CREATE TRIGGER Command


CREATE TRIGGER ON TableName FOR DELETE \| INSERT \| UPDATE AS lExpression

Use CREATE TRIGGER to trap for events that cause records in a table to be deleted, added, or changed. Delete, Insert, or Update triggers can be created only for a table that has been added to a database. Use CREATE DATABASE to create a database, and ADD TABLE to add a table to a database.

The following table describes the events that cause a Delete, Insert, or Update trigger to occur.

Event	Result
Delete Trigger	DELETE is issued. A record is marked for deletion from the Table Menu in a Browse window or Edit window. Note that issuing ZAP does not cause the Delete trigger to occur.
Insert Trigger	APPEND FROM is issued. APPEND FROM ARRAY is issued. APPEND BLANK is issued. A record is appended from the Table Menu in a Browse window or Edit window. IMPORT is issued. INSERT – SQL is issued. RECALL is issued. A record is recalled from the Table Menu in a Browse window or Edit window.
Update Trigger	GATHER is issued. The following rules apply to triggers created with CREATE TRIGGER: INSERT cannot be issued for a table with a trigger. However, INSERT – SQL can be used. Issuing PACK does not cause any triggers to occur. Issuing ZAP does not cause a Delete trigger to occur. No trigger occurs if you update a record marked for deletion. A trigger may not occur immediately, depending on the current buffering mode: REPLACE is issued. REPLACE FROM ARRAY is issued. UPDATE – SQL is issued. Other event that causes a record to be modified, such as when a Form changes the contents of a field.

If table buffering is in effect, the Update trigger occurs when TABLEUPDATE( ) is issued and each buffered record is updated in the table.

Note
When a trigger is called, the Alias is always that of the cursor being updated, regardless of the Alias selected in the code that caused the trigger to fire.

The following example creates an Update trigger which prevents values greater than 50 from being entered in the maxordamt field in the customer table. An error message is generated when the first REPLACE command is executed because the value for the maxordamt field is greater than 50. The second REPLACE command does not generate an error because the value for the maxordamt field is less than or equal to 50.

	Copy Code
CLOSE DATABASES OPEN DATABASE (HOME(2) + 'data\testdata') USE customer && Open customer table * Set trigger on maxordamt field to fail with values <= 50 CREATE TRIGGER ON customer FOR UPDATE AS maxordamt <= 50 ON ERROR && Restore the system error handler WAIT WINDOW "Press a key to test trigger with value of 60"+CHR(13); +"When you get the error message, press Ignore." REPLACE maxordamt WITH 60 && Displays an error message ? maxordamt WAIT WINDOW "Press a key to test with value of 50." REPLACE maxordamt WITH 50 && Value is accepted ? maxordamt DELETE TRIGGER ON customer FOR UPDATE && Remove the trigger

Copy Code

CLOSE DATABASES
OPEN DATABASE (HOME(2) + 'data\testdata')
USE customer  && Open customer table
* Set trigger on maxordamt field to fail with values <= 50
CREATE TRIGGER ON customer FOR UPDATE AS maxordamt <= 50
ON ERROR  && Restore the system error handler
WAIT WINDOW "Press a key to test trigger with value of 60"+CHR(13);
 +"When you get the error message, press Ignore."
REPLACE maxordamt WITH 60    && Displays an error message
? maxordamt
WAIT WINDOW "Press a key to test with value of 50."
REPLACE maxordamt WITH 50    && Value is accepted
? maxordamt
DELETE TRIGGER ON customer FOR UPDATE  && Remove the trigger

Reference

ADD TABLE Command
AERROR( ) Function
CREATE DATABASE Command
DELETE TRIGGER Command
DISPLAY DATABASE Command
LIST DATABASE Command
OPEN DATABASE Command

Other Resources

Commands (Visual FoxPro)
Language Reference (Visual FoxPro)

Parameters

Remarks

Example

See Also

Reference

Other Resources