Transactions.
A transaction is a logical group of one or more SQL statements.
Transactions are used in various scenarios such as banking, ecommerce, social networks, booking tickets, etc.
A transaction has four important properties.
- Atomicity
- Consistency
- Isolation
- Durability
Atomicity
Either all SQL statements or none are applied to the database.
Consistency
Transactions always leave the database in a consistent state.
Isolation
Multiple transaction can occur at the same time without adversely affecting the other.
Durability
Changes of a successful transaction persist even after a system crash.
These four properties are commonly acronymed as ACID.
Indexes
In scenarios like, searching for a word in dictionary, we use index to easily search for the word. Similarly, in databases, we maintain indexes to speed up the search for data in a table.