How Can MySQL Triggers Handle Duplicate Entries and Signal Exceptions to a C# Application?

How to Handle Duplicate Entries with Triggers in MySQL

When maintaining a table with unique constraints that extend beyond the limitations of key length, such as in the case of lengthy parameter strings, triggers can be employed to enforce these constraints. This article addresses how to handle duplicate entries and throw exceptions to notify your application.

Throwing Exceptions

To throw an exception and return an error to your C# code, you can utilize the SIGNAL statement within your trigger. The following code demonstrates how:

DECLARE msg VARCHAR(255);
IF (SomeTestToFail = "FAIL!") THEN
    SET msg = "DIE: You broke the rules... I will now Smite you, hold still...";
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
END IF;

This code sets an error message and signals an exception with SQL state '45000'. You can then capture this exception in your C# code.

Allowing Inserts

To allow inserts if no duplicate entry exists, you can use the following code:

IF num_rows = 0 THEN
    -- Allow insert
END IF;

This code checks if the count of duplicate entries (num_rows) is 0. If so, the insert is allowed to proceed.

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