Joins between two tables

To understand how a simple inner join is computed, consider the following query. It answers the question: which product sizes have been ordered in the same quantity as the quantity in stock?

SELECT DISTINCT Name, Size,
    SalesOrderItems.Quantity
FROM Products JOIN SalesOrderItems
ON Products.ID = SalesOrderItems.ProductID
  AND Products.Quantity = SalesOrderItems.Quantity;

Name	Size	Quantity
Baseball Cap	One size fits all	12
Visor	One size fits all	36

You can interpret the query as follows. Note that this is a conceptual explanation of the processing of this query, used to illustrate the semantics of a query involving a join. It does not represent how SQL Anywhere actually computes the result set.

Create a cross product of the Products table and SalesOrderItems table. A cross product contains every combination of rows from the two tables.
Exclude all rows where the product IDs are not identical (because of the join condition Products.ID = SalesOrderItems.ProductID).
Exclude all rows where the quantity is not identical (because of the join condition Products.Quantity = SalesOrderItems.Quantity).
Create a result table with three columns: Products.Name, Products.Size, and SalesOrderItems.Quantity.
Exclude all duplicate rows (because of the DISTINCT keyword).

Joins between two tables

See also