Sql Excel : Relational Databases

Relational databases, which were invented in the 1970s, are now the storehouse of
mountains of data available to businesses. To a large extent, the popularity of
relational databases rests on what are called ACID properties of transactions:
Atomicity
Consistency
Isolation
Durability
These properties basically mean that when data is stored or updated in a database,
it really is changed. The databases have transaction logs and other capabilities to
ensure that changes really do happen and that modified data is visible when the
data modification step completes. (The data should even survive major failures
such as the operating system crashing.) In practice, databases support
transactions, logs, replication, concurrent access, stored procedures, security, and
a host of features suitable for designing real-world applications.
From our perspective, a more important attribute of relational databases is their
ability to take advantage of the hardware they are running on—multiple
processors, memory, and disk. When you run a query, the optimization engine
first translates the SQL query into the appropriate lower-level algorithms that
exploit the available resources. The optimization engine is one of the reasons why
SQL is so powerful: the same query running on a slightly different machine or
slightly different data might have very different execution plans. The SQL remains
the same; it is the optimization engine that chooses the best way to execute the
code.