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Begin adding documentation for version 3.0. (CVS 1509)

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set rcsid {$Id: datatype3.tcl,v 1.1 2004/05/31 18:22:26 drh Exp $}
source common.tcl
header {Datatypes In SQLite Version 3}
puts {
<h2>Datatypes In SQLite Version 3</h2>
<h3>1. Storage Classes</h3>
<P>Version 2 of SQLite stores all column values as ASCII text.
Version 3 enhances this by providing the ability to store integer and
real numbers in a more compact format and the capability to store
BLOB data.</P>
<P>Each value stored in an SQLite database (or manipulated by the
database engine) has one of the following storage classes:</P>
<UL>
<LI><P><B>NULL</B>. The value is a NULL value.</P>
<LI><P><B>INTEGER</B>. The value is a signed integer, stored in 1,
2, 3, 4, 6, or 8 bytes depending on the magnitude of the value.</P>
<LI><P><B>REAL</B>. The value is a floating point value, stored as
an 8-byte IEEE floating point number.</P>
<LI><P><B>TEXT</B>. The value is a text string, stored using the
database encoding (UTF-8, UTF-16BE or UTF-16-LE).</P>
<LI><P><B>BLOB</B>. The value is a blob of data, stored exactly as
it was input.</P>
</UL>
<P>As in SQLite version 2, any column in a version 3 database except an INTEGER
PRIMARY KEY may be used to store any type of value. The exception to
this rule is described below under 'Strict Affinity Mode'.</P>
<P>All values supplied to SQLite, whether as literals embedded in SQL
statements or values bound to pre-compiled SQL statements
are assigned a storage class before the SQL statement is executed.
Under circumstances described below, the
database engine may convert values between numeric storage classes
(INTEGER and REAL) and TEXT during query execution.
</P>
<P>Storage classes are initially assigned as follows:</P>
<UL>
<LI><P>Values specified as literals as part of SQL statements are
assigned storage class TEXT if they are enclosed by single or double
quotes, INTEGER if the literal is specified as an unquoted number
with no decimal point or exponent, REAL if the literal is an
unquoted number with a decimal point or exponent and NULL if the
value is a NULL.</P>
<LI><P>Values supplied using the sqlite3_bind_* APIs are assigned
the storage class that most closely matches the native type bound
(i.e. sqlite3_bind_blob() binds a value with storage class BLOB).</P>
</UL>
<P>The storage class of a value that is the result of an SQL scalar
operator depends on the outermost operator of the expression.
User-defined functions may return values with any storage class. It
is not generally possible to determine the storage class of the
result of an expression at compile time.</P>
<h3>2. Column Affinity</h3>
<p>
In SQLite version 3, the type of a value is associated with the value
itself, not with the column or variable in which the value is stored.
(This is sometimes called
<a href="http://www.cliki.net/manifest%20type%50system">
manifest typing</a>.)
All other SQL databases engines that we are aware of use the more
restrict system of static typing where the type is associated with
the container, not the value.
</p>
<p>
In order to maximize compatibility between SQLite and other database
engines, SQLite support the concept of "type affinity" on columns.
The type affinity of a column is the recommended type for data stored
in that column. The key here is that the type is recommended, not
required. Any column can still store any type of data, in theory.
It is just that some columns, given the choice, will prefer to use
one storage class over another. The preferred storage class for
a column is called its "affinity".
</p>
<P>Each column in an SQLite 3 database is assigned one of the
following type affinities:</P>
<UL>
<LI><P>TEXT.</P>
<LI><P>NUMERIC.</P>
<LI><P>INTEGER.</P>
<LI><P>NONE.</P>
</UL>
<P>A column with TEXT affinity stores all data using storage classes
NULL, TEXT or BLOB. If numerical data is inserted into a column with
TEXT affinity it is converted to text form before being stored.</P>
<P>A column with NUMERIC affinity may contain values using all five
storage classes. When text data is inserted into a NUMERIC column, an
attempt is made to convert it to an integer or real number before it
is stored. If the conversion is successful, then the value is stored
using the INTEGER or REAL storage class. If the conversion cannot be
performed the value is stored using the TEXT storage class. No
attempt is made to convert NULL or blob values.</P>
<P>A column that uses INTEGER affinity behaves in the same way as a
column with NUMERIC affinity, except that if a real value with no
floating point component (or text value that converts to such) is
inserted it is converted to an integer and stored using the INTEGER
storage class.</P>
<P>A column with affinity NONE does not prefer one storage class over
another. It makes no attempt to coerce data before
it is inserted.</P>
<h4>2.1 Determination Of Column Affinity</h4>
<P>The type affinity of a column is determined by the declared type
of the column, according to the following rules:</P>
<OL>
<LI><P>If the datatype contains the string &quot;INT&quot; then it
is assigned INTEGER affinity.</P>
<LI><P>If the datatype of the column contains any of the strings
&quot;CHAR&quot;, &quot;CLOB&quot;, or &quot;TEXT&quot; then that
column has TEXT affinity. Notice that the type VARCHAR contains the
string &quot;CHAR&quot; and is thus assigned TEXT affinity.</P>
<LI><P>If the datatype contains the string &quot;BLOB&quot;
then the column has affinity NONE.</P>
<LI><P>Otherwise, the affinity is NUMERIC. Notice that a column
where no datatype is specified is given affinity NUMERIC.</P>
</OL>
<P>If a table is created using a "CREATE TABLE &lt;table&gt; AS
SELECT..." statement, then all columns have no datatype specified
and they are given no affinity.</P>
<h4>2.2 Column Affinity Example</h4>
<blockquote>
<PRE>CREATE TABLE t1(
t AFFINITY TEXT,
nu AFFINITY NUMERIC,
i AFFINITY INTEGER,
no AFFINITY NONE
);
-- Storage classes for the following row:
-- TEXT, REAL, INTEGER, TEXT
INSERT INTO t1 VALUES('500.0', '500.0', '500.0', '500.0');
-- Storage classes for the following row:
-- TEXT, REAL, INTEGER, REAL
INSERT INTO t1 VALUES(500.0, 500.0, 500.0, 500.0);</PRE>
</blockquote>
<h3>3. Comparison Expressions</h3>
<P>Like SQLite version 2, version 3
features the binary comparison operators '=',
'&lt;', '&lt;=', '&gt;=' and '!=', an operation to test for set
membership, 'IN', and the ternary comparison operator 'BETWEEN'.</P>
<P>The results of a comparison depend on the storage classes of the
two values being compared, according to the following rules:</P>
<UL>
<LI><P>A value with storage class NULL is considered less than any
other value (including another value with storage class NULL).</P>
<LI><P>An INTEGER or REAL value is less than any TEXT or BLOB value.
When an INTEGER or REAL is compared to another INTEGER or REAL, a
numerical comparison is performed.</P>
<LI><P>A TEXT value is less than a BLOB value. When two TEXT values
are compared, the C library function memcmp() is usually used to
determine the result. However this can be overriden, as described
under 'User-defined collation Sequences' below.</P>
<LI><P>When two BLOB values are compared, the result is always
determined using memcmp().</P>
</UL>
<P>SQLite may attempt to convert values between the numeric storage
classes (INTEGER and REAL) and TEXT before performing a comparison.
For binary comparisons, this is done in the cases enumerated below.
The term “expression” used in the bullet points below means any
SQL scalar expression or literal other than a column value.</P>
<UL>
<LI><P>When a column value is compared to the result of an
expression, the affinity of the column is applied to the result of
the expression before the comparison takes place.</P>
<LI><P>When two column values are compared, if one column has
INTEGER or NUMERIC affinity and the other does not, the NUMERIC
affinity is applied to any values with storage class TEXT extracted
from the non-NUMERIC column.</P>
<LI><P>When the results of two expressions are compared, the NUMERIC
affinity is applied to both values before the comparison takes
place.</P>
</UL>
<h4>3.1 Comparison Example</h4>
<blockquote>
<PRE>CREATE TABLE t1(
a AFFINITY TEXT,
b AFFINITY NUMERIC,
c AFFINITY NONE
);
-- Storage classes for the following row:
-- TEXT, REAL, TEXT
INSERT INTO t1 VALUES('500', '500', '500');
-- 60 and 40 are converted to “60” and “40” and values are compared as TEXT.
SELECT a &lt; 60, a &lt; 40 FROM t1;
1|0
-- Comparisons are numeric. No conversions are required.
SELECT b &lt; 60, b &lt; 600 FROM t1;
0|1
-- Both 60 and 600 (storage class NUMERIC) are less than '500' (storage class TEXT).
SELECT c &lt; 60, c &lt; 600 FROM t1;
0|0</PRE>
</blockquote>
<P>
In SQLite, the expression "a BETWEEN b AND c" is currently
equivalent to "a &gt;= b AND a &lt;= c". SQLite will continue to
treat the two as exactly equivalent, even if this means that
different affinities are applied to 'a' in each of the comparisons
required to evaluate the expression.</P>
<P>Expressions of the type "a IN (SELECT b ....)" are handled by
the three rules enumerated above for binary comparisons (e.g. in a
similar manner to "a = b"). For example if 'b' is a column value
and 'a' is an expression, then the affinity of 'b' is applied to 'a'
before any comparisons take place.</P>
<P>SQLite currently treats the expression "a IN (x, y, z)" as
equivalent to "a = z OR a = y OR a = z". SQLite will continue to
treat the two as exactly equivalent, even if this means that
different affinities are applied to 'a' in each of the comparisons
required to evaluate the expression.</P>
<h3>4. Operators</h3>
<P>All mathematical operators (which is to say, all operators other
than the concatenation operator &quot;||&quot;) apply NUMERIC
affinity to all operands prior to being carried out. If one or both
operands cannot be converted to NUMERIC then the result of the
operation is NULL.</P>
<P>For the concatenation operator, TEXT affinity is applied to both
operands. If either operand cannot be converted to TEXT (because it
is NULL or a BLOB) then the result of the concatenation is NULL.</P>
<h3>5. Sorting, Grouping and Compound SELECTs</h3>
<P>When values are sorted by an ORDER by clause, values with storage
class NULL come first, followed by INTEGER and REAL values
interspersed in numeric order, followed by TEXT values usually in
memcmp() order, and finally BLOB values in memcmp() order. No storage
class conversions occur before the sort.</P>
<P>When grouping values with the GROUP BY clause values with
different storage classes are considered distinct, except for INTEGER
and REAL values which are considered equal if they are numerically
equal. No affinities are applied to any values as the result of a
GROUP by clause.</P>
<P>The compound SELECT operators UNION,
INTERSECT and EXCEPT perform implicit comparisons between values.
Before these comparisons are performed an affinity may be applied to
each value. The same affinity, if any, is applied to all values that
may be returned in a single column of the compound SELECT result set.
The affinity applied is the affinity of the column returned by the
left most component SELECTs that has a column value (and not some
other kind of expression) in that position. If for a given compound
SELECT column none of the component SELECTs return a column value, no
affinity is applied to the values from that column before they are
compared.</P>
<h3>6. Other Affinity Modes</h3>
<P>The above sections describe the operation of the database engine
in 'normal' affinity mode. SQLite version 3 will feature two other affinity
modes, as follows:</P>
<UL>
<LI><P><B>Strict affinity</B> mode. In this mode if a conversion
between storage classes is ever required, the database engine
returns an error and the current statement is rolled back.</P>
<LI><P><B>No affinity</B> mode. In this mode no conversions between
storage classes are ever performed. Comparisons between values of
different storage classes (except for INTEGER and REAL) are always
false.</P>
</UL>
<h3>7. User-defined Collation Sequences</h3>
<P>By default, when SQLite compares two
text values, the result of the comparison is determined using
memcmp(), regardless of the encoding of the string. SQLite v3
provides the ability for users to supply arbitrary comparison
functions, known as user-defined collation sequences, to be used
instead of memcmp().</P>
}
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