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C++Builder Database Architecture
database server supports, unless you intend for the result set to be updatable. If you want an
updatable result set, you must use Local SQL syntax so that the database engine can
determine which database tables to actually update.
The SQL statement can be a static SQL statement or one that includes parameters that are
dynamically replaced with real values. A query that uses replaceable parameters (known as a
dynamic SQL query) uses a colon to delineate those parameters, like so:
SELECT * FROM ORDERS
WHERE CustomerNumber=:CustNo
In this example, CustNo is the name of the replaceable parameter. You supply these named
parameters using TQuery’s Params property.
TIP
When editing TQuery’s SQL property, you can edit your SQL using
C++Builder’s full-blown Code Editor. You do this by clicking the Code
Editor button from within the SQL property editor. You’ll find
C++Builder’s Code Editor to be much more versatile than the TMemo
component that’s used to edit the property by default.
The Constrained property enables you to control what updates may be made to a live result
set. If you set Constrained to true, updates that would cause a row to be excluded from the
result set are not permitted. That is, if you set up a Query component to return only those
customers whose last names begin with A, an attempt to change the LastName column in a row
to start with B will fail. This works much the same as the WITH CHECK option on SQL VIEWs.
Tasks
To establish a live, or updatable, result set, two things must happen. First, you must set
TQuery’s RequestLive property to true. Second, the SQL you use to define the query must
conform to certain rules. These rules are different, depending on whether you are querying
local tables. For local tables, the SQL must
■ Use Local SQL syntax only.
■ Involve only one table.
■ Not have an ORDER BY clause.
■ Not contain aggregate functions.
■ Not contain calculated fields.
■ Use a WHERE clause involving comparisons of column names to scalar constants
only. Operators supported include LIKE, >, =, and