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row of the CUSTOMERS table currently being checked by the DELETE statement. The subquery in this example is a correlated subquery, as described in 9. Outer references will often be found in subqueries of a DELETE statement, because they implement the "join" between the table(s) in the subquery and the target table of the DELETE statement. In the SQL1 standard, a restriction on the use of subqueries in a DELETE statement prevented the target table from appearing in the FROM clause of a subquery or any of its subqueries at any level of nesting. This prevents the subqueries from referencing the target table (some of whose rows may already have been deleted), except for outer references to the row currently being tested by the DELETE statement's search condition. The SQL2 standard eliminated this restriction by specifying that the DELETE statement should treat such a subquery as applying to the entire target table, before any rows have been deleted. This places more overhead on the DBMS (which must handle the subquery processing and row deletion more carefully), but the behavior of the statement is well defined by the standard.

When a dimension attribute is singled out for type 3 treatment, it is modeled as a pair of columns. One column is designated to hold the current value; the other is designated to hold the old value. In the case of customer regions, they might be called region_current and region_previous, as depicted in Figure 8-4. When a change to the source value occurs, both attributes are updated. The new value is placed in the column representing the current value; this is a standard type 1 response. The old value is moved to the column representing the previous value. No additional rows are created. Since each fact is linked to a single dimension row containing both values, it is possible to create reports that use either value to group all data.

Figure A-2.

CUSTOMER Legend: SK 1 2 3 Surrogate Key Type 1 Type 2 Type 3 Pair NK Natural Key customer_key customer_id customer_name region_current region_previous address_line1 . . . SK NK 2 3 3 2

Typically, the values of data items stored in a database are modified when corresponding changes occur in the outside world. For example, in the sample database: When a customer calls to change the quantity on an order, the QTY column in the appropriate row of the ORDERS table must be modified. When a manager moves from one office to another, the MGR column in the OFFICES table and the REP_OFFICE column in the SALESREPS table must be changed to reflect the new assignment. When sales quotas are raised by 5 percent in the New York sales office, the QUOTA column of the appropriate rows in the SALESREPS table must be modified. In each case, data values in the database are updated to maintain the database as an accurate model of the real world. The smallest unit of data that can be modified in a database is a single column of a single row.

Appendix:

A dimensional design can accommodate heterogeneous attributes through the use of generic columns. For a given row, what is held in each column is determined by its type. Auxiliary tables can be used to map the purpose of each column to each type. This approach is very flexible for storing information but less nimble when it comes to querying the information. This may seem contrary to the purpose of a dimensional design, but it can be powerful when all interaction will take place through a custom-developed front-end application. Though not a common feature of a dimensional design, it may be worth considering in rare situations.

The UPDATE statement, shown in Figure 10-6, modifies the values of one or more columns in selected rows of a single table. The target table to be updated is named in the statement, and you must have the required permission to update the table as well as each of the individual columns that will be modified. The WHERE clause selects the rows of the table to be modified. The SET clause specifies which columns are to be updated and calculates the new values for them.

mation in the Knowledge Base is available from Microsoft s TechNet and from Microsoft Support Online. Microsoft has made its extensive Knowledge Base available online via the Internet. To access Support Online, point your browser to support.microsoft.com. The first time you visit the site, you ll be able to specify the type of user you are and receive the appropriate search options. Then, you ll be able to search for information on any Microsoft product by entering text search strings. Figure A-3 shows the Microsoft Product Support website (support.microsoft.com).

The use of generic attributes seeks to capture the diversity of various types using a single set of columns. In a dimension table, a set of columns describing core attributes is supplemented by a set of multipurpose columns, the purpose of which will vary by type. For example, a business might choose to handle location data in this manner. There may be several types of facilities factories, warehouses, stores, and offices. Each has a

FACILITY facility_key facility_type name attribute_1 attribute_2 attribute_3 attribute_4