Hey, everyone, welcome back to part eight of topic four in our database class. In this video, we're going to build on our knowledge of crowsfoot symbols by seeing how we can add cardinalities to those crowsfoot symbols to convey more information in our entity relationship models. Finally, now that we'd have this concept down of minimum and maximum cardinalities, we can expand our set of crowsfoot symbols to reflect or to convey information about just maximum cardinalities, but minimum cardinalities as well. So what you're seeing here are the standard set of crowsfoot symbols that we will be using to convey information about the relationships between two tables for the rest of our time together. So as you can see, there are only four options. So it's not to overwhelm me. And additionally, you can see that we only have three distinct symbols. Okay. So we've already seen this one. This is the concept of many. Now if I, let me draw this out here. If I draw this, and I say what number is that, hopefully all of you will say that's a one. At least in whatever I would use, like the Arabic numerals. Right. So that's a one, some countries in Europe will draw it like this, which I think is interesting. But that's clear, right? This is one. This symbol, this perpendicular line that crosses our relationship line indicates one. And it looks like the number one. Right. And using that same concept, if I said what do you think this symbol represents? It's a zero. So visually, it's very easy to identify these things. So our three possible symbols then to create this complete set of combinations of possibilities are three symbols are zero, one, or many. Kind of looks like a close foot. So all you need to do is know those three and that you can interpret any of these line endings that you might see here. What's not illustrated here is that these would be connected to some entity. So maybe that's like an employee table or this one's like a department table or a customer table or whatever. So these line ends are going to be connected to some table. And they tell us something. We'll use this one here as an example. So imagine that, now just the sake of illustration that this is an employee table. So we have these two symbols. And one of them is going to tell us about the minimum cardinality. And the other one is going to tell us about the maximum cardinality. Right. So in this case, what we need to remember is that the symbol that touches the entity. So you can see the symbol here that's touching the entity where our entity is indicated by this rectangle. Those are the boundaries of the entity. That is the maximum cardinality. And the one that is not touching the entity is the minimum. So this represents a minimum cardinality of one and a maximum cardinality of many. That's why we describe it as a one to many relationship. So here's a zero to one. Here's a zero to many. Here's a one and only one or a mandatory one. Or exactly one. Whichever you prefer. So let's build out these concepts a little bit and see if we can learn to interpret some of these diagrams. Now what we see illustrated here is these symbols flowing in one way. That is, they're oriented only in one way, but we need to develop the mental flexibility to read these minimum maximum carnalities, regardless of the orientation in which these relationship lines and symbols have been drawn. So I will happily illustrate what I'm talking about with some lovely tables. So let's imagine that we have some tables that are related to each other here. I'll just draw some various tables. I'll do five of them so that we can have a complete illustrative example. So I just imagine that each of these is a table. And then have some relationship lines that interconnect naturally. So what we saw illustrated on the previous slide would be something that looks like this. But we need to have the mental flexibility to see that regardless of rotation. So regardless of the orientation in which we might draw something. We need to be able to read that relationship. I'm just drawing some examples up here. And of course we would have symbols on the other side as well. And I'll just draw a variety of them for the sake of allowing your minds to figure this stuff out. So on the previous slide, we saw things did horizontally. But as we see here, that was just one of several possibilities, right? We need to be able to have the mental flexibility to rotate these things in our minds and still read it. So if I fill in some values here with the sake of being able to describe things, let's say that this is, I don't know, a table. And this one over here is an order table. So what does this say? How can we read this relationship? Remember we read these relationships in two directions. So if I'm reading it in the direction from customer to order, this direction, customer to order, I would say each customer places one to many orders. So this is a one to many relationship in that direction. Each customer places one to many orders, which makes sense, right? Because someone can't be your customer unless they ordered something from you. Am I really a customer? I just walk into a store and walk back out. I don't know. I didn't buy anything. So this gets to the semantic definition of what is a customer. So philosophy. Now, if we took a look at the same relationship in the opposite direction, that is, in the direction from order to customer, we would read it as each order is placed by one and only one customer. So the minimum cardinality in that direction is one and the maximum cardinality is one. Interesting. So we have to have the mental flexibility to read these relationships. Both directions, regardless of the way that the lines and the symbols are rotated. If we have some other concept illustrated here, I don't know what I want to call this. Maybe this is, since I can't think of anything, entity. Say what is the relationship between customer and entity? So in this case, reading in this direction, we would say each customer can have zero to many entities, whatever that is. I think reading in this direction, we would say each entity is associated with zero to one customers. So the minimum cardinality here is zero, maximum cardinality is one. Minimum cardinality there is zero, maximum cardinality is many. Minimum cardinality is zero, minimum cardinality one, minimum cardinality one, minimum zero, set. So you will notice looking out here at these possibilities, actually, let me. I don't know, this is fine. If you look at all these possible combinations of minimum and maximum cardinalities, note the following. The many symbol can only appear as a maximum cardinality. Okay. And the zero symbol can only appear as a minimum cardinality. It doesn't make sense to have a maximum cardinality of zero, because that means the maximum number of rows that can participate in the relationship with another table is zero. So there's no relationship. Right. If I say that to the maximum number of connections is zero, well, then they're not related. Similarly, I cannot say that the minimum cardinality is many. Right. It has to be zero or one. So interesting things. Clear that out and return back to our slides. So again, just three symbols. One symbol indicates the concept of zero. One symbol indicates the concept of one. And the third symbol indicates the concept of many. With those, we can create these four possibilities. And these are the only four possibilities that we use for minimum maximum cardinalities to convey information about the nature of the relationship between two tables in our database design. Cool. So let's take a look at some additional ones here now that we have the knowledge and understanding of what all these various symbols mean. So here we see a relationship between a customer and an order. And this is very similar to what I illustrated just a moment ago. This tells us when we read the relationship in this case from left to right in the direction from customer to order. That's each customer places one to many orders. Minimum cardinality is one. Maximum cardinality is many. If we take a look at that same relationship in the opposite direction, the direction from order to customer. We say each order is placed by one and only one customer. That is an order cannot exist. Unless it is associated with the customer, that's the minimum cardinality. And each order cannot be associated with more than one customer. That's the maximum cardinality. So this tells us something about the way that this business operates. In order for an order to be placed or for an order to exist, it must be linked to exactly one customer. No more and no fewer. So what we've been seeing here, these types of diagrams, the ones that I illustrated a moment ago, these are entity relationship diagrams or our diagrams or ERDs. I just different words for the same thing. I like to call them ERDs because it's fast, but I know we get stuck in this world of too many acronyms. So you can call it an ER diagram, an ERD, an entity relationship diagram. It's all the same thing. But just to summarize and revisit in this world, we represent entities with rectangles. We use lines to represent relationships between entities. And we specify the cardinalities using this set of three different crowsfoot symbols, which we cleverly combine to convey both the minimum and maximum cardinalities for the relationship in both directions that we may want to read it.