Greetings, I require to impose a restriction on my table wherein if a worker's status is "in service", it is prohibited to alter it to "unavailable".

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To impose this restriction on your table, you can add a constraint using SQL. Here's an example code:

ALTER TABLE workers ADD CONSTRAINT CK_WorkerStatus CHECK (status <> 'unavailable' OR status IS NULL);

This constraint will ensure that the status column cannot be updated to 'unavailable' if the current status of the worker is 'in service'. If you try to update the status column in this scenario, the constraint will be violated and the update will fail. However, if the current status is null, the constraint will not apply and any value can be inserted for that particular worker.