Transactions and concurrency control in SQL are really important when you’re working with databases because they help keep data accurate and consistent even when multiple users or processes are working on it at the same time. So first let’s talk about transactions. A transaction is like a bundle of operations you group together so they all succeed or fail as one unit. Imagine you're transferring money from one bank account to another. You don’t want the money to be taken from one account and not show up in the other, right? That’s why we use transactions

In SQL a transaction usually has four properties, called ACID, which stand for Atomicity Consistency Isolation and Durability. These make sure the transaction is handled properly. Atomicity means that all parts of the transaction must be completed or none at all. Consistency ensures that the database is left in a good state after the transaction is done. Isolation means transactions are separated from each other so they don’t mess each other up if they happen at the same time. Durability ensures that once the transaction is finished, the changes are saved permanently even if there’s a system crash

Concurrency control comes into play when multiple transactions are happening at the same time. Without control over this it could cause big problems. For instance two users might try to update the same data at the same time. One might overwrite the changes of the other or read data that’s not yet committed. This is where locking and isolation levels help. SQL uses locks to make sure only one transaction can change a piece of data at a time. There are different kinds of locks like shared locks where multiple transactions can read data but not write and exclusive locks where only one can write

Isolation levels control how visible changes in one transaction are to others before the transaction is finished. The stricter the isolation the less likely you’ll have problems like dirty reads where one transaction sees data from another transaction that isn’t finished. But strict isolation can slow things down so there's a trade-off between speed and safety

So in short transactions help you bundle operations to make sure everything works together or nothing happens at all and concurrency control helps manage how multiple transactions interact to avoid conflicts.

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