Security, Transactions, and Views

Security Achieved through GRANT & REVOKE Assumes the database can recognize its users and verify their identity can also be controlled through the use of views - subsets of data usually maintained by the database administrator or DBA

Views A view is representation of an existing table which corresponds to the SELECT statement that created it. The view can then be manipulated much like an actual table. A view is not a separate table or entity. It’s more like a mask of the actual table.

Uses of a VIEW Hiding sensitive data from users Preserving a previous table schema Presenting data to users in a desired format. Simplify a complex query

Creating a VIEW CREATE VIEW view_name [(col_name…)] AS SELECT _statement

Dropping a VIEW DROP VIEW view_name Only drops the view… not the table.

More about Views The view displays like any table and the data you see is the actual data in the table(s). A view is more for viewing rather than updating since an update could disqualify a record from the view. Updates made to a view are made to the table(s) and any changes made to the table(s) are reflected in the view.

Naming View Columns Column names are inherited from the underlying tables. New names can be assigned Columns must be renamed when using arithmetic expressions or when more than one column has the same name.

Formatting It is sometimes necessary to alter the output of a query for the sake of readability such as in report generation. This can also be applied to a view creation which users will share.

SUBSTRING Returns a part of a character or binary string. SUBSTRING (expression, start, length) SUBSTRING (‘SQL Programming’, 1, 3) = SQL

CONVERT Changes one datatype to another. CONVERT(datatype[length], expression) CONVERT(char(2), ‘SQL’) = SQ CONVERT(int, ‘10’) = 10

Using them together... Select substring(title_id, 1, 2) as alpha convert(int, substring(title_id, 3, 4)) as num from titles

Transactions are... Transaction - Logical unit of work Transaction Management - ensuring that a set of SQL statements is treated as a unit - an indivisible entity

Transactions A transaction is a set of SQL statements that represent a unit of work or a procedural operation. A transaction is not complete unless all off its steps are followed through. This can be critical to maintaining data integrity such as when an account must be credited while debiting another.

Why transactions? Transactions are necessary for the purpose of concurrency control and recovery concurrency control - allowing multiple users simultaneous access recovery - allowing the database system to return the database to a reliable state after a failure.

Concurrency Lost-update problem Locking database system puts a lock on accessed data so it cannot be altered until lock is released.

Locking Since many users may be trying to access the same data simultaneously the DBMS has a locking mechanism which locks data which is in use. This provides a solution to concurrency problems which would arise if locking were not available.

2 Types of Locks Exclusive - for UPDATE, INSERT, and DELETE (write operations) - no other transaction can acquire lock until original is released Shared - applied during non-update or read operations - usually SELECT - prevents write operations from acquiring lock - allows other read operations to share lock

Recovery Allows a database to bounce back after a system failure must decide what transactions are incomplete which transactions completed but were not written and must be redone

User-defined Transactions Allows user to define any number of SQL statements as a transaction and instruct the database to process them as one unit.

Defining a Transaction A transaction starts with the keyword BEGIN BEGIN SQL statement SQL statement SQL statement COMMIT

Finishing the Transaction If the transaction goes successfully then the COMMIT command will commit the changes to the database. However, if an error occurs the ROLLBACK command can be used to restore the database to its state prior to the transaction.