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Exchanging Data: Databases - Coggle Diagram
Exchanging Data: Databases
SQL
Key words
UPDATE
Updates existing rows in a table
VALUES
Specifies the values of an INSERT INTO statement
INSERT INTO
Inserts new rows in a table
SET
Specifies which columns and values that should be updated in a table
MODIFY
Specifies the columns or fields to be modified in an ALTER statement
DROP
Deletes a column, constraint, database, index, table, or view
ALTER
Adds, deletes, or modifies columns in a table, or changes the data type of a column in a table
DELETE
Deletes rows from a table
CREATE
Creates a database, index, view, table, or procedure
Serialisation methods (transaction processing)
Timestamp ordering
Every object has a read and write timestamp which are updated whenever the object is read or written
When a user tries to save an update, if the read timestamp is not the same as it was when they started the transaction, the DBMS knows another user has accessed the same object
Commitment ordering
Transactions are ordered in terms of their dependencies on one another as well as the time they were initiated
Redundancies are duplicate hardware located in a different geographical area that mirror every transaction that takes place on the main system so that if the main system fails, the transactions can still be processed
DBMS
Database Management System, has to prevent deadlock
Serialisation ensures that transactions do not overlap in time and therefore cannot interfere with each other or lead to updates being lost