### Berkeley DB Reference Guide: Access Methods |

# Logical join

A logical join is a method of retrieving data from a primary database using criteria stored in a set of secondary indexes. A logical join requires that your data be organized as a primary database which contains the primary key and primary data field, and a set of secondary indexes. Each of the secondary indexes 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:

>     Primary key:		Primary data:
>     apple		Convenience Store
>     blueberry	Farmer's Market
>     peach		Shopway
>     pear		Farmer's Market
>     raspberry	Shopway
>     strawberry	Farmer's Market

We would then create a secondary index with the key color, and, as the data items, the names of fruits of different colors.

>     Secondary key:		Secondary data:
>     blue		blueberry
>     red		apple
>     red		raspberry
>     red		strawberry
>     yellow		peach
>     yellow		pear

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 function:

1. The actual data should be stored in the database represented by the DB object used to invoke this function. Generally, this DB object is called the primary.

2. Secondary indexes should be stored in separate databases, whose keys are the values of the secondary indexes and whose data items are the primary keys corresponding to the records having the designated secondary key value. It is acceptable (and expected) that there may be duplicate entries in the secondary indexes.

   These duplicate entries should be sorted for performance reasons, although it is not required. For more information see the DB_DUPSORT flag to the DB->set_flags function.

What the DB->join function 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:

>     Secondary key:		Secondary data:
>     expensive	blueberry
>     expensive	peach
>     expensive	pear
>     expensive	strawberry
>     inexpensive	apple
>     inexpensive	pear
>     inexpensive	raspberry

The DB->join function provides logical 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, e.g., 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 indexes, 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.

### Example

Consider the following three databases:

personnel
  - key = SSN
  - data = record containing name, address, phone number, job title

lastname
  - key = lastname
  - data = SSN

jobs
  - key = job title
  - data = SSN

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 referenced by the handle txn.

>     DBC *name_curs, *job_curs, *join_curs;
>     DBC *carray[3];
>     DBT key, data;
>     int ret, tret;

>     name_curs = NULL;
>     job_curs = NULL;
>     memset(&key, 0, sizeof(key));
>     memset(&data, 0, sizeof(data));

>     if ((ret =
>         name_db->cursor(name_db, txn, &name_curs)) != 0)
>     	goto err;
>     key.data = "smith";
>     key.size = sizeof("smith");
>     if ((ret =
>         name_curs->c_get(name_curs, &key, &data, DB_SET)) != 0)
>     	goto err;

>     if ((ret = job_db->cursor(job_db, txn, &job_curs)) != 0)
>     	goto err;
>     key.data = "manager";
>     key.size = sizeof("manager");
>     if ((ret =
>         job_curs->c_get(job_curs, &key, &data, DB_SET)) != 0)
>     	goto err;

>     carray[0] = name_curs;
>     carray[1] = job_curs;
>     carray[2] = NULL;

>     if ((ret =
>         pers_db->join(pers_db, carray, &join_curs, 0)) != 0)
>     	goto err;
>     while ((ret =
>         join_curs->c_get(join_curs, &key, &data, 0)) == 0) {
>     	/* Process record returned in key/data. */
>     }

>     /*
>      * If we exited the loop because we ran out of records,
>      * then it has completed successfully.
>      */
>     if (ret == DB_NOTFOUND)
>     	ret = 0;

>     err:
>     if (join_curs != NULL &&
>         (tret = join_curs->c_close(join_curs)) != 0 && ret == 0)
>     	ret = tret;
>     if (name_curs != NULL &&
>         (tret = name_curs->c_close(name_curs)) != 0 && ret == 0)
>     	ret = tret;
>     if (job_curs != NULL &&
>         (tret = job_curs->c_close(job_curs)) != 0 && ret == 0)
>     	ret = tret;

>     return (ret);

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 logical join call. This code then loops over the join cursor getting the personnel records of each one until there are no more.

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