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+<a href="../session/intro.html"><h2>Session Module C Interface</h2></a><h2>Generate A Changeset From A Session Object</h2><blockquote><pre>int sqlite3session_changeset(
+  sqlite3_session *pSession,      /* Session object */
+  int *pnChangeset,               /* OUT: Size of buffer at *ppChangeset */
+  void **ppChangeset              /* OUT: Buffer containing changeset */
+);
+</pre></blockquote><p>
+Obtain a changeset containing changes to the tables attached to the 
+session object passed as the first argument. If successful, 
+set *ppChangeset to point to a buffer containing the changeset 
+and *pnChangeset to the size of the changeset in bytes before returning
+SQLITE_OK. If an error occurs, set both *ppChangeset and *pnChangeset to
+zero and return an SQLite error code.</p>
+
+<p>A changeset consists of zero or more INSERT, UPDATE and/or DELETE changes,
+each representing a change to a single row of an attached table. An INSERT
+change contains the values of each field of a new database row. A DELETE
+contains the original values of each field of a deleted database row. An
+UPDATE change contains the original values of each field of an updated
+database row along with the updated values for each updated non-primary-key
+column. It is not possible for an UPDATE change to represent a change that
+modifies the values of primary key columns. If such a change is made, it
+is represented in a changeset as a DELETE followed by an INSERT.</p>
+
+<p>Changes are not recorded for rows that have NULL values stored in one or 
+more of their PRIMARY KEY columns. If such a row is inserted or deleted,
+no corresponding change is present in the changesets returned by this
+function. If an existing row with one or more NULL values stored in
+PRIMARY KEY columns is updated so that all PRIMARY KEY columns are non-NULL,
+only an INSERT is appears in the changeset. Similarly, if an existing row
+with non-NULL PRIMARY KEY values is updated so that one or more of its
+PRIMARY KEY columns are set to NULL, the resulting changeset contains a
+DELETE change only.</p>
+
+<p>The contents of a changeset may be traversed using an iterator created
+using the <a href="../session/sqlite3changeset_start.html">sqlite3changeset_start()</a> API. A changeset may be applied to
+a database with a compatible schema using the <a href="../session/sqlite3changeset_apply.html">sqlite3changeset_apply()</a>
+API.</p>
+
+<p>Within a changeset generated by this function, all changes related to a
+single table are grouped together. In other words, when iterating through
+a changeset or when applying a changeset to a database, all changes related
+to a single table are processed before moving on to the next table. Tables
+are sorted in the same order in which they were attached (or auto-attached)
+to the sqlite3_session object. The order in which the changes related to
+a single table are stored is undefined.</p>
+
+<p>Following a successful call to this function, it is the responsibility of
+the caller to eventually free the buffer that *ppChangeset points to using
+<a href="../c3ref/free.html">sqlite3_free()</a>.</p>
+
+<p><h3>Changeset Generation</h3></p>
+
+<p>Once a table has been attached to a session object, the session object
+records the primary key values of all new rows inserted into the table.
+It also records the original primary key and other column values of any
+deleted or updated rows. For each unique primary key value, data is only
+recorded once - the first time a row with said primary key is inserted,
+updated or deleted in the lifetime of the session.</p>
+
+<p>There is one exception to the previous paragraph: when a row is inserted,
+updated or deleted, if one or more of its primary key columns contain a
+NULL value, no record of the change is made.</p>
+
+<p>The session object therefore accumulates two types of records - those
+that consist of primary key values only (created when the user inserts
+a new record) and those that consist of the primary key values and the
+original values of other table columns (created when the users deletes
+or updates a record).</p>
+
+<p>When this function is called, the requested changeset is created using
+both the accumulated records and the current contents of the database
+file. Specifically:</p>
+
+<p><ul>
+  <li> For each record generated by an insert, the database is queried
+       for a row with a matching primary key. If one is found, an INSERT
+       change is added to the changeset. If no such row is found, no change 
+       is added to the changeset.</p>
+
+<p>  <li> For each record generated by an update or delete, the database is 
+       queried for a row with a matching primary key. If such a row is
+       found and one or more of the non-primary key fields have been
+       modified from their original values, an UPDATE change is added to 
+       the changeset. Or, if no such row is found in the table, a DELETE 
+       change is added to the changeset. If there is a row with a matching
+       primary key in the database, but all fields contain their original
+       values, no change is added to the changeset.
+</ul></p>
+
+<p>This means, amongst other things, that if a row is inserted and then later
+deleted while a session object is active, neither the insert nor the delete
+will be present in the changeset. Or if a row is deleted and then later a 
+row with the same primary key values inserted while a session object is
+active, the resulting changeset will contain an UPDATE change instead of
+a DELETE and an INSERT.</p>
+
+<p>When a session object is disabled (see the <a href="../session/sqlite3session_enable.html">sqlite3session_enable()</a> API),
+it does not accumulate records when rows are inserted, updated or deleted.
+This may appear to have some counter-intuitive effects if a single row
+is written to more than once during a session. For example, if a row
+is inserted while a session object is enabled, then later deleted while 
+the same session object is disabled, no INSERT record will appear in the
+changeset, even though the delete took place while the session was disabled.
+Or, if one field of a row is updated while a session is disabled, and 
+another field of the same row is updated while the session is enabled, the
+resulting changeset will contain an UPDATE change that updates both fields.
+</p><p>See also lists of
+  <a href="../session/objlist.html">Objects</a>,
+  <a href="../session/constlist.html">Constants</a>, and
+  <a href="../session/funclist.html">Functions</a>.</p>
+