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    <p>Using two separate tables looks like a sensible approach here - the two tables seem to have very little in common. Combining unrelated objects into a single table and using NULLs for the columns that don't apply is typically a bad design.</p> <p>More generally though, if you have two types objects that are different in some respects but also share some common features then another alternative you could look into is a technique that is sometimes called "table inheritance":</p> <ul> <li><a href="https://stackoverflow.com/questions/386652/techniques-for-database-inheritance">Techniques for database inheritance?</a></li> <li><a href="http://www.sqlteam.com/article/implementing-table-inheritance-in-sql-server" rel="nofollow noreferrer">Implementing Table Inheritance in SQL Server</a></li> </ul> <p>In your first approach you use one table that contains all the data. In your second approach you use two tables that are completely separate. The inheritance approach uses three tables - one for the common columns, and an extra table for each specialized type. You then use a foreign key relationship to relate the record in the common table to a record in one of the specific tables so that you can join to get the complete object. There are a few different ways to create this relationship that are described in the article I linked to above.</p> <p>But don't overuse table inheritence. Two tables may happen to share a couple of columns for example (ID, insertion_date) but be otherwise two conceptually different things. Then it probably would not be appropriate to use this technique to factor out the common columns. I don't know enough about your specific situation to say whether it makes sense to use this technique on 'Wall posts' and 'Share a Link' posts, but you might want to consider it.</p> <p>To give an example of when it could be useful to use this technique rather than having two separate tables: If you want to query for a user's ten most recent posts (regardless of whether they are wall posts or shared links) then it is useful to have a single table where you can get the IDs for these posts with a single simple query. If you have two completely separate tables with no relationship between them the query is more complex - you must first get the top 10 from each table, union the results, then get the top 10 from the union.</p>
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