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