a norm across all MVC frameworks?
In database design it is generally accepted that numeric id's are preferred, because
they are easier to index, and thus easier to "follow" or check when creating links (foreign keys).
when editing/updating records, you have a unique (and efficient) identifier
So therefore it is advised to give all tables a unique numeric key, always.
Now this numeric key has no meaning whatsoever to your application, it is a "implementation detail" of your database layer. Also to make sure every table has an id, unless you explicitly ask not to.
I think this would indeed fall under the "convention over configuration" nomer: why explicitly specify an id for each table if you each table should have one.
The timestamps is different: this is interesting for some tables, but for same tables it is not important at all. It also depends on your application.
Note that this is not at all related to MVC. The M in MVC is a container for data, but in MVC it is actually not really important how the Model gets filled. In other words: the ORM part is not part of MVC. You will see that in most MVC implementations there is no ORM, or definitely not as tightly integrated as with Rails.
So in short: imho ommitting the 'id' from the migration is not a secret, it is just to make life easier, saves you some more typing, and it makes sure you follow a good convention unless you explicitly do not want to.
This is probably due to the fact that relational databases tend to use integer primary keys, and doing otherwise introduces complexities. I guess the reason it's hidden in rails is so that creating tables with integer primary keys does not require any special configuration, and having to write it into rails migrations invites inexperienced developers to play around with it (which is probable not a good idea).
Additionally, I think rails tries to abstract away things like numeric ids, if you want to create associations in a migration you do not need to specify foreign keys, you can simply write the name of the object you want to relate the table to.
I never thinked about the id field because almost every table have an id....
Check the documentation about migrations where they say:
A primary key column called id will also be added implicitly, as it's
the default primary key for all Active Record models. The timestamps
macro adds two columns, created_at and updated_at. These special
columns are automatically managed by Active Record if they exist.
If you want to check your table columns, just go to rails console and type Model.column_names
I think it's clear that if you don't add a primary key, then rails will add one generic key for you so that it can index your record and have a control over it, so basically it isn't stating that there WILL be an ID field, since I don't believe this has to be imperative, but rather optional in the event you do not provide a primary key.
It's a Rails convention to hide the I'd attribute to discourage and remove temptation of playing with it.
id attr is automatically generated with auto_increment to ad normalization to your data(to make each record unique an accessible). Injecting your own values would eventually corrupt and break the magic of ActiveRecord.
When mapping an ER diagram to a relational schema, my textbook says that in step.. whatever.. a new relation S should be created for multivalued attributes. But if the multivalued attribute is the primary key of R... that leaves the R with no primary key and S with no primary key?
This is an excellent question and something that always bothered me about the textbook explanations of how to eliminate "complex" types.
The question you need to ask is: What is being identified by the sets of values? What are you trying to model? Most database architects working with SQL would probably say that you ought to invent a new attribute to identify the sets of things that would have made up your multivalued attribute.
Another solution is to embrace "complex" types as first class attributes in their own right - not "multivalued" attributes but sets or arrays that can be assigned to a variable as a single value just like any other value. The Tutorial D language permits relation-valued types withing relations. e.g.:
VAR r BASE RELATION {foo RELATION {bar INTEGER} } KEY {foo};
where foo is a relvar nested within r.
SQL however doesn't support anything like this. Nested tables are supported in SQL but are not usually allowed to be part of keys so in SQL you always have to create a new identifying attribute. In a true RDBMS you arguably shouldn't have to create another attribute because any supported type ought to support being part of a key - if it didn't then you wouldn't even be able to project on that attribute because the result wouldn't contain a key.