3 SQL databases walked into a NoSQL bar
3 SQL databases walked into a NoSQL bar. A little while later, they walked out because they couldn't find a table.
In order to be funny this joke requires the listener to know what a NoSql Database is, as well as how they differ from Sql Databases.
A NoSql database is a database that stores data in a unstructured format. One of the biggest differences between Sql databases and NoSql databases is that Sql databases have tables. The joke, therefore is a pun, and the humor is derived from the fact that database tables are not the same as physical tables.