Without knowing all of the information I would suggest there are a few ways you can approach this.
This can be enforced by MySQL by adding a
UNIQUE key constraint.
ALTER TABLE user ADD UNIQUE (email)
or expressed as Varien DDL:
$installer->getIdxName('youmodule/modelalias', array('email'), Varien_Db_Adapter_Interface::INDEX_TYPE_UNIQUE),
array('email'), array('type' => Varien_Db_Adapter_Interface::INDEX_TYPE_UNIQUE)
Alpha entry only
- To enforce the schema you could create a Backend Model (read more here: https://magento.stackexchange.com/a/6423/336) to validate the input
- You could also use a backend model to strip non-alpha characters out instead of validating
- You could use a MySQL
TRIGGER to enforce the schema as well on INSERT or UPDATE
However, to my knowledge, there is no built-in column type in MySQL that will enforce this schema for you.