When developing most applications, you end up having to interact with a database. If you’re application is large enough and warrants the design and effort, it will no doubt be tiered as well. If your application is tiered, one of your layers will probably include a database API. A database API is used to restrict access to the database, a funnel which all requests from your application must pass through. Implementing this additional layer of abstraction normally yields in higher performance, increased security and easier maintenance.
In most cases, you’re application code will require a fairly standard interface into your database to retrieve information. A simple way to achieve that consistent interface is using the DECODE function in your PL/SQL procedures and functions. There are always cases where you need to do something special, and in those cases it is probably a perfect scenario for this methods counter-part – the string building technique.
For a simple but practical example, consider the following table definition:
CREATE TABLE MyTable ( PrimaryKeyID NUMBER NOT NULL, ForeignKeyID NUMBER NOT NULL, Name VARCHAR2 (255) NOT NULL)
As I have mentioned before, the Oracle DECODE function provides similar functionality to an IF-THEN-ELSE code block. Lets examine an example which relates to MyTable above.
PROCEDURE Get (pDetails OUT grcDetails, pPrimaryKeyID IN MyTable.PrimaryKeyID%TYPE DEFAULT NULL, pForeignKeyID IN MyTable.ForeignKeyID%TYPE DEFAULT NULL, pName IN MyTable.Name%TYPE DEFAULT NULL) IS BEGIN OPEN pDetails FOR SELECT * FROM MyTable WHERE PrimaryKeyID = DECODE(pPrimaryKeyID, NULL, PrimaryKeyID, 0, PrimaryKeyID, pPrimaryKeyID) AND ForeignKeyID = DECODE(pForeignKeyID, NULL, ForeignKeyID, 0, ForeignKeyID, pForeignKeyID) AND LOWER(Name) LIKE DECODE(pName, NULL, LOWER(Name), '%'||LOWER(pName)||'%'); END;
Working top down, the second line declares pDetails (the parameter name) as being an OUT parameter. As the name suggests, an OUT parameter is a way for you to pass information out of your procedure. In some aspects it would be similar to returning a value out of a typical programming function. The difference to an OUT parameter is that you can have many OUT parameters in a single procedure, where as a typical programming function can only return a single value at a time.
The next thing you’ve probably noticed is this weird thing in there, grcDetails. Just as the IN parameters below it, grcDetails is the type of the parameter. It would be akin to defining a paramter in a traditional function as accepting an integer or string type. In this case, grcDetails is declared in the package header:
TYPE grcDetails IS REF CURSOR RETURN MyTable%ROWTYPE;
The important thing to notice about grcDetails is that it is a REF CURSOR and its type is the ROWTYPE of MyTable. The benefit of defining it as a ROWTYPE is that if tomorrow the definition of the table changed in some way, you don’t have to worry about changing any code to match the new table definition – it simply continues to work. This is the same reason why the INPUT parameters above have their type defined against the field that they represent. Of course, if you are passing in or out a value which isn’t based on a field – you would simply define it as a standard PL/SQL type.
Next, you have probably noticed that each IN parameter has a DEFAULT value. As with most programming languages, providing a DEFAULT value allows the programmer to optionally supply that parameter. For our example, this is a pivotal point as it simplifies the use of this method; more on that later.
Moving on to the actual SQL statement, the flexibility of the DECODE function starts to show. Since the DECODE function acts like an IF-THEN-ELSE block, you’re actually seeing an inline IF-THEN-ELSE IF-ELSE block per field. Lets take a practical example of that statement and assume we passed in pPrimaryKeyID with a value of 1, while the other two input parameter’s are not passed in (thus taking their default value of NULL). The SQL statement would be parsed and executed as follows:
SELECT *FROM MyTableWHERE PrimaryKeyID = 1 AND ForeignKeyID = ForeignKeyID AND LOWER(Name) LIKE LOWER(Name);
Of course, the net effect of that SQL statement is that the first WHERE condition is used, while the second and third conditions are nullified as the left and right half of each expression are equivalent. So by passing in a NULL value for the pForeignKeyID and pName parameters, they are effectively removed from the SQL statement by evaluating to themselves. Of course, you could also pass in two, three or none of the parameters to the procedure as well. In which case you would get either a restricted set from the statement or all records in the table.
As mentioned above, the DEFAULT value of each IN parameter is critical to this methods success. By providing a default value, the programmer no longer needs to call the procedure with all parameters; all, some or none are also perfectly valid combinations. Since the input parameter’s default value is NULL, the following DECODE function call is all that is required to negate the parameter in the WHERE clause:
PrimaryKeyID = DECODE(pPrimaryKeyID, NULL, PrimaryKeyID, pPrimaryKeyID)
By now, you have probably noticed some extra parameters in the example GET procedure listed above. The additional values are used to exclude any other ‘not important’ values from the statement. In the example procedure, NULL and the value 0 are considered unimportant; whilst all other values are considered useful.
If you’re wondering why you’d want to exclude other values, it might be to make another section of your application simpler. Some programming languages don’t support nullable primitive types. If you use such a language and you intend to pass in all parameters to your procedure in all circumstances (this is the ‘simpler’ above); then all values will have a value of some sort. In the case of a primitive such as an integer, you might find that its default uninitialised value is zero. If that is the case and you don’t require the ability to filter on a zero value, then excluding it within the DECODE function makes things simpler.
Don’t think you’re limited to using this method on SELECT statements, it will work a treat on DELETE too. Consider the following DEL procedure:
PROCEDURE Del (pRowCount OUT NUMBER, pRoomTypeID IN RoomTypes.ROOMTYPEID%TYPE DEFAULT NULL, pBuildingID IN RoomTypes.BUILDINGID%TYPE DEFAULT NULL) IS BEGIN IF (pRoomTypeID IS NOT NULL OR pBuildingID IS NOT NULL) THEN DELETE FROM RoomTypes WHERE RoomTypeID = DECODE(pRoomTypeID, NULL, RoomTypeID, 0, RoomTypeID, pRoomTypeID) AND BuildingID = DECODE(pBuildingID, NULL, BuildingID, 0, BuildingID, pBuildingID); ELSE RAISE_APPLICATION_ERROR(-20001, 'At least one parameter must be supplied'); END IF; pRowCount := SQL%ROWCOUNT; END;
There is a caveat to using this method for building dynamic SQL within Oracle, it cannot handle columns which are nullable. If you consider the use of the DEFAULT value on all of the input parameters, it will become clear. Within Orcale PL/SQL, it is not possible to use an equality (=) operator to compare a NULL value. As a simple example, take the two simple SQL statements:
SELECT * FROM DUALSELECT * FROM DUAL WHERE NULL = NULL
The first SQL statement above will return the expected row from the DUAL table, while the second statement will return no results as you cannot compare the NULL value in that manner. If a comparison against a NULL is required, it must be handled using the IS NULL clause.
Other than not being able to use this method against columns which are nullable, its a really convenient way to write dynamic SQL in Oracle. Next time you need a little flexibility and you don’t want to go down the string building path, try using the Oracle DECODE function to produce your dynamic SQL.