Databases and the United States Legal Code
In the United States, facts by themselves are not protected by copyright. Therefore, data, as a collection of facts, is not protected by U.S. copyright law.
Databases as a whole can be protected by copyright as a compilation, but only under certain conditions. The first is that mere collection of data is not enough. The arrangement and selection of data must be sufficiently creative or original.
As the Supreme Court put it in Feist Publications v. Rural Telephone:
"Factual compilations... may possess the requisite originality. The compilation author typically chooses which facts to include, in what order to place them, and how to arrange the collected data so that they may be used effectively by readers. These choices as to selection and arrangement, so long as they are made independently by the compiler and entail a minimal degree of creativity, are sufficiently original that Congress may protect such compilations through the copyright laws." Feist Publications, Inc. v. Rural Telephone Service Co., 499 U. S. 340 (1991)
The key point is that while a sufficiently creative and/or original arrangement or selection of data can be protected as a compilation by copyright, the factual content will not be.
European Database Rights and Copyright
Unlike the United States, the European Union does provide legal protection for data under the Database Directive. This act defines what a database is and gives the measures for which databases are treated under copyright ("by reason of the selection or arrangement of their contents, constitute the author's own intellectual creation") and under sui generis database rights when not under copyright. This is the case when the contents of a database are selected by an objective criteria, instead of through originality or creativity. In Europe, database rights protect the time and effort invested in obtaining, verifying and entering the contents into the database.
For the full legal text of European Union Database rights, see the European Parliament Database Directive.
Because legal protection for data varies by national jurisdiction, choosing and applying a license for your data is the first and most crucial step to making your open. Leaving your dataset unlicensed creates uncertainty about the use and re-use of your data. Licensing your data explicitly states that your data is open for re-use.
Applying the License to Your Data
In the United States, researchers can use the Creative Commons Zero (CC0) license to openly share their datasets worldwide. To apply a CC0 license to your work, fill out the CC0 Waiver on the Creative Commons website. Once you complete a waiver, the site will generate a customized HTML code for your work. When you insert the code into any webpage, it will display the relevant CC0 logo and link back to a summary of the selected license and the full legal text. This will show your users that you have waived any potential rights to your database and they may use and re-use the data as they see fit.
For more information on CC0 and data sharing, see CC0 use for data.