Know How to Read an ER Model
Software applications store data in databases. Database administrators, system architects and developers design the database layout. The ER model is the visual of how the data is stored. An ER model is an entity relationship model.
Being able to read and ER model is like reading a map; some elements of a map are more readily understandable than others. Learning to read an ER model is like other information we learn; it helps to start with an overview and continue learning with understanding the finer points and intricacies.
An entity is also referred to as a table, and tables store data. For example, imagine a database that stores customer information. The tables in a customer database might consist of a customer table, an address table and a phone number table. Tables contain one or more columns, and, using our customer database example, the columns on a customer table might include: company name, company website and company president. The address table might include these columns of data: street name, suite number, city, state and zip code. On the ER diagram, there would be a rectangle with the name "Customer" just above it, and the columns would appear inside the rectangle.
The 'R' of the ER model refers to the the relationships which the tables have to each other. For example, a table containing customer data will have a relationship with the table containing orders data (customers place orders), which means that a row in the orders table must contain a reference to the associated customer who placed the order. This relationship will appear on the ER diagram as a line between the two tables.
As each company gets added or created through the application, a row of information for that company gets added to the tables. Columns and rows in a database can be compared to an Excel file where the column headers represent what data is stored and each row of the spreadsheet represents the data itself. Tables in a database are made up of columns and rows. A primary key is used to uniquely identify each row in each table. The primary key is comprised of a single column or set of columns. No two rows can have the same value for the primary key. An example would be using the customer name field as the primary key on the customer table.
A foreign key is a referential constraint between two tables. The foreign key identifies a column or a set of columns as a reference to another table. Using the customer database example, a column on the address table is needed to refer back to the customer table. This relationship links the data and the tables together. Having a sense of the data model is like knowing the primary highways through a state or province. Once you get familiar with reading a data model, you’ll find you won’t want to work on an application without seeing the model. It would be like leaving your house without at GPS or road atlas – the data model is navigation tool into the inner working of an application.