SQLite: Check-in [4ff0f578]

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Overview

Comment:	Update documentation to better explain the typelessness of SQLite and to describe the distinction between text and numeric data. (CVS 711)
Downloads:	Tarball \| ZIP archive \| SQL archive
Timelines:	family \| ancestors \| descendants \| both \| trunk
Files:	files \| file ages \| folders
SHA1:	4ff0f578eca4a8672cf570923e6c015c3ba6b9f4
User & Date:	drh 2002-08-14 00:08:13

Context

2002-08-14
00:10		Fix for ticket #132: make the working directory the last choice for where to write temporary files, not the first choice. (CVS 712) check-in: 26a4e7e7 user: drh tags: trunk
00:08		Update documentation to better explain the typelessness of SQLite and to describe the distinction between text and numeric data. (CVS 711) check-in: 4ff0f578 user: drh tags: trunk
2002-08-13
23:02		Make the distinction between text and numeric data. (CVS 710) check-in: 310ac4fb user: drh tags: trunk

Changes

Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to main.mk.


download.html:	$(TOP)/www/download.tcl
	tclsh $(TOP)/www/download.tcl >download.html

omitted.html:	$(TOP)/www/omitted.tcl
	tclsh $(TOP)/www/omitted.tcl >omitted.html





# Files to be published on the website.
#
DOC = \
  index.html \
  sqlite.html \
  changes.html \
................................................................................
  mingw.html \
  tclsqlite.html \
  download.html \
  speed.html \
  faq.html \
  formatchng.html \
  conflict.html \
  omitted.html


doc:	$(DOC)
	mkdir -p doc
	mv $(DOC) doc

install:	sqlite libsqlite.a sqlite.h
	mv sqlite /usr/bin


download.html:	$(TOP)/www/download.tcl
	tclsh $(TOP)/www/download.tcl >download.html

omitted.html:	$(TOP)/www/omitted.tcl
	tclsh $(TOP)/www/omitted.tcl >omitted.html

datatypes.html:	$(TOP)/www/datatypes.tcl
	tclsh $(TOP)/www/datatypes.tcl >datatypes.html


# Files to be published on the website.
#
DOC = \
  index.html \
  sqlite.html \
  changes.html \
................................................................................
  mingw.html \
  tclsqlite.html \
  download.html \
  speed.html \
  faq.html \
  formatchng.html \
  conflict.html \
  omitted.html \
  datatypes.html

doc:	$(DOC)
	mkdir -p doc
	mv $(DOC) doc

install:	sqlite libsqlite.a sqlite.h
	mv sqlite /usr/bin

Added www/datatypes.tcl.

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#
# Run this script to generated a datatypes.html output file
#
set rcsid {$Id: datatypes.tcl,v 1.1 2002/08/14 00:08:13 drh Exp $}

puts {<html>
<head>
  <title>Datatypes In SQLite</title>
</head>
<body bgcolor="white">
<h1 align="center">
Datatypes In SQLite
</h1>
}
puts "<p align=center>
(This page was last modified on [lrange $rcsid 3 4] UTC)
</p>"

puts {<h2>1.0 &nbsp; Typelessness</h2>
<p>
SQLite is "typeless".  This means that you can store any
kind of data you want in any column of any table, regardless of the
declared datatype of that column.  
(See the one exception to this rule in section 2.0 below.)
This behavior is a feature, not
a bug.  A database is suppose to store and retrieve data and it 
should not matter to the database what format that data is in.
The strong typing system found in most other SQL engines and
codified in the SQL language spec is a misfeature -
it is an example of the implementation showing through into the
interface.  SQLite seeks to overcome this misfeature by allowing
you to store any kind of data into any kind of column and by
allowing flexibility in the specification of datatypes.
</p>

<p>
A datatype to SQLite is any sequence of zero or more names
optionally followed by a parenthesized lists of one or two
signed integers.  Notice in particular that a datatype may
be <em>zero</em> or more names.  That means that an empty
string is a valid datatype as far as SQLite is concerned.
So you can declare tables where the datatype of each column
is left unspecified, like this:
</p>

<blockquote><pre>
CREATE TABLE ex1(a,b,c);
</pre></blockquote>

<p>
Even though SQLite allows the datatype to be omitted, it is
still a good idea to include it in your CREATE TABLE statements,
since the data type often serves as a good hint to other
programmers about what you intend to put in the column. And
if you ever port your code to another database engine, that
other engine will probably require a datatype of some kind.
SQLite accepts all the usual datatypes.  For example:
</p>

<blockquote><pre>
CREATE TABLE ex2(
  a VARCHAR(10),
  b NVARCHAR(15),
  c TEXT,
  d INTEGER,
  e FLOAT,
  f BOOLEAN,
  g CLOB,
  h BLOB,
  i TIMESTAMP,
  j NUMERIC(10,5)
  k VARYING CHARACTER (24),
  l NATIVE VARYING CHAR(16)
);
</pre></blockquote>

<p>
And so forth.  Basically any sequence of names optionally followed by 
one or two signed integers in parentheses will do.
</p>

<h2>2.0 &nbsp; The INTEGER PRIMARY KEY</h2>

<p>
One exception to the typelessness of SQLite is a column whose type
is INTEGER PRIMARY KEY.  (And you must use "INTEGER" not "INT".
A column of type INT PRIMARY KEY is typeless just like any other.)
INTEGER PRIMARY KEY columns must contain a 32-bit signed integer.  Any
attempt to insert non-integer data will result in an error.
</p>

<p>
INTEGER PRIMARY KEY columns can be used to implement the equivalent
of AUTOINCREMENT.  If you try to insert a NULL into an INTEGER PRIMARY
KEY column, the column will actually be filled with a integer that is
one greater than the largest key already in the table.  Or if the
largest key is 2147483647, then the column will be filled with a
random integer.  Either way, the INTEGER PRIMARY KEY column will be
assigned a unique integer.  You can retrieve this integer using
the <b>sqlite_last_insert_rowid()</b> API function or using the
<b>last_insert_rowid()</b> SQL function is a subsequent SELECT statement.
</p>

<h2>3.0 &nbsp; Comparison and Sort Order</h2>

<p>
SQLite is typeless for the purpose of deciding what data is allowed
to be stored in a column.  But some notion of type comes into play
when sorting and comparing data.  For these purposes, a column or
an expression can be one of two types: <b>numeric</b> and <b>text</b>.
The sort or comparison may give different results depending on which
type of data is being sorted or compared.
</p>

<p>
If data is of type <b>text</b> then the comparison is determined by
the standard C data comparison functions <b>memcmp()</b> or
<b>strcmp()</b>.  The comparison looks at bytes from two inputs one
by one and returns the first non-zero difference.
String are '\000' terminated so shorter
strings sort before longer strings, as you would expect.
</p>

<p>
For numeric data, this situation is more complex.  If both strings
being compared look like well-formed numbers, then they are converted
into floating point values using <b>atof()</b> and compared numerically.
If one input is not a well-formed number but the other is, then the
number is considered to be less than the non-number.  If neither inputs
is a well-formed number, then <b>strcmp()</b> is used to do the
comparison.
</p>

<p>
Do not be confused by the fact that a column might have a "numeric"
datatype.  This does not mean that the column can contain only numbers.
It merely means that if the column does contain a number, that number
will sort in numerical order.
</p>

<p>
For both text and numeric values, NULL sorts before any other value.
A comparison of any value against NULL using operators like "&lt;" or
"&gt;=" is always false.
</p>

<h2>4.0 &nbsp; How SQLite Determines Datatypes</h2>

<p>
For SQLite version 2.6.3 and earlier, all values used the numeric datatype.
The text datatype appears in version 2.7.0 and later.  In the sequel it
is assumed that you are using version 2.7.0 or later of SQLite.
</p>

<p>
For an expression, the datatype of the result is often determined by
the outermost operator.  For example, arithmatic operators ("+", "*", "%")
always return a numeric results.  The string concatenation operator
("||") returns a text result.  And so forth.  If you are ever in doubt
about the datatype of an expression you can use the special <b>typeof()</b>
SQL function to determine what the datatype is.  For example:
</p>

<blockquote><pre>
sqlite&gt; SELECT typeof('abc'+123);
numeric
sqlite&gt; SELECT typeof('abc'||123);
text
</pre></blockquote>

<p>
For table columns, the datatype is determined by the datatype declaration
of the CREATE TABLE statement.  The datatype is text if and only if
the type declaration contains one or more of the following strings:
</p>

<blockquote>
BLOB<br>
CHAR<br>
CLOB</br>
TEXT
</blockquote>

<p>
The search for these strings in the type declaration is case insensitive,
of course.  If any of the above strings occur anywhere in the type
declaration, then the datatype of the column is text.  Otherwise the
datatype is numeric.  Note in particular that the datatype for columns
with an empty type declaration is numeric.
</p>
}

puts {
<p><hr /></p>
<p><a href="index.html"><img src="/goback.jpg" border=0 />
Back to the SQLite Home Page</a>
</p>

</body></html>}

Changes to www/index.tcl.

#
# Run this TCL script to generate HTML for the index.html file.
#
set rcsid {$Id: index.tcl,v 1.64 2002/08/13 20:45:41 drh Exp $}

puts {<html>
<head><title>SQLite: An SQL Database Engine In A C Library</title></head>
<body bgcolor=white>
<h1 align=center>SQLite: An SQL Database Engine In A C Library</h1>
<p align=center>}
puts "This page was last modified on [lrange $rcsid 3 4] UTC<br>"
................................................................................

<p>The following documentation is currently available:</p>

<p><ul>
<li><a href="faq.html">Frequently Asked Questions</a> are available online.</li>
<li>Information on the <a href="sqlite.html">sqlite</a>
    command-line utility.</li>

<li>The <a href="lang.html">SQL Language</a> subset understood by SQLite.</li>
<li>The <a href="c_interface.html">C/C++ Interface</a>.</li>
<li>The <a href="tclsqlite.html">Tcl Binding</a> to SQLite.</li>
<li>The <a href="arch.html">Architecture of the SQLite Library</a> describes
    how the library is put together.</li>
<li>A description of the <a href="opcode.html">virtual machine</a> that
    SQLite uses to access the database.</li>

#
# Run this TCL script to generate HTML for the index.html file.
#
set rcsid {$Id: index.tcl,v 1.65 2002/08/14 00:08:13 drh Exp $}

puts {<html>
<head><title>SQLite: An SQL Database Engine In A C Library</title></head>
<body bgcolor=white>
<h1 align=center>SQLite: An SQL Database Engine In A C Library</h1>
<p align=center>}
puts "This page was last modified on [lrange $rcsid 3 4] UTC<br>"
................................................................................

<p>The following documentation is currently available:</p>

<p><ul>
<li><a href="faq.html">Frequently Asked Questions</a> are available online.</li>
<li>Information on the <a href="sqlite.html">sqlite</a>
    command-line utility.</li>
<li>SQLite is <a href="datatypes.html">typeless</a>.
<li>The <a href="lang.html">SQL Language</a> subset understood by SQLite.</li>
<li>The <a href="c_interface.html">C/C++ Interface</a>.</li>
<li>The <a href="tclsqlite.html">Tcl Binding</a> to SQLite.</li>
<li>The <a href="arch.html">Architecture of the SQLite Library</a> describes
    how the library is put together.</li>
<li>A description of the <a href="opcode.html">virtual machine</a> that
    SQLite uses to access the database.</li>

Changes to www/lang.tcl.

#
# Run this Tcl script to generate the sqlite.html file.
#
set rcsid {$Id: lang.tcl,v 1.41 2002/06/25 01:09:13 drh Exp $}

puts {<html>
<head>
  <title>Query Language Understood By SQLite</title>
</head>
<body bgcolor=white>
<h1 align=center>
................................................................................
followed by the name of a new table and a parenthesized list of column
definitions and constraints.  The table name can be either an identifier
or a string.  Tables names that begin with "<b>sqlite_</b>" are reserved
for use by the engine.</p>

<p>Each column definition is the name of the column followed by the
datatype for that column, then one or more optional column constraints.

The datatype for the column is (usually) ignored and may be omitted.

All information is stored as null-terminated strings.
The UNIQUE constraint causes an index to be created on the specified
columns.  This index must contain unique keys.
The DEFAULT constraint
specifies a default value to use when doing an INSERT.
</p>

#
# Run this Tcl script to generate the sqlite.html file.
#
set rcsid {$Id: lang.tcl,v 1.42 2002/08/14 00:08:13 drh Exp $}

puts {<html>
<head>
  <title>Query Language Understood By SQLite</title>
</head>
<body bgcolor=white>
<h1 align=center>
................................................................................
followed by the name of a new table and a parenthesized list of column
definitions and constraints.  The table name can be either an identifier
or a string.  Tables names that begin with "<b>sqlite_</b>" are reserved
for use by the engine.</p>

<p>Each column definition is the name of the column followed by the
datatype for that column, then one or more optional column constraints.
SQLite is <a href="datatypes.html">typeless</a>.
The datatype for the column does not constraint what data may be put
in that column.
All information is stored as null-terminated strings.
The UNIQUE constraint causes an index to be created on the specified
columns.  This index must contain unique keys.
The DEFAULT constraint
specifies a default value to use when doing an INSERT.
</p>

Added www/omitted.tcl.

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#
# Run this script to generated a omitted.html output file
#
set rcsid {$Id: omitted.tcl,v 1.1 2002/08/14 00:08:14 drh Exp $}

puts {<html>
<head>
  <title>SQL Features That SQLite Does Not Implement</title>
</head>
<body bgcolor="white">
<h1 align="center">
SQL Features That SQLite Does Not Implement
</h1>
}
puts "<p align=center>
(This page was last modified on [lrange $rcsid 3 4] UTC)
</p>"

puts {
<p>
Rather than try to list all the features of SQL92 that SQLite does
support, it is much easier to list those that it does not.
The following are features of of SQL92 that SQLite does not implement.
</p>

<table cellpadding="10">
}

proc feature {name desc} {
  puts "<tr><td valign=\"top\"><b><nobr>$name</nobr></b></td>"
  puts "<td valign=\"top\">$desc</td></tr>"
}

feature {RIGHT and FULL OUTER JOIN} {
  LEFT OUTER JOIN is implemented, but not RIGHT OUTER JOIN or
  FULL OUTER JOIN.
}

feature {CHECK constraints} {
  CHECK constraints are parsed but they are not enforced.
  NOT NULL and UNIQUE constraints are enforced, however.
}

feature {FOREIGN KEY constraints} {
  FOREIGN KEY constraints are parsed but are ignored.
}

feature {GRANT and REVOKE} {
  Since SQLite reads and writes an ordinary disk file, the
  only access permissions that can be applied are the normal
  file access permissions of the underlying operating system.
  The GRANT and REVOKE commands commonly found on client/server
  RDBMSes are not implemented because they would be meaningless
  for an embedded database engine.
}

feature {DELETE, INSERT, and UPDATE on VIEWs} {
  VIEWs in SQLite are read-only.  But you can create a trigger
  that fires on an attempt to DELETE, INSERT, or UPDATE a view and do
  what you need in the body of the trigger.
}

feature {ALTER TABLE} {
  To change a table you have to delete it (saving its contents to a temporary
  table) and recreate it from scratch.
}

feature {The COUNT(DISTINCT X) function} {
  You can accomplish the same thing using a subquery, like this:<br />
  &nbsp;&nbsp;SELECT count(x) FROM (SELECT DISTINCT x FROM tbl);
}

feature {Variable subqueries} {
  Subqueries must be static.  They are evaluated only once.  They must not,
  therefore, refer to variables in the containing query.
}

puts {
</table>

<p>
If you find other SQL92 features that SQLite does not support, please
send e-mail to <a href="mailto:drh@hwaci.com">drh@hwaci.com</a> so they
can be added to this list.
</p>
<p><hr /></p>
<p><a href="index.html"><img src="/goback.jpg" border=0 />
Back to the SQLite Home Page</a>
</p>

</body></html>}