How to run alter query to modify mysql database using hook_schema? I want to change data-type of a field in for a particular table in database. I  want to use hook_schema_alter() to do so.
The ALTER query is:

ALTER TABLE `abcd` CHANGE `session_number` `session_number` INT( 255 ) NOT NULL COMMENT 'Session number to get number.'


Is it possible to implement such query using hook ? If yes please guide me.
Tonnes of thanx in advance.
Changes to the schema are typically handled with hook_update_N and you will normally locate this file in the module.install file.

For example with the update from Drupal 6.x-7.x the some of the base permissions were rename:

/**
 * Rename blog and forum permissions to be consistent with other content types.
 */
function system_update_7000() {
  $result = db_query("SELECT rid, perm FROM {permission} ORDER BY rid");
  foreach ($result as $role) {
    $renamed_permission = $role-&gt;perm;
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )create\ blog\ entries(?=,|$)/', 'create blog content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )edit\ own\ blog\ entries(?=,|$)/', 'edit own blog content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )edit\ any\ blog\ entry(?=,|$)/', 'edit any blog content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )delete\ own\ blog\ entries(?=,|$)/', 'delete own blog content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )delete\ any\ blog\ entry(?=,|$)/', 'delete any blog content', $renamed_permission);

    $renamed_permission = preg_replace('/(?&lt;=^|,\ )create\ forum\ topics(?=,|$)/', 'create forum content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )delete\ any\ forum\ topic(?=,|$)/', 'delete any forum content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )delete\ own\ forum\ topics(?=,|$)/', 'delete own forum content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )edit\ any\ forum\ topic(?=,|$)/', 'edit any forum content', $renamed_permission);
    $renamed_permission = preg_replace('/(?&lt;=^|,\ )edit\ own\ forum\ topics(?=,|$)/', 'edit own forum content', $renamed_permission);

    if ($renamed_permission != $role-&gt;perm) {
      db_update('permission')
        -&gt;fields(array('perm' =&gt; $renamed_permission))
        -&gt;condition('rid', $role-&gt;rid)
        -&gt;execute();
    }
  }
}