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A hierarchy bridge table captures recursive instance relationships, so the dimension table will not need to. This means that a normal-looking dimension table can be used one that does not include a pig s ear or flattened relationships. Stars for various processes can refer to this dimension table directly, in the standard way. In fact, it is possible to build and implement a standard star that completely ignores the recursive hierarchy, and add a bridge in a future project. An example for the case of the company ownership hierarchy is shown in Figure 10-5. A standard company dimension appears in this design, with one row for each company. The shaded area of the diagram encloses a star schema for the orders process. The fact table in this star contains an order_dollars fact and refers to the company dimension directly via a company_key. This relationship captures the specific company with which an order transaction occurred, regardless of its place in any hierarchy. This star can be used to study orders by company in the usual way. To the right of the orders star, the figure shows a hierarchy bridge table for the company dimension. The specifics of how this bridge table is designed and used will be dealt with

Full backups every Sunday evening Differential backups completed on weeknights, Monday through Saturday, at 9:00 P.M. Transaction log backups performed every hour between the hours of 9:00 A.M. and 6:00 P.M.

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COMPANY company_ key 1 2 3 4 5 6 7 8 company_ name Company 1 Company 2 Company 3 Company 4 Company 5 Company 6 Company 7 Company 8

Assuming a complete loss of the database occurs on Thursday at 11:53 A.M., we ll need to restore from several backups and then recover the database to bring it back online. The exact sequence of steps is as follows: 1. Restore from the full database backup. 2. Restore differential backup from Wednesday. 3. Restore transaction log backups from 9:00 A.M. and 10:00 A.M. 4. Restore the transaction log backup from 11:00 A.M. with recovery. Figure 7-3 illustrates the database restoration and recovery operations required to bring this installation back online to the latest point in time allowable by the backups. When performing a backup, SQL Server also provides the ability to save an undo log file to disk. This file is used mainly for the purpose of establishing and maintaining standby servers (a topic we ll cover in 8, Advanced Data Protection Tactics ). Fortunately, as we ll see later in this chapter, the tools and utilities included with SQL Server 7 can make these complex operations much easier to manage. Now that we have an overview of the processes involved, let s move on to look at how we can use the specific backup types.

shortly. The important point here is that the star can be used without the bridge. If a user wants to group orders by the companies that placed them, the bridge is not needed. This means that the bridge table does not need to be built at the same time as the star; it can be added later. Without it, of course, there is no representation of the relationships between companies.

SELECT HAIR, EYES, COUNT(*) FROM PEOPLE GROUP BY HAIR, EYES HAIR ----Brown NULL NULL Brown Brown Brown EYES -----Blue Blue NULL NULL Brown Brown COUNT(*) -------1 2 2 3 2 2

Differential backups contain only the data that has changed since the time of the last full backup. Therefore, they are generally smaller than full backups and require less time to perform. When recovering data, however, you should keep in mind that you must have a working copy of the most recent full backup before you can begin restoring data. If several differential backups have been performed since the last full backup, you will likely want to use the most recent one. However, any of the backups can be used if, for example, you want to restore to a point before the last differential backup was made or if one or more of the backups is corrupt, missing, or otherwise unusable. Differential backups provide a good way to optimize backup plans, but do support some of the functions of transaction log backups (covered next).

The structure of a hierarchy bridge table is quite simple, but it is crucial to understand the nature of its contents. If you have dealt with a recursive relationship in an operational system, you may find the content of the bridge table to be familiar. Similar tables are often constructed to ease the process of operational reporting.

Transaction log backups frequently are used in circumstances where other backup types are not feasible. The transaction log stores information about the various database transactions that have been performed on the server since the log was last truncated. Generally, backing up the transaction log will take much less time than backing up the entire database, because only information about data that was modified is required. Addi-

Although this behavior of NULLs in grouping columns is clearly specified in the ANSI/ISO standard, it is not implemented in all SQL dialects. It's a good idea to build a small test table and check the behavior of your DBMS brand before counting on a specific behavior.