Specifying the WHERE clause for update/delete

Sometimes multiple users are accessing the same tables at the same time. In these situations, you need to decide when to allow your application to update the database. If you allow your application to always update the database, it could overwrite changes made by other users:

The sample display is titled Where Clause for Update / Delete. Three radio buttons offer the choices of Key Columns, Key and Updatable Columns, and Key and Modified Columns. In the sample, the button for Key and Updatable Columns is selected.

You can control when updates succeed by specifying which columns PowerBuilder includes in the WHERE clause in the UPDATE or DELETE statement used to update the database:

UPDATE table...
SET column = newvalue
WHERE col1 = value1
AND col2 = value2 ...

DELETE
FROM table
WHERE col1 = value1
AND col2 = value2 ...

Using timestamps Some DBMSs maintain timestamps so you can ensure that users are working with the most current data. If the SELECT statement for the DataWindow object contains a timestamp column, PowerBuilder includes the key column and the timestamp column in the WHERE clause for an UPDATE or DELETE statement regardless of which columns you specify in the Where Clause for Update/Delete box.

If the value in the timestamp column changes (possibly due to another user modifying the row), the update fails.

To see whether you can use timestamps with your DBMS, see Connecting to Your Database.

Choose one of the options in Table 21-1 in the Where Clause for Update/Delete box. The results are illustrated by an example following the table.

**Table 21-1: Specifying the WHERE clause for UPDATE and DELETE**
Option	Result
Key Columns	The WHERE clause includes the key columns only. These are the columns you specified in the Unique Key Columns box. The values in the originally retrieved key columns for the row are compared against the key columns in the database. No other comparisons are done. If the key values match, the update succeeds. Caution Be very careful when using this option. If you tell PowerBuilder only to include the key columns in the WHERE clause and someone else modified the same row after you retrieved it, their changes will be overwritten when you update the database (see the example following this table). Use this option only with a single-user database or if you are using database locking. In other situations, choose one of the other two options described in this table.
Key and Updatable Columns	The WHERE clause includes all key and updatable columns. The values in the originally retrieved key columns and the originally retrieved updatable columns are compared against the values in the database. If any of the columns have changed in the database since the row was retrieved, the update fails.
Key and Modified Columns	The WHERE clause includes all key and modified columns. The values in the originally retrieved key columns and the modified columns are compared against the values in the database. If any of the columns have changed in the database since the row was retrieved, the update fails.

Example

Consider this situation: a DataWindow object is updating the Employee table, whose key is Emp_ID; all columns in the table are updatable. Suppose the user has changed the salary of employee 1001 from $50,000 to $65,000. This is what happens with the different settings for the WHERE clause columns:

If you choose Key Columns for the WHERE clause, the UPDATE statement looks like this:
```
UPDATE Employee
SET Salary = 65000
WHERE Emp_ID = 1001
```
This statement will succeed regardless of whether other users have modified the row since your application retrieved the row. For example, if another user had modified the salary to $70,000, that change will be overwritten when your application updates the database.
If you choose Key and Modified Columns for the WHERE clause, the UPDATE statement looks like this:
```
UPDATE Employee
SET Salary = 65000
WHERE Emp_ID = 1001	
   AND Salary = 50000
```
Here the UPDATE statement is also checking the original value of the modified column in the WHERE clause. The statement will fail if another user changed the salary of employee 1001 since your application retrieved the row.
If you choose Key and Updatable Columns for the WHERE clause, the UPDATE statement looks like this:
```
UPDATE Employee
SET Salary = 65000
WHERE Emp_ID = 1001	
   AND Salary = 50000	
   AND Emp_Fname = original_value	
   AND Emp_Lname = original_value	
   AND Status = original_value	
   ...
```
Here the UPDATE statement is checking all updatable columns in the WHERE clause. This statement will fail if any of the updatable columns for employee 1001 have been changed since your application retrieved the row.