Creating multiple tables and table relationships
[Instructor] In this section, we're going to explore some of the relationships between tables. Microsoft SQL Server is a relational. If it asks you to create a diagram say yes, then add the tables and voila! ApexSQL Search is a FREE SQL Server Management Studio and Visual Studio add-in that, among other features, has the View Dependencies feature. In my test database I want to create a table which records names of users. The check I want to enforce is that the user name must be one of the LastName's of a .
Let's demonstrate how this works. What happens if we try to add another address for a user who already has one? How about if we try to add an address for a user who doesn't exist?
Here we get a different error. If you're wondering why we can add a user without an address but can't add an address without a user, this is down to the modality of the relationship between the two entities. Don't worry about exactly what this means for now, just think of it as another aspect of entity relationships.
Adding this clause, and setting it to CASCADE basically means that if the row being referenced is deleted, the row referencing it is also deleted.
Determining what to do in situations where you delete a row that is referenced by another row is an important design decision, and is part of the concept of maintaining referential integrity.
Create Relationships Between Tables on a Diagram (Visual Database Tools)
One-to-Many Okay, time to get back to our different table relationship types with a look at one-to-many. A one-to-many relationship exists between two entities if an entity instance in one of the tables can be associated with multiple records entity instances in the other table. The opposite relationship does not exist; that is, each entity instance in the second table can only be associated with one entity instance in the first table.
A book has many reviews. A review belongs to only one book. Let's set up the necessary data.
Table of Contents
There's a key difference worth pointing out in the statement for our reviews table however: In other words a book can have many reviews. Now we have created our books and reviews tables, let's add some data to them. Since a column in reviews references data in books we must first ensure that the data exists in the books table for us to reference. We set up the table in this way for our example because we wanted to focus on the one-to-many relationship type. If we had added such a Foreign Key to reviews we'd effectively be setting up a Many-to-Many relationship between books and users, which is what we'll look at next.
Many-to-Many A many-to-many relationship exists between two entities if for one entity instance there may be multiple records in the other table, and vice versa. A user can check out many books. A book can be checked out by many users over time.
In order to implement this sort of relationship we need to introduce a third, cross-reference, table. We already have our books and users tables, so we just need to create the cross-reference table: Each row of the checkouts table uses these two Foreign Keys to create an association between rows of users and books.
If a foreign key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering.
How to create database diagram with Microsoft SQL Server Management Studio - SSMS Tutorials
The table opens in Table Designer. From the Table Designer menu, click Relationships. In the Foreign-key Relationships dialog box, click Add. Click the relationship in the Selected Relationship list. Click Tables and Columns Specification in the grid to the right and click the ellipses In the Tables and Columns dialog box, in the Primary Key drop-down list, choose the table that will be on the primary-key side of the relationship.
In the grid beneath, choose the columns contributing to the table's primary key.
In the adjacent grid cell to the left of each column, choose the corresponding foreign-key column of the foreign-key table.
Table Designer suggests a name for the relationship. To change this name, edit the contents of the Relationship Name text box. Choose OK to create the relationship. On the Standard bar, click New Query. Copy and paste the following example into the query window and click Execute.