If you need to retrieve data from a database, whether it's for a report, the web, or some other application, then SQL is the language to learn.
Updated to include information about ‘case’ statements and how they can be used.
If you are interested in social engineering and want to know more about the principles behind it, as well as the different types of attack, you can find details here.
If password security is something that concerns you, details of the various attacks against them can be found here.
If you are looking to automate system tasks, then the PowerShell scripting language can help. Covered here are some of the language basics, including information on variables, operators, decision making and loops.
Updated to include information on hash tables in PowerShell.
If you are interested in learning more regarding the various different types of attack aimed at networks, you can read about them here.
If you deal with software, web, or mobile applications and want to know more regarding cyber attacks aimed at them, you can read about the various types of attack here.
Updated to include information on interfaces.
Updated to include information on exporting data to a text file from an Oracle database.
Updated to include information on arrays and how they can be used.