T-SQL ENHANCEMENTS Hierarchical queries Declarative syntax for tree-based

T-SQL ENHANCEMENTS Hierarchical queries Declarative syntax for tree-based queries PIVOT Declarative syntax aggregations across columns and converting columns to rows APPLY New JOINsyntax made for use with user-defined functions and XML TOP Row count based on an expression Transaction abort TRY/CATCHsyntax for handling errors SNAPSHOT Isolation SQL Server changes the state of a database by performing a transaction on it. Each transaction is a unit of work consisting of one or more steps. A perfect transaction is ACID, meaning it is atomic, consistent, isolated, and durable. In short, this means that the result of performing two transactions on a database, even if they are performed simultaneously by interleaving some of the steps that make them up, will not corrupt the database. Atomic means that a transaction will perform all of its steps or fail and perform none of its steps. Consistent means that the transaction must not leave the results of a partial calculation in the database; for example, if a transaction is to move money from one account to another, it must not terminate after having subtracted money from one account but not having added it to another. Isolated means that none of the changes a transaction makes to a database become visible to other transactions until the transaction making the changes completes, and then they all appear simultaneously. Durable means that changes made to the database by a transaction that completes are permanent, typically by being written to a medium like a disk. A transaction need not always be perfect. The isolation level of a transaction determines how close to perfect it is. Prior to SQL Server 2005, SQL Server provided four levels of isolation: READ UNCOMMITTED, REPEATABLE READ, READ COMMITTED, and SERIALIZABLE. A SERIALIZABLE transaction is a perfect transaction. Functionally, a database could always use SERIALIZABLE that is, perfect transactions, but doing so would typically adversely affect performance. Judicious use of isolation levels other than SERIALIZABLE, when analysis of an application shows that it does not require perfect transactions, will improve performance in these cases. SQL Server uses the isolation level of a transaction to control concurrent access to data through a set of read and write locks. It applies these

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