Creating Entity Relationship Diagrams

Identify data stores The data stores are easy to identify if you have already created a data flow diagram If you are working from a document list the data stores are usually identified by; “filed into”, “from the …. file” etc. Name of data storeM

Add the data stores to the ERD diagram For each data store draw a rectangle with the name of the data store inside Product Sales Customer Salesmen

Look at the data in each store (entity)to find fields (attributes) they have in common by which they can be linked Product ProductID Product Stock level Price Sales Inv No CustID Date ProductID Quantity Customer CustID Name Address Salesmen PayNo Name Address Tel In this system: - Customer links to Sales via CustID Product links to Sales via ProductID Salesmen has no connection

Just looking at the linking fields If you found a link value in one table, how many records with the same link value at the opposite end might be found over time? If “many” is possible then the connecting line (relation) should have a “Crows foot” connector at that end. Repeat this in the opposite direction

N-Ary Relations Sales Inv No CustID Date ProductID Quantity Customer CustID Name Address CustIDNameAddress 1FredThe house 2JaneThe castle InvNoCustIDDateProdIDQuantity 4101/01/ /01/ /03/1439 A CustID only ever appears one in the Customer table but the same CustID may appear many times in the Sales table.

The Final Entity Relationship Table One customer may be involved in many sales but one sale only involves one customer. One product may be involved in many sales invoices but one Sales invoice product may only identify one product in the Product table. Product ProductID Product Stock level Price Sales Inv No CustID Date ProductID Quantity Customer CustID Name Address