SQL Bible [Electronic resources] نسخه متنی

Bitwise Operators

Bitwise operators perform bit operations
on integer data types; all bitwise operators introduced into SQL by RDBMS
vendors are listed in
Table
11-6. To understand the results of the bitwise operations one must
understand the basics of Boolean algebra.

The operands for bitwise operators can be
of either the integer data type or the binary string data type (except for
IMAGE data type) category. Data type
compatibility is given in
Table
11-7. Bitwise operations are not typical for a high-level language such
as SQL, and one might be hard-pressed to come up with a usage example.

One of the possible uses could be a
complex bit mask made for color — after all RDBMS now supports more than just
text and numeric data. Consider a combination of zeroes and ones, for example
0101. When a binary
AND is applied to this number and to
another binary number, for example 1101, it will produce the following result:

SELECT 5 & 13 result result
------ 5

The binary representation of 5 is
0101. In this example of logical
AND, only 1 and 1 produce 1, any other
combinations — namely 1 and 0, or 0 and 0 — produce 0.

0101 (decimal 5) 1101 (decimal
13) ---- 0101 (decimal 5)

To decode or encode a pixel in an image
stored in your database you would use bit mask and shifting. Another possible
use of the
XOR (exclusive
OR) operator would be to encrypt data based
on some numeric key.

In the database world, safety of data is
of paramount importance; it comes right after getting the software running at
all. There are different safety aspects, discussed in
Chapter
12, including unauthorized access, malicious data manipulations, and so
on. Here we're going to briefly touch the data corruption issue.

Sometimes a technical glitch occurs and
data gets corrupted — somewhere in the gazillions of zeroes and ones
representing data on your hard drive, a couple of bits get flipped, exchange
places, or disappear altogether. You could blame it on a power fluctuation,
cosmic rays, earthquakes, and such, but the question remains — how to restore
data to a consistent state.

One of the technologies out there is
RAID (Redundant Array of Independent Disks). While RAID does not apply directly
to SQL, it does have something important to do with RDBMS and Boolean
algebra.

The idea behind RAID is to combine
several relatively inexpensive disk drives into an array that works as a single
unit, thus increasing the reliability and speed of operations. There are
different types of RAID architecture (ways these disks can be combined),
ranging from RAID0 to RAID5 (the first, RAID0, being just another name for a
single disk).

What makes RAID relevant to Boolean
logic is that the fundamental technology behind it is striping.
RAID partitions each of the drives participating in the array into
stripes that are interleaved in a sequence.

To visualize that, imagine a stack of
wafers where you butter the first, third, and fifth wafers, and spread jam over
the second and fourth. In a similar manner, RAID enables you to perform
different operations on the data spread across more than one disk (drive)
simultaneously, greatly improving input/output operations. In RAID3, RAID4, and
RAID5 configurations, an additional benefit of safety comes from
parity, where one drive is dedicated to storing parity
information: In case of one drive failure the whole information can be
recovered by using a Boolean exclusive OR (XOR) operation of the bit
information on the remaining drives.

Consider the previous example, but
performing only an
XOR operation instead of the logical
AND. The
XOR operation follows rules of Boolean
logic:
0
XOR
0
=
0,
0
XOR
1
=
1,
1
XOR
1
=
0, and
1
XOR
0
=
1. Imagine that the first number is
written on disk 1, the second is written on disk 2, and their
XOR result (parity) — on disk 3. Now,
disk 2 (containing value
1101) has failed. By performing an
XOR operation between data on disk 1 and
data on disk 3, you could easily restore the corrupted values:

0101 0101 1101 1000 ---- ----
1000 1101

Of course, if more than one drive fails,
then this safety system would not work. The bet is on the calculated risk, when
failure of more than one drive at the same time is extremely unlikely.

All this is a fairly low level that
normally is outside of the SQL programming domain, and chances are that if you
find yourself in need of more in-depth coverage of Boolean algebra, you will
reach for a topic-specific book, and not the one on general SQL issues.

Only Microsoft SQL Server provides bitwise
operators; the IBM DB2 UDB dialect of SQL does not have bit operations support
built into the language itself, and Oracle 9i has a
BITAND function that works identically to
SQL Server's bitwise
AND.