Learningcomputer.com SQL Server 2008 – Entity Relationships in a Database

Some Terms to get us going Entity relationship diagram (ERD) shows the db structure An Entity or Table is any object that we store information on An Attribute or a field is a specific value in a Table like SSN in an Employee table A Row or Record is one instance on an object in a Table A Primary Key (PK) is a field that can server as a unique identifier in a table A Foreign Key (FK) is a field in a child table that stores related info from a parent table e.g. Customer and Orders Relationship is an association between tables

Customer and SalesOrderHeader (AdventureWorks2008)

Types of Database Relationship One to One One to Many (Most common) Many to Many

One - One Relationship In this relationship, a row in table A can have no more than one matching row in table B, and vice versa Not very common Divide a table with many columns into two for performance Isolate part of a table for security reasons Example Account and Account Ext table

One - Many Relationship This is the most common type of relationship. In this type of relationship, a row in table A can have one to many matching rows in table B, but a row in table B can have only one matching row in table A Typically the PK of the primary (parent) table matches the same data (FK) in the secondary (child) table. Example is Customer and SalesOrderHeader table shown earlier

Many to Many Relationship In this type, a row in table A can have many matching rows in table B, and vice versa You create such a relationship by defining a third table, called a junction table, whose primary key consists of the foreign keys from both table A and table B Example would be Students and Teachers tables

College database I have created a new database called College First I will create and populate student data using TSQL Second I will create and populate teacher data using TSQL Now remember this is a many – many relationship so what do we need? We need a junction table I will create the junction table called student_teacher which will contain a field for studentid and one for teacherid Demo

What in the world is a Join? In order to understand relationships, you have to know Joins Joins are an integral component of relational database design and usage Joins let you match data from multiple tables; based on significant key information A typical join condition specifies a foreign key from one table and its associated primary key in the other table Types of Joins are INNER and OUTER

Inner Join A join that displays only the rows that have a match in both joined tables Most common type of join in practice For example, you can join the Customers and SalesOrderHeader tables to create a result set that shows the Orders for each Customer Null values do not match other null values so they are ignored You can have any number of joins in one statement as long as there is a valid relationship between the tables

Syntax for Inner Join Old Syntax is SQL-87: SELECT * FROM SALES.CUSTOMER C, SALES.SALESORDERHEADER S WHERE C.CUSTOMERID=S.CUSTOMERID New Syntax (ANSI SQL or SQL-92): SELECT * FROM SALES.CUSTOMER C INNER JOIN SALES.SALESORDERHEADER S ON C.CUSTOMERID=S.CUSTOMERID Demo

Outer Join A join that includes rows even if they do not have related rows in the joined table Outer joins are typically used to find corrupt data or unmatched data After the results, you can run a delete query to remove the problem records Three variations of an outer join to specify the unmatched rows to be included

Types of Outer Joins LEFT OUTER JOIN RIGHT OUTER JOIN FULL OUTER JOIN

Left Outer Join All rows from the first-named table (the "left" table, which appears leftmost in the JOIN clause) are included. Unmatched rows in the right table do not appear. We are going to use Customer and SalesOrderHeader tables Basically we are interested in finding out Customers that have yet to place an order Demo

Right Outer Join All rows in the second-named table (the "right" table, which appears rightmost in the JOIN clause) are included. Unmatched rows in the left table are not included. Similar in concept to Left Outer Join except it is reversed

Full Outer Join All rows in all joined tables are included, whether they are matched or not. For example, a full outer join between students and teachers shows all students and all teachers, even those that have no match in the other table. This type of join is not very common

Referential integrity Referential integrity is a system of rules that ensure relationships between related tables are valid You cannot enter a value in the foreign key column of the child table (Orders) if that value does not exist in the primary key of the parent table (Customers) You cannot delete a row from a primary table (Customers) if rows matching it exist in a related table (Orders) In SQL Server you can enforce this via Cascade Updates and Delete or through the use of Triggers