With your database tables now converted into tables, you’re ready to analyze the relationships between those tables. Cardinality refers to the quantity of elements that interact between two related tables. Identifying the cardinality helps make sure you’ve divided the data into tables most efficiently.
Each entity can potentially have a relationship with every other one, but those relationships are typically one of three types:
When there’s only one instance of Entity A for every instance of Entity B, they are said to have a one-to-one relationship (often written 1:1). You can indicate this kind of relationship in an ER diagram with a line with a dash on each end:
Unless you have a good reason not to, a 1:1 relationship usually indicates that you’d be better off combining the two tables’ data into a single table.
However, you might want to create tables with a 1:1 relationship under a particular set of circumstances. If you have a field with optional data, such as “description,” that is blank for many of the records, you can move all of the descriptions into their own table, eliminating empty space and improving database performance.
To guarantee that the data matches up correctly, you’d then have to include at least one identical column in each table, most likely the primary key.
These relationships occur when a record in one table is associated with multiple entries in another. For example, a single customer might have placed many orders, or a patron may have multiple books checked out from the library at once. One-to-many (1:M) relationships are indicated with what’s called “Crow’s foot notation,” as in this example:
To implement a 1:M relationship as you set up a database, simply add the primary key from the “one” side of the relationship as an attribute in the other table. When a primary key is listed in another table in this manner, it’s called a foreign key. The table on the “1” side of the relationship is a considered a parent table to the child table on the other side.
When multiple entities from a table can be associated with multiple entities in another table, they are said to have a many-to-many (M:N) relationship. This might happen in the case of students and classes, since a student can take many classes and a class can have many students.
In an ER diagram, these relationships are portrayed with these lines:
Unfortunately, it’s not directly possible to implement this kind of relationship in a database. Instead, you have to break it up into two one-to-many relationships.
To do so, create a new entity between those two tables. If the M:N relationship exists between sales and products, you might call that new entity “sold_products,” since it would show the contents of each sale. Both the sales and products tables would have a 1:M relationship with sold_products. This kind of go-between entity is called a link table, associative entity, or junction table in various models.
Each record in the link table would match together two of the entities in the neighboring tables (it may include supplemental information as well). For instance, a link table between students and classes might look like this:
Mandatory or not?
Another way to analyze relationships is to consider which side of the relationship has to exist for the other to exist. The non-mandatory side can be marked with a circle on the line where a dash would be. For instance, a country has to exist for it to have a representative in the United Nations, but the opposite is not true:
Two entities can be mutually dependent (one could not exist without the other).
Sometimes a table points back to itself. For example, a table of employees might have an attribute “manager” that refers to another individual in that same table. This is called a recursive relationship.
A redundant relationship is one that is expressed more than once. Typically, you can remove one of the relationships without losing any important information. For instance, if an entity “students” has a direct relationship with another called “teachers” but also has a relationship with teachers indirectly through “classes,” you’d want to remove the relationship between “students” and “teachers.” It’s better to delete that relationship because the only way that students are assigned to teachers is through classes.
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