How do I go about making an ERD in which the 3 tables I have are not linked in any way, so there is not a foreign key amongst the tables?
One table for registered users
One table for storing events (e.g event name, location, time/date etc.)
One table for image storing (for the gallery)
You have answer right into your question:
3 tables are not linked in any way
So you just need to add desired entities on your ERD diagram and no relations. Your Entity Relationship Diagram will be boring and looks like:
Related
I would like to know if it's mandatory for my database to have the same tables as my ERD diagram?
In my ERD I have 5 tables...:
I have a database that only has 2 tables:
Booking (C_Name, C_Phone, B_Date, S_Time, Ach,)
C_NAME: customer name
C_Phone: customer phone number
B_Date date of booking
S_Time: booking start time
Ach: achieved
Food(Meal_Name, Meal_Cost)
The customer information is already in the booking table. Is it necessary to create an order table? For the employee table, there is a relationship between other tables, however, this information is not required in my database...
Any ideas? Thanks
Definitely not. An ERD diagram is usually for the logical data model. The most obvious situation is when it contains a many-to-many relationship. Such a relationship would generally be represented using a junction/association table.
Under other circumstances, you might want to combine different but similar entities into a single table (for instance, to handle one-of relationships). Or, you may decide to represent some relationships using nested tables or similar mechanisms. There may be more arcane requirements that call for splitting tables as well.
For your particular model, I would expect one table per entity. You need additional junction/association tables to handle the employee relationships to order and meal.
Title is the question. When should a relationship in an ER diagram be given its own table in a RDBMS? For instance, one mail courier(with attributes eid and surname) can deliver a number of packages but a package(attributes,pid, sent_By, going_to) can only have one mail courier. Would it make sense to make a table for the relationship called delivers(with an attribute of the time that the package was delivered)? or should the eid of the mail_courier and time_delivered from the deliver relationship be added to the package entity? Also, what would be an example when you would not want to add the attributes to the package entity?
I think what you are trying is to create a one-to-many relationship between two entities. And for that, there is no need to create a separate table; as you mentioned in your question, just add those two attributes to the package table.
Where you would need to create a separate table is when you want to achieve many-to-many relationship between two entities. For example, take twitter's followers. One user can have many followers and a follower can follow many users. You can't do that the relational way without creating a new table with just those two columns.
How do we translate something like this into SQL?
Entity A -thick line- relation -simple line- Entity B
Its easy enough to write any of the other connections, but somehow I can't seem to figure it out when it comes to 1 thick line and a simple one, like shown aboove
I have a primary key which is the date of a football season (Entity A - Season) and an entity (Entity B - Football team) which has 2 primary keys which are it's name and primary key of the Season entity. But 'cause of that doubt I have I can't relate them properly.
Relations do not typically form independent tables (diamonds). However, for a many-many relationship, you will usually see them in a separate tables. Depending on your notation (there are many) your diagram could represent a many-many relationship or a 1:1 relationship.
Strong entities (your rectangles) get tables.
In your ER diagram, you will also typically see attributes for each table in circles connected by lines to the entity itself. Those attributes are turned into columns for each table. Attributes which are underlined in the diagram are representative of a primary key for a particular entity.
Additional or strange constraints that aren't typically easily represented in an ER diagram are usually put as side notes.
To answer your question, you must know whether or not it's a many-many relationship; if so, you would create a SeasonClub table with the two different primary keys inside it.
Are there any hard and fast rules against creating a junction table out of a table's primary key? Say I have a table with a structure similar to:
In this instance there are a list of items that can be sold together, but should be marked as dangerous. Any one item can have multiple other items with which it is dangerous. All of the items are uniquely identified using their itemId. Is it OK to refer a table to itself in a many-to-many relationship? I've seen other examples on SO, but they weren't SQL specific really.
This is the correct design for your problem, as long as your combinations can only be two-item combos.
In database design a conceptual design that renders a relation in many-to-many form is converted into 2 one-to-many in physical design. Say for example a Student could take one or many courses and a course could have many students, so that's many to many. So, in actual design it would be a Student table, Course table then CourseTaken table that has both the primary key of Student and Course table thus creating 2 one to many relayionship. In your case altough the two tables are one and the same but you have the virtual third table to facilitate the 2 one to many relationship so that to me is still very viable approach.
I have a trip (primary: idTrip), where I can link more packages (primary: idPackage), so, I got a relationship table to link trips with packages (primary: idRelTripPackage). (relationship n-to-n)
And next I got a registrations table (primary: idRegistration). How do I best link those (1-to-1 relationship)?
I add two columns in the registrations table (idTrip, idPackage)?
I add a relationship table where i link idRegistration, idTrip, idPackage?
I add a relationship table where i link idRegistration, idRelTripPackage?
Am I right in thinking the relation from Registrations is to RelTripPackage, and its definitely one-to-one. There are a couple of options:
1: As it really is a one-to-one there's not really anything to stop you putting the Registrations data directly onto RelTripPackage, or doing the vice-versa and putting idPackage and idTrip straight onto Registrations as FKs, with a unique key across the two FK columns to ensure there aren't duplicates.
2: If do want the two separate tables then just add idRetTripPackage to Registrations as an FK, and then add a unique constraint on it - again to ensure uniqueness.
There's no need for a separate relationship table as its a 1-1 relationship - They only really become relevant when you are using an n-n. The rest of the time FKs should be placed directly on the child table.
If you follow that logic, you will
add tables and Relations every time you need to add Relations
end up with confusing or duplicate Relations (multiple paths between any two tables)
However the problem (limiting factor) is that the tables you are starting with are not actually normalised. Since the starting position does not have a good basis, you will end up with far more Relations (in tables) than there actually are between the Entities. So the best advice is, the Best practice is, before you attempt this current extension, step back and normalise the data, the existing tables. Then the extension will be much easier, and you will end up with less tables.
if you provide info re the tables (Person, Trip, Package, etc); what exactly is a Registration, etc ... I can provide more explicit answers.
Generally any attribute that is 1::1 with the PK of an Entity should be an attribute in that entity. Any attribute that is 1::0-1 with the PK of an Entity should be in a separate table.
ER Diagram
Based on the information provided, this is your ▶Entity Relation Diagram◀. As long as you use Relational Identifiers, all the Relations you have identified thus far are supported directly (otherwise, if you use IDs, you will need more Relations and tables).
Readers who are unfamiliar with the Relational Database Modelling standard may find ▶IDEF1X Notation◀ useful.