<?php
$installer = $this;
$installer->startSetup();

$installer->run("
    ALTER TABLE {$this->getTable('slideshow/slideshow')}
        ADD `from_date` datetime NULL default NULL,
        ADD `to_date` datetime NULL default NULL;
    ");
$installer->endSetup();
<?php

$this->startSetup();
$this->getConnection()->addColumn(
    $this->getTable('slideshow/slideshow'),
    'from_date',
    'datetime NULL default NULL'
);

$this->getConnection()->addColumn(
    $this->getTable('slideshow/slideshow'),
    'to_date',
    'datetime NULL default NULL'
);

$this->endSetup();

Change sql query to check if table already exist
Change sql query to check if table already exist

Change sql query to check if table already exist