As documented in http://drupal.org/node/146939, type 'text' columns cannot have default values. This restriction is imposed by MySQL. However, as described at http://drupal.org/node/159330, MySQL treats text columns as if they have an implicit default value of the empty string. These two behaviors encourage non-portable programming practices but it is easy to avoid those practices and do things correctly.
Suppose you have the following table definition:
$schema['mytable'] = array(
'fields' => array(
'id' => array('type' => 'int'),
'info' => array('type' => 'text', 'not null' => TRUE)),
);
The 'info' column is type text. It is not null and, as is required, has no default value. To write rows to this table, you always need to provide a value for the 'info' column even if it is the empty string. Here are correct examples:
$row->id = $whatever;
$row->info = '';
// using an explicit INSERT statement:
db_query("INSERT INTO {mytable} (id, info) VALUES (%d, '%s')", $row->id, $row->info);
// using drupal_write_record():
drupal_write_record('mytable', $row);
You cannot perform these inserts without a value for 'info'. Here are incorrect examples:
<?php
$row->id = $whatever;
// $rows->info is not defined!
// INCORRECT explicit INSERT statement:
db_query("INSERT INTO {mytable} (id) VALUES (%d)", $row->id);
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