Why is Consistency Important in ACID if Schemas Already Enforce Rules?

Actually, most RDBMS enforce constraints at the operation level instead of the transaction level for consistency. Some do allow deferred constraints that only get checked at the end of a transaction, so in some cases, you may not have tight consistency with every operation.

Schemas alone don’t enforce consistency— they define what a consistent state looks like. If your database lacks a consistency layer, it could allow transactions to lead to inconsistent states, which would be a disaster. For example, without consistency enforced, you might end up with records missing critical keys, leading to a messy situation. Having 'C' ensures that transactions only lead to consistent states through valid schema definitions.

In many cases, you shouldn't be inserting data outside of a transaction anyway. For example, SQL Server automatically creates a transaction if there isn't one present. Once you run your statement, it either keeps the transaction open or commits it if everything goes smoothly.

You're right only if you think of transactions in a narrow scope, like just one table. But if your transaction spans multiple tables or processes (like ensuring a unit of work is consistent), it can get tricky. For instance, if you're working on a database of people and their various attributes, missing some can lead to inconsistencies that the schema alone won't correct. It's all about ensuring that the entire transaction is successful or it fails as a whole. Good schema enforcement helps here too.

Good question! ACID's properties outline what we want from transactions. Consistency is one of those key properties, and the schema constraints define what consistency means for your data. Just because you have constraints doesn’t mean you can ignore consistency in transactions. You want all four ACID properties to ensure reliable database operations. Also, treating a single operation outside of a transaction is like the same as handling it within one—that's how you ensure those properties are met.