The CONSTRUCT sub-dialog
The CONSTRUCT sub-dialog provides database query by example feature,
converting search criteria entered by the user into an SQL WHERE condition that can
be used to execute a SELECT statement.
Defining query by example fields
CONSTRUCT sub-dialog requires a character string variable to hold the
WHERE clause, and a list of screen fields where the user can enter search criteria.
PRIVATE DEFINE sql_condition STRING
...
DIALOG cust_query()
CONSTRUCT BY NAME sql_condition
ON customer.cust_name, customer.cust_address
BEFORE FIELD cust_name
...
END CONSTRUCT
...
END DIALOGMake sure the character string variable is large enough to store all possible SQL conditions.
It is better to use a STRING data
type to avoid any size problems.
CONSTRUCT uses the field data types defined
in the current form file to produce the SQL conditions. This is different from other interactive
instructions, where the data types of the program variables define the way to handle input/display.
It is strongly recommended (but not mandatory) that the form field data types correspond to
the data types of the program variables used for input. This is implicit if both form fields and
program variables are based on the database schema file.
The CONSTRUCT clause can be used in two forms:
CONSTRUCT BY NAME string-variable ON column-listCONSTRUCT string-variable ON column-list FROM field-list
The BY NAME clause implicitly binds the form fields to the columns, where the
form field identifiers match the column names specified in the column-list after the
ON keyword. You can specify the individual column names (separated by commas)
or use the tablename.* shortcut to include all columns defined for a table in
the database schema file.
The FROM clause explicitly binds the form fields listed after the
FROM keyword with the column definitions listed after the ON
keyword.
In both cases, the name of the columns in column-list will be used to produce the SQL condition in string-variable.
Identifying a CONSTRUCT sub-dialog
The name of a CONSTRUCT sub-dialog can be used to qualify sub-dialog actions with a prefix. In order to
identify the CONSTRUCT sub-dialog with a specific name, use the
ATTRIBUTES clause to set the NAME attribute:
CONSTRUCT BY NAME sql_condition ON customer.*
ATTRIBUTES (NAME = "q_cust")
...Control blocks in CONSTRUCT
A
Query By Example declared with the CONSTRUCT clause
can raise the following triggers:
In the singular CONSTRUCT instruction, BEFORE CONSTRUCT and
AFTER CONSTRUCT blocks are typically used as initialization and
finalization blocks. In DIALOG block, BEFORE CONSTRUCT and
AFTER CONSTRUCT blocks will be executed each time the focus goes to
(BEFORE) or leaves (AFTER) the group of fields defined
by this sub-dialog.