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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html><head><title>Datatypes In SQLite version 2</title><style type="text/css">body { margin: auto; font-family: "Verdana" "sans-serif"; padding: 8px 1%;}a { color: #45735f }a:visited { color: #734559 }.logo { position:absolute; margin:3px; }.tagline { float:right; text-align:right; font-style:italic; width:240px; margin:12px; margin-top:58px;}.toolbar { font-variant: small-caps; text-align: center; line-height: 1.6em; margin: 0; padding:1px 8px;}.toolbar a { color: white; text-decoration: none; padding: 6px 12px; }.toolbar a:visited { color: white; }.toolbar a:hover { color: #80a796; background: white; }.content { margin: 5%; }.content dt { font-weight:bold; }.content dd { margin-bottom: 25px; margin-left:20%; }.content ul { padding:0px; padding-left: 15px; margin:0px; }/* rounded corners */.se { background: url(images/se.png) 100% 100% no-repeat #80a796}.sw { background: url(images/sw.png) 0% 100% no-repeat }.ne { background: url(images/ne.png) 100% 0% no-repeat }.nw { background: url(images/nw.png) 0% 0% no-repeat }</style><meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head><body><div><!-- container div to satisfy validator --><a href="index.html"><img class="logo" src="images/SQLite.gif" alt="SQLite Logo" border="0"></a><div><!-- IE hack to prevent disappearing logo--></div><div class="tagline">Small. Fast. Reliable.<br>Choose any three.</div><table width=100% style="clear:both"><tr><td> <div class="se"><div class="sw"><div class="ne"><div class="nw"> <div class="toolbar"> <a href="about.html">About</a> <a href="sitemap.html">Sitemap</a> <a href="docs.html">Documentation</a> <a href="download.html">Download</a> <a href="copyright.html">License</a> <a href="news.html">News</a> <a href="http://www.sqlite.org/cvstrac/index">Developers</a> <a href="support.html">Support</a> </div></div></div></div></div></td></tr></table> <h2>Datatypes In SQLite Version 2</h2><h3>1.0 Typelessness</h3><p>SQLite is "typeless". This means that you can store anykind of data you want in any column of any table, regardless of thedeclared datatype of that column. (See the one exception to this rule in section 2.0 below.)This behavior is a feature, nota 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 andcodified in the SQL language spec is a misfeature -it is an example of the implementation showing through into theinterface. SQLite seeks to overcome this misfeature by allowingyou to store any kind of data into any kind of column and byallowing flexibility in the specification of datatypes.</p><p>A datatype to SQLite is any sequence of zero or more namesoptionally followed by a parenthesized lists of one or twosigned integers. Notice in particular that a datatype maybe <em>zero</em> or more names. That means that an emptystring is a valid datatype as far as SQLite is concerned.So you can declare tables where the datatype of each columnis 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 isstill a good idea to include it in your CREATE TABLE statements,since the data type often serves as a good hint to otherprogrammers about what you intend to put in the column. Andif you ever port your code to another database engine, thatother 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 NATIONAL VARYING CHARACTER(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><h3>2.0 The INTEGER PRIMARY KEY</h3><p>One exception to the typelessness of SQLite is a column whose typeis 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. Anyattempt to insert non-integer data will result in an error.</p><p>INTEGER PRIMARY KEY columns can be used to implement the equivalentof AUTOINCREMENT. If you try to insert a NULL into an INTEGER PRIMARYKEY column, the column will actually be filled with a integer that isone greater than the largest key already in the table. Or if thelargest key is 2147483647, then the column will be filled with arandom integer. Either way, the INTEGER PRIMARY KEY column will beassigned a unique integer. You can retrieve this integer usingthe <b>sqlite_last_insert_rowid()</b> API function or using the<b>last_insert_rowid()</b> SQL function in a subsequent SELECT statement.</p><h3>3.0 Comparison and Sort Order</h3><p>SQLite is typeless for the purpose of deciding what data is allowedto be stored in a column. But some notion of type comes into playwhen sorting and comparing data. For these purposes, a column oran expression can be one of two types: <b>numeric</b> and <b>text</b>.The sort or comparison may give different results depending on whichtype of data is being sorted or compared.</p><p>If data is of type <b>text</b> then the comparison is determined bythe standard C data comparison functions <b>memcmp()</b> or<b>strcmp()</b>. The comparison looks at bytes from two inputs oneby one and returns the first non-zero difference.Strings are '\000' terminated so shorterstrings sort before longer strings, as you would expect.</p><p>For numeric data, this situation is more complex. If both inputslook like well-formed numbers, then they are convertedinto floating point values using <b>atof()</b> and compared numerically.If one input is not a well-formed number but the other is, then thenumber is considered to be less than the non-number. If neither inputsis a well-formed number, then <b>strcmp()</b> is used to do thecomparison.</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 numberwill 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 "<" or">=" is always false.</p><h3>4.0 How SQLite Determines Datatypes</h3><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 itis 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 bythe outermost operator. For example, arithmetic operators ("+", "*", "%")always return a numeric results. The string concatenation operator("||") returns a text result. And so forth. If you are ever in doubtabout 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> SELECT typeof('abc'+123);numericsqlite> SELECT typeof('abc'||123);text</pre></blockquote><p>For table columns, the datatype is determined by the type declarationof the CREATE TABLE statement. The datatype is text if and only ifthe 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 typedeclaration, then the datatype of the column is text. Notice thatthe type "VARCHAR" contains "CHAR" as a substring so it is consideredtext.</p><p>If none of the strings above occur anywhere in the type declaration,then the datatype is numeric. Note in particular that the datatype for columnswith an empty type declaration is numeric.</p><h3>5.0 Examples</h3><p>Consider the following two command sequences:</p><blockquote><pre>CREATE TABLE t1(a INTEGER UNIQUE); CREATE TABLE t2(b TEXT UNIQUE);INSERT INTO t1 VALUES('0'); INSERT INTO t2 VALUES(0);INSERT INTO t1 VALUES('0.0'); INSERT INTO t2 VALUES(0.0);</pre></blockquote><p>In the sequence on the left, the second insert will fail. In this case,the strings '0' and '0.0' are treated as numbers since they are being inserted into a numeric column but 0==0.0 which violates the uniquenessconstraint. However, the second insert in the right-hand sequence works. Inthis case, the constants 0 and 0.0 are treated a strings which means thatthey are distinct.</p><p>SQLite always converts numbers into double-precision (64-bit) floatsfor comparison purposes. This means that a long sequence of digits thatdiffer only in insignificant digits will compare equal if theyare in a numeric column but will compare unequal if they are in a textcolumn. We have:</p><blockquote><pre>INSERT INTO t1 INSERT INTO t2 VALUES('12345678901234567890'); VALUES(12345678901234567890);INSERT INTO t1 INSERT INTO t2 VALUES('12345678901234567891'); VALUES(12345678901234567891);</pre></blockquote><p>As before, the second insert on the left will fail because the comparisonwill convert both strings into floating-point number first and the onlydifference in the strings is in the 20-th digit which exceeds the resolutionof a 64-bit float. In contrast, the second insert on the right will workbecause in that case, the numbers being inserted are strings and arecompared using memcmp().</p><p>Numeric and text types make a difference for the DISTINCT keyword too:</p><blockquote><pre>CREATE TABLE t3(a INTEGER); CREATE TABLE t4(b TEXT);INSERT INTO t3 VALUES('0'); INSERT INTO t4 VALUES(0);INSERT INTO t3 VALUES('0.0'); INSERT INTO t4 VALUES(0.0);SELECT DISTINCT * FROM t3; SELECT DISTINCT * FROM t4;</pre></blockquote><p>The SELECT statement on the left returns a single row since '0' and '0.0'are treated as numbers and are therefore indistinct. But the SELECT statement on the right returns two rows since 0 and 0.0 are treateda strings which are different.</p><hr><small><i>This page last modified 2007/11/12 14:22:22 UTC</i></small></div></body></html>?? 快捷鍵說明
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