SQL is short for Structured Query Language. From its words you can tell, it is a tool to query something from somewhere. It is a declarative language but not procedural language. It means that you just use it to tell what you want to get from somewhere rather than tell how to your results are acheived.
SELECT * FROM ABC_DATABASEThe above is to get all columns from a database called ABC_DATABASE. It simply states that you want all columns in a certain database, but not how it is extracted from the database.
Some basics about relational database. In simply terms, the data saved in the database have relations between tables, but these data also have its own attributes as the format of columns. The relationship between different tables are defined through primary and foreign keys. Tables are arranged in schemas, which is a namespace for database objects.
Generally, the full qualified name for an object saved in the databased is as [server_name.][databse_name.][schema_name.][object_name]. However, if the work is done in the same database, it can just use [schema_name.][object_name] to refer to the object.
If you are interested in seeing some SQL query codes, you may visit my Github repository here: https://github.com/eddyzhang2018/SQL
Eddy Zhang 