Hey, everyone. Welcome back. In this video, part three of topic four in our database class, I'm going to discuss attributes or columns if you prefer, and identifiers or keys if you prefer in the context of the entity relationship model. All right. So aside from the entities themselves, we know that they consist of attributes, and this is certainly true in the entity relationship models. So these are depicted in our diagrams, the attributes that are out there. And remember, it is the collection of values for the attributes that we've chosen that will represent the instances of our entities. Okay. So if I have information in a project table about the name of the project, it's start date, it's project type, and it's project description. And that's all I have. That's all the information that I store about my projects in the database. Well, then that is that collection of values is a project as far as the database is concerned. So we naturally then need to be careful about buying an appropriate set of attributes to use to collectively describe an entity. This is it connects back to our requirements analysis discussion from earlier. And we need to do a good job there because we need to ensure feel comfortable that so we have a complete set of attributes so that we can store all of the data values that are necessary to allow people in the organization to carry out their tasks and to perform their work. So the set of attributes is a key thing. We need to ensure that we're recording everything that we need to record, and they're not having an excessive number of useless attributes that are just taking up space. And as we know, attributes have data types, right, then the data type constraints, it defines and constrains what sort of values can be stored in the attribute. And we know that attributes have other properties as well. They have things like a null status. Do we allow null values for an attribute or do we disallow null values? We can define defaults. If these are other properties of an attribute, what is the default value if no value is provided? We can attach constraints to attributes. For example, in addition to using the data type to control the acceptable set of values that can be entered into an attribute, we could add, say, a check constraint. And that would be an additional property of the attribute, right? So we can limit its set of acceptable values. So all of these decisions need to be made during the design phase of the overall database design process. Once we get into that component design stage, we need to make these decisions. We need to say, what is the maximum length for a project description? Can the value of project description be null? What is an acceptable range of values for the start date for a project? Can I say that this project started on January 4th, 1782? Or do I want to impose some sort of constraint on that attribute that will prevent those types of data in accuracies from creeping in there? So these are all decisions that are made at the attribute level during this design stage of the broader database design process. But we're already familiar with this concept. Similarly, in the entity relationship modeling world, we have identifiers and we're aware of this concept already in the form or with the name keys. So we know about primary keys, we know about foreign keys, and we know of course that keys are a type of attribute. They serve many purposes as we've already learned, right, to primary keys, have a major role in ensuring uniqueness among all the rows. Within a table. So we use primary keys for that basis. We also use primary keys as one end of the connection that links rows in one table to the rows in another table. Primary key in one table will be connected to a foreign key in another, and we use matching values of those linkages in order to figure out which rows are related to which other rows in some other table. But in the parlance of entity relationship modeling, we can say that a key will identify one or more specific instances in the entity class. So value of the key attribute will identify one or more rows of data or instances of the entity. Same thing. So some examples you might use for like the United States federal government and we're keeping track of our citizens or people that are working in the country we may use social security number, where I don't know an elementary school or a primary school or a college or university, we might use something like a student ID to keep track of students. Companies may have an employee ID. Email addresses can be used. There's a lots of things. What the point is that these are all possibilities for keys. And we use keys for the purpose of uniqueness in the case of primary keys. And also for the purpose of keeping track of how instances in one entity class are related to instances in another entity class. That is how the rows in one table are related to the rows in another table. So just reloading these ideas into your mind because they are very common during the database design process uniqueness matters. Our keys may be unique or non unique. If a key attribute is identified as a primary key, it is by definition unique, but we can specify unique status for foreign keys as well if we choose to do so. Like in a one to one relationship as an example. So we know that uniqueness means if a key attribute is marked as unique, then the data value for that attribute must be unique among all rows in the table. Or in the language of entity relationship modeling among all instances of the entity. And so we can see that the key is the key that is called the key. 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