Can Triggers Prevent INSERT Operations in MySQL?

Can Triggers Cause INSERTs to Fail?

In MySQL, triggers such as BEFORE INSERT can modify data before its insertion into a table. However, it is not immediately evident if they can prevent the INSERT operation from succeeding due to validation errors.

MySQL Trickery

However, blog posts have revealed a technique to circumvent this limitation. For example, a MySQL 5.0 or 5.1 trigger can force an attempt to access a non-existent column, which triggers an error and aborts the INSERT operation.

Example Trigger

Here's an example trigger that employs this trick:

CREATE TRIGGER mytabletriggerexample
BEFORE INSERT
FOR EACH ROW BEGIN
IF(NEW.important_value) < (fancy * dancy * calculation) THEN
    DECLARE dummy INT;

    SELECT Your meaningful error message goes here INTO dummy 
        FROM mytable
      WHERE mytable.id=new.id
END IF; END;

Other RDBMS

While MySQL's solution involves a workaround, other RDBMS may provide more direct methods for failing INSERTs in triggers.

THROW EXCEPTION

For instance, Microsoft SQL Server includes the THROW EXCEPTION syntax. It allows triggers to raise exceptions that can abort the INSERT operation.

db2

db2 supports the SIGNAL statement, which signals a user-defined condition that can terminate the INSERT operation.

Note: While these techniques demonstrate triggers' ability to fail INSERTs, it's important to use caution and consider the performance implications.

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