Implementation Idea for login structure (login id with two different tables)
First of all, every thing depends on business logic you have. If employees and customers it's fine to use 2 diferrent tables, to connect them with the table WEB_USER_LOGIN I would just append and field like 'usertype'.

If your database is Oracle you can have object tables to solve this problem, if its PostgreSQL you can hava inherited tables. 

If its mysql I would use just one table with both fields, but if you prefer to use the left join it's ok but I would create a VIEW for it.