lang_expr.html

嵌入式数据库sqlite 3.5.9的文档

Parameters can take several forms:
</p

<blockquote>
<table>
<tr>
<td align="right" valign="top"><b>?</b><i>NNN</i></td><td width="20"></td>
<td>A question mark followed by a number <i>NNN</i> holds a spot for the
NNN-th parameter.  NNN must be between 1 and 
<a href="limits.html#max_variable_number">SQLITE_MAX_VARIABLE_NUMBER</a>.</td>
</tr>
<tr>
<td align="right" valign="top"><b>?</b></td><td width="20"></td>
<td>A question mark that is not followed by a number holds a spot for
the next unused parameter.</td>
</tr>
<tr>
<td align="right" valign="top"><b>:</b><i>AAAA</i></td><td width="20"></td>
<td>A colon followed by an identifier name holds a spot for a named
parameter with the name AAAA.  Named parameters are also numbered.
The number assigned is the next unused number.  To avoid confusion,
it is best to avoid mixing named and numbered parameters.</td>
</tr>
<tr>
<td align="right" valign="top"><b>@</b><i>AAAA</i></td><td width="20"></td>
<td>An "at" sign works exactly like a colon.</td>
</tr>
<tr>
<td align="right" valign="top"><b>$</b><i>AAAA</i></td><td width="20"></td>
<td>A dollar-sign followed by an identifier name also holds a spot for a named
parameter with the name AAAA.  The identifier name in this case can include
one or more occurances of "::" and a suffix enclosed in "(...)" containing
any text at all.  This syntax is the form of a variable name in the Tcl
programming language.</td>
</tr>
</table>
</blockquote>

<p>Parameters that are not assigned values using

<a href="c3ref/bind_blob.html">sqlite3_bind()</a> are treated
as NULL.</p>


<a name="like"></a>

<p>The LIKE operator does a pattern matching comparison. The operand
to the right contains the pattern, the left hand operand contains the
string to match against the pattern. 


A percent symbol <font color="#2c2cf0"><big>%</big></font> in the pattern matches any
sequence of zero or more characters in the string.  An underscore
<font color="#2c2cf0"><big>_</big></font> in the pattern matches any single character in the
string.  Any other character matches itself or it's lower/upper case
equivalent (i.e. case-insensitive matching).  (A bug: SQLite only
understands upper/lower case for 7-bit Latin characters.  Hence the
LIKE operator is case sensitive for 8-bit iso8859 characters or UTF-8
characters.  For example, the expression <b>'a'&nbsp;LIKE&nbsp;'A'</b>
is TRUE but <b>'&aelig;'&nbsp;LIKE&nbsp;'&AElig;'</b> is FALSE.).</p>


<p>If the optional ESCAPE clause is present, then the expression
following the ESCAPE keyword must evaluate to a string consisting of
a single character. This character may be used in the LIKE pattern
to include literal percent or underscore characters. The escape
character followed by a percent symbol, underscore or itself matches a
literal percent symbol, underscore or escape character in the string,
respectively. The infix LIKE operator is implemented by calling the
user function <a href="#likeFunc"> like(<i>X</i>,<i>Y</i>)</a>.</p>

The LIKE operator is not case sensitive and will match upper case
characters on one side against lower case characters on the other.  
(A bug: SQLite only understands upper/lower case for 7-bit Latin
characters.  Hence the LIKE operator is case sensitive for 8-bit
iso8859 characters or UTF-8 characters.  For example, the expression
<b>'a'&nbsp;LIKE&nbsp;'A'</b> is TRUE but
<b>'&aelig;'&nbsp;LIKE&nbsp;'&AElig;'</b> is FALSE.).</p>

<p>The infix LIKE
operator is implemented by calling the user function <a href="#likeFunc">
like(<i>X</i>,<i>Y</i>)</a>.  If an ESCAPE clause is present, it adds
a third parameter to the function call. If the functionality of LIKE can be
overridden by defining an alternative implementation of the
like() SQL function.</p>
</p>


<a name="glob"></a>

<p>The GLOB operator is similar to LIKE but uses the Unix
file globbing syntax for its wildcards.  Also, GLOB is case
sensitive, unlike LIKE.  Both GLOB and LIKE may be preceded by
the NOT keyword to invert the sense of the test.  The infix GLOB 
operator is implemented by calling the function

<a href="lang_expr.html#glob">glob(<i>X</i>,<i>Y</i>)</a> and can be modified by overriding
that function.</p>


<a name="regexp"></a>

<p>The REGEXP operator is a special syntax for the regexp()
user function.  No regexp() user function is defined by default
and so use of the REGEXP operator will normally result in an
error message.  If a user-defined function named "regexp"
is added at run-time, that function will be called in order
to implement the REGEXP operator.</p>


<a name="match"></a>

<p>The MATCH operator is a special syntax for the match()
user function.  The default match() function implementation
raises and exception and is not really useful for anything.
But extensions can override the match() function with more
helpful logic.</p>

<p>A column name can be any of the names defined in the 
<a href="lang_createtable.html">CREATE TABLE</a>
statement or one of the following special identifiers: "<b>ROWID</b>",
"<b>OID</b>", or "<b>_ROWID_</b>".
These special identifiers all describe the
unique integer key (the "row key") associated with every 
row of every table.
The special identifiers only refer to the row key if the 
<a href="lang_createtable.html">CREATE TABLE</a>
statement does not define a real column with the same name.  Row keys
act like read-only columns.  A row key can be used anywhere a regular
column can be used, except that you cannot change the value
of a row key in an 
<a href="lang_update.html">UPDATE</a> or 
<a href="lang_insert.html">INSERT</a> statement.
"SELECT * ..." does not return the row key.</p>

<p>
<a href="lang_select.html">SELECT</a> statements can appear in expressions as either the
right-hand operand of the IN operator, as a scalar quantity, or
as the operand of an EXISTS operator.
As a scalar quantity or the operand of an IN operator,
the SELECT should have only a single column in its
result.  Compound SELECTs (connected with keywords like UNION or
EXCEPT) are allowed.
With the EXISTS operator, the columns in the result set of the 
<a href="lang_select.html">SELECT</a> are
ignored and the expression returns TRUE if one or more rows exist
and FALSE if the result set is empty.
If no terms in the 
<a href="lang_select.html">SELECT</a> expression refer to value in the containing
query, then the expression is evaluated once prior to any other
processing and the result is reused as necessary.  If the 
<a href="lang_select.html">SELECT</a> expression
does contain variables from the outer query, then the 
<a href="lang_select.html">SELECT</a> is reevaluated
every time it is needed.</p>

<p>When a SELECT is the right operand of the IN operator, the IN
operator returns TRUE if the result of the left operand is any of
the values generated by the select.  The IN operator may be preceded
by the NOT keyword to invert the sense of the test.</p>

<p>When a SELECT appears within an expression but is not the right
operand of an IN operator, then the first row of the result of the
SELECT becomes the value used in the expression.  If the SELECT yields
more than one result row, all rows after the first are ignored.  If
the SELECT yields no rows, then the value of the SELECT is NULL.</p>

<p>A CAST expression changes the datatype of the <expr> into the
type specified by &lt;type&gt;. 
&lt;type&gt; can be any non-empty type name that is valid
for the type in a column definition of a 
<a href="lang_createtable.html">CREATE TABLE</a> statement.</p>

<p>Both simple and aggregate functions are supported.  A simple
function can be used in any expression.  Simple functions return
a result immediately based on their inputs.  Aggregate functions
may only be used in a SELECT statement.  Aggregate functions compute
their result across all rows of the result set.</p>


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This page last modified 2008/04/27 17:48:52 UTC
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