Berkeley DB supports "equality" (also known as "natural"), joins on secondary indices. An equality join is a method of retrieving data from a primary database using criteria stored in a set of secondary indices. It requires the data be organized as a primary database which contains the primary key and primary data field, and a set of secondary indices. Each of the secondary indices is indexed by a different secondary key, and, for each key in a secondary index, there is a set of duplicate data items that match the primary keys in the primary database.
For example, let's assume the need for an application that will return the names of stores in which one can buy fruit of a given color. We would first construct a primary database that lists types of fruit as the key item, and the store where you can buy them as the data item:
We would then create a secondary index with the key color, and, as the data items, the names of fruits of different colors.
This secondary index would allow an application to look up a color, and then use the data items to look up the stores where the colored fruit could be purchased. For example, by first looking up blue, the data item blueberry could be used as the lookup key in the primary database, returning Farmer's Market.
Your data must be organized in the following manner in order to use the DB->join method:
What the DB->join method does is review a list of secondary keys, and, when it finds a data item that appears as a data item for all of the secondary keys, it uses that data items as a lookup into the primary database, and returns the associated data item.
If there were a another secondary index that had as its key the cost of the fruit, a similar lookup could be done on stores where inexpensive fruit could be purchased:
The DB->join method provides equality join functionality. While not strictly cursor functionality, in that it is not a method off a cursor handle, it is more closely related to the cursor operations than to the standard DB operations.
It is also possible to do lookups based on multiple criteria in a single operation. For example, it is possible to look up fruits that are both red and expensive in a single operation. If the same fruit appeared as a data item in both the color and expense indices, then that fruit name would be used as the key for retrieval from the primary index, and would then return the store where expensive, red fruit could be purchased.
Consider the following three databases:
Consider the following query:
Return the personnel records of all people named smith with the job title manager.
This query finds are all the records in the primary database (personnel) for whom the criteria lastname=smith and job title=manager is true.
Assume that all databases have been properly opened and have the handles: pers_db, name_db, job_db. We also assume that we have an active transaction to which the handle txn refers.
DBC *name_curs, *job_curs, *join_curs; DBC *carray; DBT key, data; int ret, tret;
The name cursor is positioned at the beginning of the duplicate list for smith and the job cursor is placed at the beginning of the duplicate list for manager. The join cursor is returned from the join method. This code then loops over the join cursor getting the personnel records of each one until there are no more.