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+set rcsid {$Id: different.tcl,v 1.8 2006/12/18 14:12:21 drh Exp $}
+source common.tcl
+header {Distinctive Features Of SQLite}
+puts {
+<p>
+This page highlights some of the characteristics of SQLite that are
+unusual and which make SQLite different from many other SQL
+database engines.
+</p>
+}
+proc feature {tag name text} {
+  puts "<a name=\"$tag\" />"
+  puts "<p><b>$name</b></p>\n"
+  puts "<blockquote>$text</blockquote>\n"
+}
+
+feature zeroconfig {Zero-Configuration} {
+  SQLite does not need to be "installed" before it is used. 
+  There is no "setup" procedure.  There is no
+  server process that needs to be started, stopped, or configured.
+  There is
+  no need for an administrator to create a new database instance or assign
+  access permissions to users.
+  SQLite uses no configuration files.
+  Nothing needs to be done to tell the system that SQLite is running.
+  No actions are required to recover after a system crash or power failure.
+  There is nothing to troubleshoot.
+  <p>
+  SQLite just works.
+  <p>
+  Other more familiar database engines run great once you get them going.
+  But doing the initial installation and configuration can be
+  intimidatingly complex.
+}
+
+feature serverless {Serverless} {
+  Most SQL database engines are implemented as a separate server
+  process.  Programs that want to access the database communicate
+  with the server using some kind of interprocess communcation
+  (typically TCP/IP) to send requests to the server and to receive
+  back results.  SQLite does not work this way.  With SQLite, the
+  process that wants to access the database reads and writes
+  directly from the database files on disk.  There is no intermediary
+  server process.
+  <p>
+  There are advantages and disadvantages to being serverless.  The
+  main advantage is that there is no separate server process
+  to install, setup, configure, initialize, manage, and troubleshoot.
+  This is one reason why SQLite is a "zero-configuration" database
+  engine.  Programs that use SQLite require no administrative support
+  for setting up the database engine before they are run.  Any program
+  that is able to access the disk is able to use an SQLite database.
+  <p>
+  On the other hand, a database engine that uses a server can provide
+  better protection from bugs in the client application - stray pointers
+  in a client cannot corrupt memory on the server.  And because a server
+  is a single persistent process, it is able control database access with
+  more precision, allowing for finer grain locking and better concurrancy.
+  <p>
+  Most SQL database engines are client/server based.  Of those that are
+  serverless, SQLite is the only one that this author knows of that
+  allows multiple applications to access the same database at the same time.
+}
+
+feature onefile {Single Database File} {
+  An SQLite database is a single ordinary disk file that can be located
+  anywhere in the directory hierarchy.  If SQLite can read
+  the disk file then it can read anything in the database.  If the disk
+  file and its directory are writable, then SQLite can change anything
+  in the database.   Database files can easily be copied onto a USB
+  memory stick or emailed for sharing.
+  <p>
+  Other SQL database engines tend to store data as a large collection of
+  files.  Often these files are in a standard location that only the
+  database engine itself can access.  This makes the data more secure,
+  but also makes it harder to access.  Some SQL database engines provide
+  the option of writing directly to disk and bypassing the filesystem
+  all together.  This provides added performance, but at the cost of
+  considerable setup and maintenance complexity.
+}
+
+feature small {Compact} {
+  When optimized for size, the whole SQLite library with everything enabled
+  is less than 225KiB in size (as measured on an ix86 using the "size"
+  utility from the GNU compiler suite.)  Unneeded features can be disabled
+  at compile-time to further reduce the size of the library to under
+  170KiB if desired.
+  <p>
+  Most other SQL database engines are much larger than this.  IBM boasts
+  that it's recently released CloudScape database engine is "only" a 2MiB
+  jar file - 10 times larger than SQLite even after it is compressed!
+  Firebird boasts that it's client-side library is only 350KiB.  That's
+  50% larger than SQLite and does not even contain the database engine.
+  The Berkeley DB library from Sleepycat is 450KiB and it omits SQL
+  support, providing the programmer with only simple key/value pairs.
+}
+
+feature typing {Manifest typing} {
+  Most SQL database engines use static typing.  A datatype is associated
+  with each column in a table and only values of that particular datatype
+  are allowed to be stored in that column.  SQLite relaxes this restriction
+  by using manifest typing.
+  In manifest typing, the datatype is a property of the value itself, not 
+  of the column in which the value is stored.
+  SQLite thus allows the user to store
+  any value of any datatype into any column regardless of the declared type
+  of that column.  (There are some exceptions to this rule: An INTEGER
+  PRIMARY KEY column may only store integers.  And SQLite attempts to coerce
+  values into the declared datatype of the column when it can.)
+  <p>
+  As far as we can tell, the SQL language specification allows the use
+  of manifest typing.   Nevertheless, most other SQL database engines are
+  statically typed and so some people
+  feel that the use of manifest typing is a bug in SQLite.  But the authors
+  of SQLite feel very strongly that this is a feature.  The use of manifest
+  typing in SQLite is a deliberate design decision which has proven in practice
+  to make SQLite more reliable and easier to use, especially when used in
+  combination with dynamically typed programming languages such as Tcl and
+  Python.
+}
+
+feature flex {Variable-length records} {
+  Most other SQL database engines allocated a fixed amount of disk space
+  for each row in most tables.  They play special tricks for handling
+  BLOBs and CLOBs which can be of wildly varying length.  But for most
+  tables, if you declare a column to be a VARCHAR(100) then the database
+  engine will allocate
+  100 bytes of disk space regardless of how much information you actually
+  store in that column.
+  <p>
+  SQLite, in contrast, use only the amount of disk space actually
+  needed to store the information in a row.  If you store a single
+  character in a VARCHAR(100) column, then only a single byte of disk
+  space is consumed.  (Actually two bytes - there is some overhead at
+  the beginning of each column to record its datatype and length.)
+  <p>
+  The use of variable-length records by SQLite has a number of advantages.
+  It results in smaller database files, obviously.  It also makes the
+  database run faster, since there is less information to move to and from
+  disk.  And, the use of variable-length records makes it possible for
+  SQLite to employ manifest typing instead of static typing.
+}
+
+feature readable {Readable source code} {
+  The source code to SQLite is designed to be readable and accessible to
+  the average programmer.  All procedures and data structures and many
+  automatic variables are carefully commented with useful information about
+  what they do.  Boilerplate commenting is omitted.
+}
+
+feature vdbe {SQL statements compile into virtual machine code} {
+  Every SQL database engine compiles each SQL statement into some kind of
+  internal data structure which is then used to carry out the work of the
+  statement.  But in most SQL engines that internal data structure is a
+  complex web of interlinked structures and objects.  In SQLite, the compiled
+  form of statements is a short program in a machine-language like
+  representation.  Users of the database can view this 
+  <a href="opcode.html">virtual machine language</a>
+  by prepending the <a href="lang_explain.html">EXPLAIN</a> keyword
+  to a query.
+  <p>
+  The use of a virtual machine in SQLite has been a great benefit to
+  library's development.  The virtual machine provides a crisp, well-defined
+  junction between the front-end of SQLite (the part that parses SQL
+  statements and generates virtual machine code) and the back-end (the
+  part that executes the virtual machine code and computes a result.)
+  The virtual machine allows the developers to see clearly and in an
+  easily readable form what SQLite is trying to do with each statement
+  it compiles, which is a tremendous help in debugging.
+  Depending on how it is compiled, SQLite also has the capability of
+  tracing the execution of the virtual machine - printing each
+  virtual machine instruction and its result as it executes.
+}
+
+#feature binding {Tight bindings to dynamic languages} {
+#  Because it is embedded, SQLite can have a much tighter and more natural
+#  binding to high-level dynamic languages such as Tcl, Perl, Python,
+#  PHP, and Ruby.
+#  For example, 
+#}
+
+feature license {Public domain} {
+  The source code for SQLite is in the public domain.  No claim of copyright
+  is made on any part of the core source code.  (The documentation and test
+  code is a different matter - some sections of documentation and test logic
+  are governed by open-sources licenses.)  All contributors to the
+  SQLite core software have signed affidavits specifically disavowing any
+  copyright interest in the code.  This means that anybody is able to legally
+  do anything they want with the SQLite source code.
+  <p>
+  There are other SQL database engines with liberal licenses that allow
+  the code to be broadly and freely used.  But those other engines are
+  still governed by copyright law.  SQLite is different in that copyright
+  law simply does not apply.  
+  <p>
+  The source code files for other SQL database engines typically begin
+  with a comment describing your license rights to view and copy that file.
+  The SQLite source code contains no license since it is not governed by
+  copyright.  Instead of a license, the SQLite source code offers a blessing:
+  <blockquote>
+  <i>May you do good and not evil<br>
+  May you find forgiveness for yourself and forgive others<br>
+  May you share freely, never taking more than you give.</i>
+  </blockquote>
+}
+
+feature extensions {SQL language extensions} {
+  SQLite provides a number of enhancements to the SQL language 
+  not normally found in other database engines.
+  The EXPLAIN keyword and manifest typing have already been mentioned
+  above.  SQLite also provides statements such as 
+  <a href="lang_replace.html">REPLACE</a> and the
+  <a href="lang_conflict.html">ON CONFLICT</a> clause that allow for
+  added control over the resolution of constraint conflicts.
+  SQLite supports <a href="lang_attach.html">ATTACH</a> and
+  <a href="lang_detach.html">DETACH</a> commands that allow multiple
+  independent databases to be used together in the same query.
+  And SQLite defines APIs that allows the user to add new
+  <a href="capi3ref.html#sqlite3_create_function">SQL functions</a>
+  and <a href="capi3ref.html#sqlite3_create_collation">collating sequences</a>.
+}
+
+
+footer $rcsid