Understanding the CONSTRUCT instruction

The CONSTRUCT instruction provides database query by example. Query by example enables a user to query a database by specifying values (or ranges of values) for screen fields that correspond to the database columns. The runtime system converts the query values entered by the user into a boolean SQL condition that can be used in the WHERE clause of a prepared SELECT statement.

The CONSTRUCT statement produces an SQL condition corresponding to all search criteria that a user specifies in the fields. The instruction fills a character variable with that SQL condition, and you can use the content of this variable to create the WHERE clause of a SELECT statement. The SELECT statement must be executed with the dynamic SQL management instructions PREPARE or DECLARE FROM:

The CONSTRUCT instruction uses the data types of the form field to verify user input and to produce the SQL condition.

Important: The SQL condition is generated according to the current database session, which defines the type of the database server. Therefore, the program must be connected to a database server before entering the CONSTRUCT block. The generated SQL condition is specific to the database server and may not be used with other types of database servers.

If no criteria were entered, the string '1=1' is assigned to the string variable. This is a boolean SQL expression that always evaluates to true so that all rows are returned.

The CONSTRUCT dialog activates the current form. This is the form most recently displayed or, if you are using more than one window, the form currently displayed in the current window. When the CONSTRUCT statement completes execution, the form is cleared and deactivated.

By default the screen field tabbing order is defined by the order of the field names in the FROM clause; by default this is the list of column names in the ON clause when no FROM clause is specified. If needed, change the field tabbing order with the FIELD ORDER FORM option and TABINDEX field attributes.