Using the attribute table

The first task in every work is to get used to the acquired data. We should investigate what kind of data it holds and what can we work with. We should formulate the most fundamental questions for successful work. Is there enough information for my analysis? Is it of the right type and format? Are there any No-Data values I should handle? If I need additional information, can I calculate them from the existing attributes? Some of these questions can be answered by looking at the attribute table, while some of them (especially when working with large vector data) can be answered by asking QGIS. To ask QGIS about vector layers, we have to use a specific language called SQL or Structured Query Language.