Update documentation to better explain the typelessness of SQLite and to
describe the distinction between text and numeric data. (CVS 711) FossilOrigin-Name: 4ff0f578eca4a8672cf570923e6c015c3ba6b9f4
This commit is contained in:
drh
parent
a9e99aee95
commit
fbe4375cae
6
main.mk
6
main.mk
@ -299,6 +299,9 @@ download.html: $(TOP)/www/download.tcl
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omitted.html: $(TOP)/www/omitted.tcl
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tclsh $(TOP)/www/omitted.tcl >omitted.html
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datatypes.html: $(TOP)/www/datatypes.tcl
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tclsh $(TOP)/www/datatypes.tcl >datatypes.html
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# Files to be published on the website.
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#
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@ -320,7 +323,8 @@ DOC = \
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faq.html \
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formatchng.html \
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conflict.html \
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omitted.html
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omitted.html \
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datatypes.html
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doc: $(DOC)
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mkdir -p doc
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18
manifest
18
manifest
@ -1,5 +1,5 @@
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C Make\sthe\sdistinction\sbetween\stext\sand\snumeric\sdata.\s(CVS\s710)
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D 2002-08-13T23:02:57
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C Update\sdocumentation\sto\sbetter\sexplain\sthe\stypelessness\sof\sSQLite\sand\sto\ndescribe\sthe\sdistinction\sbetween\stext\sand\snumeric\sdata.\s(CVS\s711)
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D 2002-08-14T00:08:13
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F Makefile.in 6291a33b87d2a395aafd7646ee1ed562c6f2c28c
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F Makefile.linux-gcc b86a99c493a5bfb402d1d9178dcdc4bd4b32f906
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F README f1de682fbbd94899d50aca13d387d1b3fd3be2dd
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@ -14,7 +14,7 @@ F doc/report1.txt a031aaf37b185e4fa540223cb516d3bccec7eeac
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F install-sh 9d4de14ab9fb0facae2f48780b874848cbf2f895
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F libtool c56e618713c9510a103bda6b95f3ea3900dcacd6
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F ltmain.sh e9ed72eb1d690f447c13945eaf69e28af531eda1
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F main.mk 34e976d914ac76ba90a93d2b7c41c1a7498e8c59
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F main.mk 6f0f1cfd611e5ead3e7487b0e8b60ccb17d9fc0b
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F publish.sh a7a8d23e6525bd25d4f5ba9b0fc6edc107d94050
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F spec.template 238f7db425a78dc1bb7682e56e3834c7270a3f5e
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F sqlite.1 83f4a9d37bdf2b7ef079a82d54eaf2e3509ee6ea
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@ -131,19 +131,21 @@ F www/c_interface.tcl fb87b825b9032b9aa941694c5c9c5aee9424467e
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F www/changes.tcl df6f06b1aa97ef285c744bf19ec3efddf707b05f
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F www/conflict.tcl 81dd21f9a679e60aae049e9dd8ab53d59570cda2
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F www/crosscompile.tcl 3622ebbe518927a3854a12de51344673eb2dd060
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F www/datatypes.tcl c18a57dd02ba12bbede52f24ae9325f7fc1bf463
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F www/download.tcl 29aa6679ca29621d10613f60ebbbda18f4b91c49
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F www/dynload.tcl 02eb8273aa78cfa9070dd4501dca937fb22b466c
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F www/faq.tcl e5a752ff431a8408ae24a91ab88ded2dfe699e16
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F www/formatchng.tcl b4449e065d2da38b6563bdf12cf46cfe1d4d765e
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F www/index.tcl 47945cc79706e9f2caf797330412c112d54ad72b
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F www/lang.tcl 8c3d0bda030f110c754b5edbad75eddf5dbe2ed1
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F www/index.tcl 9af69527a26895ec56ad920d4c51541c3e5643a6
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F www/lang.tcl 902f677258ee63dd8f6677b54118f354a1d824b6
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F www/mingw.tcl f1c7c0a7f53387dd9bb4f8c7e8571b7561510ebc
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F www/omitted.tcl 7a6d6598e6a6a09bf54a02e0aff0f29e407d9f11
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F www/opcode.tcl 33c5f2061a05c5d227c72b84c080b3bf74c74f8b
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F www/speed.tcl 7fc83f1b018e1ecc451838449542c3079ed12425
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F www/sqlite.tcl ae3dcfb077e53833b59d4fcc94d8a12c50a44098
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F www/tclsqlite.tcl 1db15abeb446aad0caf0b95b8b9579720e4ea331
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F www/vdbe.tcl 2013852c27a02a091d39a766bc87cff329f21218
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P 92c403f485390bbd83a5be472e61c974f76d46b1
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R 4eff7fd5f3310391432b7a4cfb68c2be
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P 310ac4fbaf0ed63f98bfacb55259960be03b0c8b
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R 66d58d14066d17fd95f1b8c2d419e072
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U drh
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Z 8efada5c2bc32cebfca6eea0d127d8d7
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Z caee8504007932315996b27a5456db2c
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@ -1 +1 @@
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310ac4fbaf0ed63f98bfacb55259960be03b0c8b
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4ff0f578eca4a8672cf570923e6c015c3ba6b9f4
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199
www/datatypes.tcl
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199
www/datatypes.tcl
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@ -0,0 +1,199 @@
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#
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# Run this script to generated a datatypes.html output file
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#
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set rcsid {$Id: datatypes.tcl,v 1.1 2002/08/14 00:08:13 drh Exp $}
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puts {<html>
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<head>
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<title>Datatypes In SQLite</title>
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</head>
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<body bgcolor="white">
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<h1 align="center">
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Datatypes In SQLite
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</h1>
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}
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puts "<p align=center>
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(This page was last modified on [lrange $rcsid 3 4] UTC)
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</p>"
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puts {<h2>1.0 Typelessness</h2>
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<p>
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SQLite is "typeless". This means that you can store any
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kind of data you want in any column of any table, regardless of the
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declared datatype of that column.
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(See the one exception to this rule in section 2.0 below.)
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This behavior is a feature, not
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a bug. A database is suppose to store and retrieve data and it
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should not matter to the database what format that data is in.
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The strong typing system found in most other SQL engines and
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codified in the SQL language spec is a misfeature -
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it is an example of the implementation showing through into the
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interface. SQLite seeks to overcome this misfeature by allowing
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you to store any kind of data into any kind of column and by
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allowing flexibility in the specification of datatypes.
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</p>
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<p>
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A datatype to SQLite is any sequence of zero or more names
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optionally followed by a parenthesized lists of one or two
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signed integers. Notice in particular that a datatype may
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be <em>zero</em> or more names. That means that an empty
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string is a valid datatype as far as SQLite is concerned.
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So you can declare tables where the datatype of each column
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is left unspecified, like this:
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</p>
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<blockquote><pre>
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CREATE TABLE ex1(a,b,c);
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</pre></blockquote>
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<p>
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Even though SQLite allows the datatype to be omitted, it is
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still a good idea to include it in your CREATE TABLE statements,
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since the data type often serves as a good hint to other
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programmers about what you intend to put in the column. And
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if you ever port your code to another database engine, that
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other engine will probably require a datatype of some kind.
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SQLite accepts all the usual datatypes. For example:
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</p>
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<blockquote><pre>
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CREATE TABLE ex2(
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a VARCHAR(10),
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b NVARCHAR(15),
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c TEXT,
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d INTEGER,
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e FLOAT,
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f BOOLEAN,
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g CLOB,
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h BLOB,
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i TIMESTAMP,
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j NUMERIC(10,5)
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k VARYING CHARACTER (24),
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l NATIVE VARYING CHAR(16)
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);
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</pre></blockquote>
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<p>
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And so forth. Basically any sequence of names optionally followed by
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one or two signed integers in parentheses will do.
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</p>
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<h2>2.0 The INTEGER PRIMARY KEY</h2>
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<p>
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One exception to the typelessness of SQLite is a column whose type
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is INTEGER PRIMARY KEY. (And you must use "INTEGER" not "INT".
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A column of type INT PRIMARY KEY is typeless just like any other.)
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INTEGER PRIMARY KEY columns must contain a 32-bit signed integer. Any
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attempt to insert non-integer data will result in an error.
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</p>
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<p>
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INTEGER PRIMARY KEY columns can be used to implement the equivalent
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of AUTOINCREMENT. If you try to insert a NULL into an INTEGER PRIMARY
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KEY column, the column will actually be filled with a integer that is
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one greater than the largest key already in the table. Or if the
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largest key is 2147483647, then the column will be filled with a
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random integer. Either way, the INTEGER PRIMARY KEY column will be
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assigned a unique integer. You can retrieve this integer using
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the <b>sqlite_last_insert_rowid()</b> API function or using the
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<b>last_insert_rowid()</b> SQL function is a subsequent SELECT statement.
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</p>
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<h2>3.0 Comparison and Sort Order</h2>
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<p>
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SQLite is typeless for the purpose of deciding what data is allowed
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to be stored in a column. But some notion of type comes into play
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when sorting and comparing data. For these purposes, a column or
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an expression can be one of two types: <b>numeric</b> and <b>text</b>.
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The sort or comparison may give different results depending on which
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type of data is being sorted or compared.
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</p>
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<p>
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If data is of type <b>text</b> then the comparison is determined by
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the standard C data comparison functions <b>memcmp()</b> or
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<b>strcmp()</b>. The comparison looks at bytes from two inputs one
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by one and returns the first non-zero difference.
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String are '\000' terminated so shorter
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strings sort before longer strings, as you would expect.
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</p>
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<p>
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For numeric data, this situation is more complex. If both strings
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being compared look like well-formed numbers, then they are converted
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into floating point values using <b>atof()</b> and compared numerically.
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If one input is not a well-formed number but the other is, then the
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number is considered to be less than the non-number. If neither inputs
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is a well-formed number, then <b>strcmp()</b> is used to do the
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comparison.
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</p>
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<p>
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Do not be confused by the fact that a column might have a "numeric"
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datatype. This does not mean that the column can contain only numbers.
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It merely means that if the column does contain a number, that number
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will sort in numerical order.
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</p>
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<p>
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For both text and numeric values, NULL sorts before any other value.
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A comparison of any value against NULL using operators like "<" or
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">=" is always false.
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</p>
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<h2>4.0 How SQLite Determines Datatypes</h2>
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<p>
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For SQLite version 2.6.3 and earlier, all values used the numeric datatype.
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The text datatype appears in version 2.7.0 and later. In the sequel it
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is assumed that you are using version 2.7.0 or later of SQLite.
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</p>
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<p>
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For an expression, the datatype of the result is often determined by
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the outermost operator. For example, arithmatic operators ("+", "*", "%")
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always return a numeric results. The string concatenation operator
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("||") returns a text result. And so forth. If you are ever in doubt
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about the datatype of an expression you can use the special <b>typeof()</b>
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SQL function to determine what the datatype is. For example:
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</p>
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<blockquote><pre>
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sqlite> SELECT typeof('abc'+123);
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numeric
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sqlite> SELECT typeof('abc'||123);
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text
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</pre></blockquote>
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<p>
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For table columns, the datatype is determined by the datatype declaration
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of the CREATE TABLE statement. The datatype is text if and only if
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the type declaration contains one or more of the following strings:
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</p>
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<blockquote>
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BLOB<br>
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CHAR<br>
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CLOB</br>
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TEXT
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</blockquote>
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<p>
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The search for these strings in the type declaration is case insensitive,
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of course. If any of the above strings occur anywhere in the type
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declaration, then the datatype of the column is text. Otherwise the
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datatype is numeric. Note in particular that the datatype for columns
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with an empty type declaration is numeric.
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</p>
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}
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puts {
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<p><hr /></p>
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<p><a href="index.html"><img src="/goback.jpg" border=0 />
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Back to the SQLite Home Page</a>
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</p>
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</body></html>}
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@ -1,7 +1,7 @@
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#
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# Run this TCL script to generate HTML for the index.html file.
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#
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set rcsid {$Id: index.tcl,v 1.64 2002/08/13 20:45:41 drh Exp $}
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set rcsid {$Id: index.tcl,v 1.65 2002/08/14 00:08:13 drh Exp $}
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puts {<html>
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<head><title>SQLite: An SQL Database Engine In A C Library</title></head>
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@ -132,6 +132,7 @@ puts {<h2>Documentation</h2>
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<li><a href="faq.html">Frequently Asked Questions</a> are available online.</li>
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<li>Information on the <a href="sqlite.html">sqlite</a>
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command-line utility.</li>
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<li>SQLite is <a href="datatypes.html">typeless</a>.
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<li>The <a href="lang.html">SQL Language</a> subset understood by SQLite.</li>
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<li>The <a href="c_interface.html">C/C++ Interface</a>.</li>
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<li>The <a href="tclsqlite.html">Tcl Binding</a> to SQLite.</li>
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@ -1,7 +1,7 @@
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#
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# Run this Tcl script to generate the sqlite.html file.
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#
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set rcsid {$Id: lang.tcl,v 1.41 2002/06/25 01:09:13 drh Exp $}
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set rcsid {$Id: lang.tcl,v 1.42 2002/08/14 00:08:13 drh Exp $}
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puts {<html>
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<head>
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@ -282,7 +282,9 @@ for use by the engine.</p>
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<p>Each column definition is the name of the column followed by the
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datatype for that column, then one or more optional column constraints.
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The datatype for the column is (usually) ignored and may be omitted.
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SQLite is <a href="datatypes.html">typeless</a>.
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The datatype for the column does not constraint what data may be put
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in that column.
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All information is stored as null-terminated strings.
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The UNIQUE constraint causes an index to be created on the specified
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columns. This index must contain unique keys.
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91
www/omitted.tcl
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91
www/omitted.tcl
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@ -0,0 +1,91 @@
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#
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# Run this script to generated a omitted.html output file
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#
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set rcsid {$Id: omitted.tcl,v 1.1 2002/08/14 00:08:14 drh Exp $}
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puts {<html>
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<head>
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<title>SQL Features That SQLite Does Not Implement</title>
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</head>
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<body bgcolor="white">
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<h1 align="center">
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SQL Features That SQLite Does Not Implement
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</h1>
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}
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puts "<p align=center>
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(This page was last modified on [lrange $rcsid 3 4] UTC)
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</p>"
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puts {
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<p>
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Rather than try to list all the features of SQL92 that SQLite does
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support, it is much easier to list those that it does not.
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The following are features of of SQL92 that SQLite does not implement.
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</p>
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<table cellpadding="10">
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}
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proc feature {name desc} {
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puts "<tr><td valign=\"top\"><b><nobr>$name</nobr></b></td>"
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puts "<td valign=\"top\">$desc</td></tr>"
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}
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feature {RIGHT and FULL OUTER JOIN} {
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LEFT OUTER JOIN is implemented, but not RIGHT OUTER JOIN or
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FULL OUTER JOIN.
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}
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feature {CHECK constraints} {
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CHECK constraints are parsed but they are not enforced.
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NOT NULL and UNIQUE constraints are enforced, however.
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}
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feature {FOREIGN KEY constraints} {
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FOREIGN KEY constraints are parsed but are ignored.
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}
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feature {GRANT and REVOKE} {
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Since SQLite reads and writes an ordinary disk file, the
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only access permissions that can be applied are the normal
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file access permissions of the underlying operating system.
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The GRANT and REVOKE commands commonly found on client/server
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RDBMSes are not implemented because they would be meaningless
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for an embedded database engine.
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}
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feature {DELETE, INSERT, and UPDATE on VIEWs} {
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VIEWs in SQLite are read-only. But you can create a trigger
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that fires on an attempt to DELETE, INSERT, or UPDATE a view and do
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what you need in the body of the trigger.
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}
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feature {ALTER TABLE} {
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To change a table you have to delete it (saving its contents to a temporary
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table) and recreate it from scratch.
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}
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feature {The COUNT(DISTINCT X) function} {
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You can accomplish the same thing using a subquery, like this:<br />
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SELECT count(x) FROM (SELECT DISTINCT x FROM tbl);
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}
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feature {Variable subqueries} {
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Subqueries must be static. They are evaluated only once. They must not,
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therefore, refer to variables in the containing query.
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}
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puts {
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</table>
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<p>
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If you find other SQL92 features that SQLite does not support, please
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send e-mail to <a href="mailto:drh@hwaci.com">drh@hwaci.com</a> so they
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can be added to this list.
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</p>
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<p><hr /></p>
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<p><a href="index.html"><img src="/goback.jpg" border=0 />
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Back to the SQLite Home Page</a>
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</p>
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</body></html>}
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Loading…
x
Reference in New Issue
Block a user