A simple DB containing two tables could be:

> 
> CREATE TABLE Stock ( StockID INT StockDesc VARCHAR SupplierID INT 
>  PRIMARY KEY (StockID) FOREIGN KEY (SupplierID) REFERENCES Supplier(SupplierID))
> 
> 
> CREATE TABLE Supplier ( SupplierID INT SupplierName VARCHAR PRIMARY
>  KEY (SupplierID) )
> 
> 
> 

*(Assume each stockID has only one supplier)*

A typical SQL query would be:

> 
> SELECT StockID, StockDesc FROM Stock, Supplier WHERE Stock.SupplierID
>  = Supplier.SupplierID AND SupplierName = "Smiths";
> 
> 
> 

My fundamental understanding of Database design alerts me to the fact that I've already established that there is a relationship between STOCK and SUPPLIER tables via the DDL. Why does SQL need to restipulate that fact again in the first WHERE clause?