SAS/ACCESS 9.2 for Relational Databases: Reference, Fourth Edition

Optimizing Your SQL Usage 4 Passing Functions to the DBMS Using WHERE Clauses 47The DISTINCT operator is passed to Oracle and generates this Oracle code:select distinct custbase."STATE" from CUSTBASEOracle then passes the results from this query back to SAS.Optimizing the Passing of WHERE Clauses to the DBMSGeneral Guidelines for WHERE ClausesFollow these general guidelines for writing efficient WHERE clauses.3 Avoid the NOT operator if you can use an equivalent form.Inefficient: where zipcode not>8000Efficient: where zipcode= and =70000 and ZIPCODE12*4000.00Efficient: where SALARY>48000.003 Use DBKEY=, DBINDEX=, and MULTI_DATASRC_OPT= when appropriate. See“Using the DBINDEX=, DBKEY=, and MULTI_DATASRC_OPT= Options” on page48 for details about these options.Whenever possible, SAS/ACCESS passes WHERE clauses to the DBMS, because theDBMS processes them more efficiently than SAS does. SAS translates the WHEREclauses into generated SQL code. The performance impact can be particularlysignificant when you are accessing large DBMS tables. The following section describeshow and when functions are passed to the DBMS. For information about passingprocessing to the DBMS when you are using PROC SQL, see “Overview of OptimizingYour SQL Usage” on page 41.If you have NULL values in a DBMS column that is used in a WHERE clause, beaware that your results might differ depending on whether the WHERE clause isprocessed in SAS or is passed to the DBMS for processing. This is because DBMSs tendto remove NULL values from consideration in a WHERE clause, while SAS does not.To prevent WHERE clauses from being passed to the DBMS, use the LIBNAMEoption DIRECT_SQL= NOWHERE.Passing Functions to the DBMS Using WHERE ClausesWhen you use the SAS/ACCESS LIBNAME statement, SAS/ACCESS translatesseveral SAS functions in WHERE clauses into DBMS-specific functions so they can bepassed to the DBMS.In the following SAS code, SAS can translate the FLOOR function into a DBMSfunction and pass the WHERE clause to the DBMS.