Thanks for the question; I currently teach SQL. The general answer is that it is not a vendor specific language. It is well standardized (the latest ISO release is SQL:2011). Having said that, different vendors of database software have implemented small variations or extensions. Therefore SQL code written using one database management system (SQL Server, MySQL, PostgreSQL, and so on) is not always 100% guaranteed to work in another software. database You will sometimes need to make some edits.
SQL implementations are incompatible between vendors and do not completely follow standards. Only PostgreSQL is credited for standards compliance. In fact plenty of adjustments are required to make SQL code portable among different database systems. Additionally there are many of procedural extensions such as PL/SQL in Oracle, SQL/PSM in MySQL, SQL PL in IBM DB2. These extensions make the problem even more complicated.