How to Default a Field to the Auto-Generated ID in MySQL Using Triggers?

MySQL Trigger to Default a Field to the Value of Auto-Generated ID

In MySQL, it is possible to create triggers to perform specific operations on table data before or after modifications occur. One common use case is to default a field to the value of the auto-generated primary key (ID) in situations where it is left null upon insertion.

Problem Statement:

To achieve this, let's consider a table with three columns: id (primary key), group_id (integer), and value (varchar). The objective is to have the following behavior upon record insertion:

• If group_id is provided, do not modify it.
• If group_id is omitted, set it to the value of the newly generated id.

Trigger Implementation:

While it would be desirable to perform this operation within a single statement or trigger, MySQL limitations prevent it. One workaround is to utilize a two-step approach:

1. Create a BEFORE INSERT trigger to set group_id to id if group_id is not specified:

CREATE TRIGGER MyTrigger BEFORE INSERT ON MyTable
FOR EACH ROW BEGIN
  SET NEW.group_id = COALESCE(NEW.group_id, NEW.id);
END

2. Since MySQL does not allow modifications of column values after INSERT, an UPDATE statement must be performed immediately after insertion to handle cases where group_id was omitted:

INSERT INTO MyTable (group_id, value) VALUES (NULL, 'a');
UPDATE MyTable SET group_id = COALESCE(group_id, id) WHERE id = LAST_INSERT_ID();

Limitations:

It's important to note that the BEFORE INSERT trigger cannot access the newly generated id value as it has not yet been assigned. However, using the AFTER INSERT trigger to access the generated value is not an option as column values cannot be modified in that phase.

Therefore, the two-step approach presented above provides a solution to the desired behavior in MySQL.

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