Understanding Parameter Substitution in Update SQL Statements
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Update SQL statements use a special form of parameter substitution that enables you to substitute old or new field values in record updates. When the Update SQL editor generates its statements, it determines which field values to use. When you write the update SQL, you specify the field values to use.
When the parameter name matches a column name in the table, the new value in the field in the cached update for the record is automatically used as the value for the parameter. When the parameter name matches a column name prefixed by the string “OLD_”, then the old value for the field will be used. For example, in the update SQL statement below, the parameter :LastName is automatically filled with the new field value in the cached update for the inserted record.
INSERT INTO Names (LastName, FirstName, Address, City, State, Zip) VALUES (:LastName, :FirstName, :Address, :City, :State, :Zip)
New field values are typically used in the InsertSQL and ModifySQL statements. In an update for a modified record, the new field value from the update cache is used by the UPDATE statement to replace the old field value in the base table updated.
In the case of a deleted record, there are no new values, so the DeleteSQL property uses the “:OLD_FieldName” syntax. Old field values are also normally used in the WHERE clause of the SQL statement for a modified or deletion update to determine which record to update or delete.
In the WHERE clause of an UPDATE or DELETE update SQL statement, supply at least the minimal number of parameters to uniquely identify the record in the base table that is updated with the cached data. For instance, in a list of customers, using just a customer’s last name may not be sufficient to uniquely identify the correct record in the base table; there may be a number of records with “Smith” as the last name. But by using parameters for last name, first name, and phone number could be a distinctive enough combination. Even better would be a unique field value like a customer number.
For more information about old and new value parameter substitution, see Creating an OnUpdateRecord Event Handler in the Delphi Developer’s Guide.