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<h1 class="heading">
SQL As Understood By SQLite</h1>
<h4>
PRAGMA</h4>
<p>
The <a href="#syntax">PRAGMA command</a> is a special command used to modify the
operation of the SQLite library or to query the library for internal (non-table)
data. The PRAGMA command is issued using the same interface as other SQLite commands
(e.g. SELECT, INSERT) but is different in the following important respects:
</p>
<ul>
<li>Specific pragma statements may be removed and others added in future releases
of SQLite. Use with caution! </li>
<li>No error messages are generated if an unknown pragma is issued. Unknown pragmas
are simply ignored. This means if there is a typo in a pragma statement the library
does not inform the user of the fact. </li>
<li>Some pragmas take effect during the SQL compilation stage, not the execution stage.
This means if using the C-language sqlite3_compile(), sqlite3_step(), sqlite3_finalize()
API (or similar in a wrapper interface), the pragma may be applied to the library
during the sqlite3_compile() call. </li>
<li>The pragma command is unlikely to be compatible with any other SQL engine. </li>
</ul>
<p>
The available pragmas fall into four basic categories:</p>
<ul>
<li>Pragmas used to <a href="#schema">query the schema</a> of the current database.
</li>
<li>Pragmas used to <a href="#modify">modify the operation</a> of the SQLite library
in some manner, or to query for the current mode of operation. </li>
<li>Pragmas used to <a href="#version">query or modify the databases two version values</a>,
the schema-version and the user-version. </li>
<li>Pragmas used to <a href="#debug">debug the library</a> and verify that database
files are not corrupted. </li>
</ul>
<hr />
<a name="syntax"></a>
<h1>
PRAGMA command syntax</h1>
<p>
<table cellpadding="10">
<tr>
<td align="right" width="1%" nowrap>
<i><font color="#ff3434">sql-statement</font></i> ::=</td>
<td>
<b><font color="#2c2cf0">PRAGMA </font></b><i><font color="#ff3434">name</font></i><b><font
color="#2c2cf0"> </font></b>[<b><font color="#2c2cf0">= </font></b><i><font color="#ff3434">
value</font></i><b><font color="#2c2cf0"></font></b>]<b><font color="#2c2cf0"> </font>
</b><big>|</big><b><font color="#2c2cf0"><br />
PRAGMA </font></b><i><font color="#ff3434">function</font></i><b><font color="#2c2cf0"><big>(</big></font></b><i><font
color="#ff3434">arg</font></i><b><font color="#2c2cf0"><big>)</big></font></b></td>
</tr>
</table>
</p>
<p>
The pragmas that take an integer <b><i>value</i></b> also accept symbolic names.
The strings "<b>on</b>", "<b>true</b>", and "<b>yes</b>" are equivalent to <b>1</b>.
The strings "<b>off</b>", "<b>false</b>", and "<b>no</b>" are equivalent to <b>0</b>.
These strings are case- insensitive, and do not require quotes. An unrecognized
string will be treated as <b>1</b>, and will not generate an error. When the <i>value</i>
is returned it is as an integer.</p>
<hr />
<a name="modify"></a>
<h1>
Pragmas to modify library operation</h1>
<ul>
<a name="pragma_auto_vacuum"></a>
<li>
<p>
<b>PRAGMA auto_vacuum;
<br />
PRAGMA auto_vacuum = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or set the auto-vacuum flag in the database.</p>
<p>
Normally, when a transaction that deletes data from a database is committed, the
database file remains the same size. Unused database file pages are marked as such
and reused later on, when data is inserted into the database. In this mode the <a
href="lang_vacuum.html">VACUUM</a> command is used to reclaim unused space.</p>
<p>
When the auto-vacuum flag is set, the database file shrinks when a transaction that
deletes data is committed (The VACUUM command is not useful in a database with the
auto-vacuum flag set). To support this functionality the database stores extra information
internally, resulting in slightly larger database files than would otherwise be
possible.</p>
<p>
It is only possible to modify the value of the auto-vacuum flag before any tables
have been created in the database. No error message is returned if an attempt to
modify the auto-vacuum flag is made after one or more tables have been created.
</p>
<a name="pragma_cache_size"></a></li>
<li>
<p>
<b>PRAGMA cache_size;
<br />
PRAGMA cache_size = </b><i>Number-of-pages</i><b>;</b></p>
<p>
Query or change the maximum number of database disk pages that SQLite will hold
in memory at once. Each page uses about 1.5K of memory. The default cache size is
2000. If you are doing UPDATEs or DELETEs that change many rows of a database and
you do not mind if SQLite uses more memory, you can increase the cache size for
a possible speed improvement.</p>
<p>
When you change the cache size using the cache_size pragma, the change only endures
for the current session. The cache size reverts to the default value when the database
is closed and reopened. Use the <a href="#pragma_default_cache_size"><b>default_cache_size</b></a>
pragma to check the cache size permanently.</p>
<a name="pragma_case_sensitive_like"></a></li>
<li>
<p>
<b>PRAGMA case_sensitive_like;
<br />
PRAGMA case_sensitive_like = </b><i>0 | 1</i><b>;</b></p>
<p>
The default behavior of the LIKE operator is to ignore case for latin1 characters.
Hence, by default <b>'a' LIKE 'A'</b> is true. The case_sensitive_like pragma can
be turned on to change this behavior. When case_sensitive_like is enabled, <b>'a'
LIKE 'A'</b> is false but <b>'a' LIKE 'a'</b> is still true.</p>
<a name="pragma_count_changes"></a></li>
<li>
<p>
<b>PRAGMA count_changes;
<br />
PRAGMA count_changes = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or change the count-changes flag. Normally, when the count-changes flag is
not set, INSERT, UPDATE and DELETE statements return no data. When count-changes
is set, each of these commands returns a single row of data consisting of one integer
value - the number of rows inserted, modified or deleted by the command. The returned
change count does not include any insertions, modifications or deletions performed
by triggers.</p>
<a name="pragma_default_cache_size"></a></li>
<li>
<p>
<b>PRAGMA default_cache_size;
<br />
PRAGMA default_cache_size = </b><i>Number-of-pages</i><b>;</b></p>
<p>
Query or change the maximum number of database disk pages that SQLite will hold
in memory at once. Each page uses 1K on disk and about 1.5K in memory. This pragma
works like the <a href="#pragma_cache_size"><b>cache_size</b></a> pragma with the
additional feature that it changes the cache size persistently. With this pragma,
you can set the cache size once and that setting is retained and reused every time
you reopen the database.</p>
<a name="pragma_default_synchronous"></a></li>
<li>
<p>
<b>PRAGMA default_synchronous;</b></p>
<p>
This pragma was available in version 2.8 but was removed in version 3.0. It is a
dangerous pragma whose use is discouraged. To help dissuide users of version 2.8
from employing this pragma, the documentation will not tell you what it does.</p>
<a name="pragma_empty_result_callbacks"></a></li>
<li>
<p>
<b>PRAGMA empty_result_callbacks;
<br />
PRAGMA empty_result_callbacks = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or change the empty-result-callbacks flag.</p>
<p>
The empty-result-callbacks flag affects the sqlite3_exec API only. Normally, when
the empty-result-callbacks flag is cleared, the callback function supplied to the
sqlite3_exec() call is not invoked for commands that return zero rows of data. When
empty-result-callbacks is set in this situation, the callback function is invoked
exactly once, with the third parameter set to 0 (NULL). This is to enable programs
that use the sqlite3_exec() API to retrieve column-names even when a query returns
no data.
</p>
<a name="pragma_encoding"></a></li>
<li>
<p>
<b>PRAGMA encoding;
<br />
PRAGMA encoding = "UTF-8";
<br />
PRAGMA encoding = "UTF-16";
<br />
PRAGMA encoding = "UTF-16le";
<br />
PRAGMA encoding = "UTF-16be";</b></p>
<p>
In first form, if the main database has already been created, then this pragma returns
the text encoding used by the main database, one of "UTF-8", "UTF-16le" (little-endian
UTF-16 encoding) or "UTF-16be" (big-endian UTF-16 encoding). If the main database
has not already been created, then the value returned is the text encoding that
will be used to create the main database, if it is created by this session.</p>
<p>
The second and subsequent forms of this pragma are only useful if the main database
has not already been created. In this case the pragma sets the encoding that the
main database will be created with if it is created by this session. The string
"UTF-16" is interpreted as "UTF-16 encoding using native machine byte-ordering".
If the second and subsequent forms are used after the database file has already
been created, they have no effect and are silently ignored.</p>
<p>
Once an encoding has been set for a database, it cannot be changed.</p>
<p>
Databases created by the ATTACH command always use the same encoding as the main
database.</p>
<a name="pragma_full_column_names"></a></li>
<li>
<p>
<b>PRAGMA full_column_names;
<br />
PRAGMA full_column_names = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or change the full-column-names flag. This flag affects the way SQLite names
columns of data returned by SELECT statements when the expression for the column
is a table-column name or the wildcard "*". Normally, such result columns are named
<table-name/alias><column-name> if the SELECT statement joins two or
more tables together, or simply <column-name> if the SELECT statement queries
a single table. When the full-column-names flag is set, such columns are always
named <table-name/alias> <column-name> regardless of whether or not
a join is performed.
</p>
<p>
If both the short-column-names and full-column-names are set, then the behaviour
associated with the full-column-names flag is exhibited.
</p>
<a name="pragma_fullfsync"></a></li>
<li>
<p>
<b>PRAGMA fullfsync
<br />
PRAGMA fullfsync = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or change the fullfsync flag. This flag affects determines whether or not
the F_FULLFSYNC syncing method is used on systems that support it. The default value
is off. As of this writing (2006-02-10) only Mac OS X supports F_FULLFSYNC.
</p>
<a name="pragma_legacy_file_format"></a></li>
<li>
<p>
<b>PRAGMA legacy_file_format;
<br />
PRAGMA legacy_file_format = <i>ON | OFF</i></b></p>
<p>
This pragma sets or queries the value of the legacy_file_format flag. When this
flag is on, new SQLite databases are created in a file format that is readable and
writable by all versions of SQLite going back to 3.0.0. When the flag is off, new
databases are created using the latest file format which might to be readable or
writable by older versions of SQLite.</p>
<p>
This flag only effects newly created databases. It has no effect on databases that
already exists.</p>
<a name="pragma_page_size"></a></li>
<li>
<p>
<b>PRAGMA page_size;
<br />
PRAGMA page_size = </b><i>bytes</i><b>;</b></p>
<p>
Query or set the page-size of the database. The page-size may only be set if the
database has not yet been created. The page size must be a power of two greater
than or equal to 512 and less than or equal to 8192. The upper limit may be modified
by setting the value of macro SQLITE_MAX_PAGE_SIZE during compilation. The maximum
upper bound is 32768.
</p>
<a name="pragma_read_uncommitted"></a></li>
<li>
<p>
<b>PRAGMA read_uncommitted;
<br />
PRAGMA read_uncommitted = </b><i>0 | 1</i><b>;</b></p>
<p>
Query, set, or clear READ UNCOMMITTED isolation. The default isolation level for
SQLite is SERIALIZABLE. Any process or thread can select READ UNCOMMITTED isolation,
but SERIALIZABLE will still be used except between connections that share a common
page and schema cache. Cache sharing is enabled using the <a href="capi3ref.html#sqlite3_enable_shared_cache">
sqlite3_enable_shared_cache()</a> API and is only available between connections
running the same thread. Cache sharing is off by default.
</p>
<a name="pragma_short_column_names"></a></li>
<li>
<p>
<b>PRAGMA short_column_names;
<br />
PRAGMA short_column_names = </b><i>0 | 1</i><b>;</b></p>
<p>
Query or change the short-column-names flag. This flag affects the way SQLite names
columns of data returned by SELECT statements when the expression for the column
is a table-column name or the wildcard "*". Normally, such result columns are named
<table-name/alias>lt;column-name> if the SELECT statement joins two or
more tables together, or simply <column-name> if the SELECT statement queries
a single table. When the short-column-names flag is set, such columns are always
named <column-name> regardless of whether or not a join is performed.
</p>
<p>
If both the short-column-names and full-column-names are set, then the behaviour
associated with the full-column-names flag is exhibited.
</p>
<a name="pragma_synchronous"></a></li>
<li>
<p>
<b>PRAGMA synchronous;
<br />
PRAGMA synchronous = FULL; </b>(2)<b>
<br />
PRAGMA synchronous = NORMAL; </b>(1)<b>
<br />
PRAGMA synchronous = OFF; </b>(0)</p>
<p>
Query or change the setting of the "synchronous" flag. The first (query) form will
return the setting as an integer. When synchronous is FULL (2), the SQLite database
engine will pause at critical moments to make sure that data has actually been written
to the disk surface before continuing. This ensures that if the operating system
crashes or if there is a power failure, the database will be uncorrupted after rebooting.
FULL synchronous is very safe, but it is also slow. When synchronous is NORMAL,
the SQLite database engine will still pause at the most critical moments, but less
often than in FULL mode. There is a very small (though non-zero) chance that a power
failure at just the wrong time could corrupt the database in NORMAL mode. But in
practice, you are more likely to suffer a catastrophic disk failure or some other
unrecoverable hardware fault. With synchronous OFF (0), SQLite continues without
pausing as soon as it has handed data off to the operating system. If the application
running SQLite crashes, the data will be safe, but the database might become corrupted
if the operating system crashes or the computer loses power before that data has
been written to the disk surface. On the other hand, some operations are as much
as 50 or more times faster with synchronous OFF.
</p>
<p>
In SQLite version 2, the default value is NORMAL. For version 3, the default was
changed to FULL.
</p>
<a name="pragma_temp_store"></a></li>
<li>
<p>
<b>PRAGMA temp_store;
<br />
PRAGMA temp_store = DEFAULT;</b> (0)<b>
<br />
PRAGMA temp_store = FILE;</b> (1)<b>
<br />
PRAGMA temp_store = MEMORY;</b> (2)</p>
<p>
Query or change the setting of the "<b>temp_store</b>" parameter. When temp_store
is DEFAULT (0), the compile-time C preprocessor macro TEMP_STORE is used to determine
where temporary tables and indices are stored. When temp_store is MEMORY (2) temporary
tables and indices are kept in memory. When temp_store is FILE (1) temporary tables
and indices are stored in a file. The <a href="#pragma_temp_store_directory">temp_store_directory</a>
pragma can be used to specify the directory containing this file. <b>FILE</b> is
specified. When the temp_store setting is changed, all existing temporary tables,
indices, triggers, and views are immediately deleted.</p>
<p>
It is possible for the library compile-time C preprocessor symbol TEMP_STORE to
override this pragma setting. The following table summarizes the interaction of
the TEMP_STORE preprocessor macro and the temp_store pragma:</p>
<blockquote>
<table border="1" cellpadding="2">
<tr>
<th valign="bottom">
TEMP_STORE</th>
<th valign="bottom">
PRAGMA<br />
temp_store</th>
<th>
Storage used for<br />
TEMP tables and indices</th>
</tr>
<tr>
<td align="middle">
0</td>
<td align="middle">
<em>any</em></td>
<td align="middle">
file</td>
</tr>
<tr>
<td align="middle">
1</td>
<td align="middle">
0</td>
<td align="middle">
file</td>
</tr>
<tr>
<td align="middle">
1</td>
<td align="middle">
1</td>
<td align="middle">
file</td>
</tr>
<tr>
<td align="middle">
1</td>
<td align="middle">
2</td>
<td align="middle">
memory</td>
</tr>
<tr>
<td align="middle">
2</td>
<td align="middle">
0</td>
<td align="middle">
memory</td>
</tr>
<tr>
<td align="middle">
2</td>
<td align="middle">
1</td>
<td align="middle">
file</td>
</tr>
<tr>
<td align="middle">
2</td>
<td align="middle">
2</td>
<td align="middle">
memory</td>
</tr>
<tr>
<td align="middle">
3</td>
<td align="middle">
<em>any</em></td>
<td align="middle">
memory</td>
</tr>
</table>
</blockquote>
<br />
<a name="pragma_temp_store_directory"></a></li>
<li>
<p>
<b>PRAGMA temp_store_directory;
<br />
PRAGMA temp_store_directory = 'directory-name';</b></p>
<p>
Query or change the setting of the "temp_store_directory" - the directory where
files used for storing temporary tables and indices are kept. This setting lasts
for the duration of the current connection only and resets to its default value
for each new connection opened.
</p>
<p>
When the temp_store_directory setting is changed, all existing temporary tables,
indices, triggers, and viewers are immediately deleted. In practice, temp_store_directory
should be set immediately after the database is opened.
</p>
<p>
The value <i>directory-name</i> should be enclosed in single quotes. To revert the
directory to the default, set the <i>directory-name</i> to an empty string, e.g.,
<i>PRAGMA temp_store_directory = ''</i>. An error is raised if <i>directory-name</i>
is not found or is not writable.
</p>
<p>
The default directory for temporary files depends on the OS. For Unix/Linux/OSX,
the default is the is the first writable directory found in the list of: <b>/var/tmp,
/usr/tmp, /tmp,</b> and <b><i>current-directory</i></b>. For Windows NT, the default
directory is determined by Windows, generally <b>C:\Documents and Settings\<i>user-name</i>\Local
Settings\Temp\</b>. Temporary files created by SQLite are unlinked immediately
after opening, so that the operating system can automatically delete the files when
the SQLite process exits. Thus, temporary files are not normally visible through
<i>ls</i> or <i>dir</i> commands.</p>
</li>
</ul>
<hr />
<a name="schema"></a>
<h1>
Pragmas to query the database schema</h1>
<ul>
<a name="pragma_database_list"></a>
<li>
<p>
<b>PRAGMA database_list;</b></p>
<p>
For each open database, invoke the callback function once with information about
that database. Arguments include the index and the name the database was attached
with. The first row will be for the main database. The second row will be for the
database used to store temporary tables.</p>
<a name="pragma_foreign_key_list"></a></li>
<li>
<p>
<b>PRAGMA foreign_key_list(</b><i>table-name</i><b>);</b></p>
<p>
For each foreign key that references a column in the argument table, invoke the
callback function with information about that foreign key. The callback function
will be invoked once for each column in each foreign key.</p>
<a name="pragma_index_info"></a></li>
<li>
<p>
<b>PRAGMA index_info(</b><i>index-name</i><b>);</b></p>
<p>
For each column that the named index references, invoke the callback function once
with information about that column, including the column name, and the column number.</p>
<a name="pragma_index_list"></a></li>
<li>
<p>
<b>PRAGMA index_list(</b><i>table-name</i><b>);</b></p>
<p>
For each index on the named table, invoke the callback function once with information
about that index. Arguments include the index name and a flag to indicate whether
or not the index must be unique.</p>
<a name="pragma_table_info"></a></li>
<li>
<p>
<b>PRAGMA table_info(</b><i>table-name</i><b>);</b></p>
<p>
For each column in the named table, invoke the callback function once with information
about that column, including the column name, data type, whether or not the column
can be NULL, and the default value for the column.</p>
</li>
</ul>
<hr />
<a name="version"></a>
<h1>
Pragmas to query/modify version values</h1>
<ul>
<a name="pragma_schema_version"></a><a name="pragma_user_version"></a>
<li>
<p>
<b>PRAGMA [database.]schema_version;
<br />
PRAGMA [database.]schema_version = </b><i>integer </i><b>;
<br />
PRAGMA [database.]user_version;
<br />
PRAGMA [database.]user_version = </b><i>integer </i><b>;</b>
</p>
<p>
The pragmas schema_version and user_version are used to set or get the value of
the schema-version and user-version, respectively. Both the schema-version and the
user-version are 32-bit signed integers stored in the database header.</p>
<p>
The schema-version is usually only manipulated internally by SQLite. It is incremented
by SQLite whenever the database schema is modified (by creating or dropping a table
or index). The schema version is used by SQLite each time a query is executed to
ensure that the internal cache of the schema used when compiling the SQL query matches
the schema of the database against which the compiled query is actually executed.
Subverting this mechanism by using "PRAGMA schema_version" to modify the schema-version
is potentially dangerous and may lead to program crashes or database corruption.
Use with caution!</p>
<p>
The user-version is not used internally by SQLite. It may be used by applications
for any purpose.</p>
</li>
</ul>
<hr />
<a name="debug"></a>
<h1>
Pragmas to debug the library</h1>
<ul>
<a name="pragma_integrity_check"></a>
<li>
<p>
<b>PRAGMA integrity_check;</b></p>
<p>
The command does an integrity check of the entire database. It looks for out-of-order
records, missing pages, malformed records, and corrupt indices. If any problems
are found, then a single string is returned which is a description of all problems.
If everything is in order, "ok" is returned.</p>
<a name="pragma_parser_trace"></a></li>
<li>
<p>
<b>PRAGMA parser_trace = ON; </b>(1)<b>
<br />
PRAGMA parser_trace = OFF;</b> (0)</p>
<p>
Turn tracing of the SQL parser inside of the SQLite library on and off. This is
used for debugging. This only works if the library is compiled without the NDEBUG
macro.
</p>
<a name="pragma_vdbe_trace"></a></li>
<li>
<p>
<b>PRAGMA vdbe_trace = ON; </b>(1)<b>
<br />
PRAGMA vdbe_trace = OFF;</b> (0)</p>
<p>
Turn tracing of the virtual database engine inside of the SQLite library on and
off. This is used for debugging. See the <a href="vdbe.html#trace">VDBE documentation</a>
for more information.</p>
<a name="pragma_vdbe_listing"></a></li>
<li>
<p>
<b>PRAGMA vdbe_listing = ON; </b>(1)<b>
<br />
PRAGMA vdbe_listing = OFF;</b> (0)</p>
<p>
Turn listings of virtual machine programs on and off. With listing is on, the entire
content of a program is printed just prior to beginning execution. This is like
automatically executing an EXPLAIN prior to each statement. The statement executes
normally after the listing is printed. This is used for debugging. See the <a href="vdbe.html#trace">
VDBE documentation</a> for more information.</p>
</li>
</ul>
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