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SQL Anywhere 12.0.0 » SQL Anywhere Server - SQL Usage » Querying and modifying data » Joins: Retrieving data from several tables » How joins work

 

Joining 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).

For a description of how outer joins are computed, see Outer joins.