How can I Prevent Invalid Data Entries in MySQL Tables Using Triggers?

Preventing Inserts and Updates with Triggers in MySQL

Unlike other database systems, MySQL does not enforce check constraints natively. As such, it becomes necessary to explore alternative methods to prevent invalid data from entering tables. Triggers provide a robust mechanism to address this issue.

In the given example, we want to restrict the agency attribute in table foo to the values 1-5. A naive approach would involve creating a trigger that checks the value of new.agency before allowing the insert or update. However, simply ignoring the insertion or update is insufficient.

A more effective solution involves using the SIGNAL syntax. By raising an error (e.g., SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'your error message'), you can prevent the operation from executing and provide a meaningful error message to the user.

Here's an updated example of the trigger using the SIGNAL syntax:

create trigger agency_check
before insert on foo
for each row
begin
  if (new.agency < 1 or new.agency > 5) then
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Agency value must be between 1 and 5';
  end if 
end

This trigger will effectively prevent any attempt to insert or update the foo table with an invalid agency value.

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