What is a Storage Definition Language (SDL)?
You may want to ask your teacher to define "SDL".  I don't think the term appears in the SQL standard.  I found this reference (see page 3), and I disagree: SQL does not define the physical structure, the physical bytes per field, the physical field order, the physical sorting, or "mixed records", whatever that might mean.  SQL doesn't define fields at all, or records, or say anything about how the data are stored on the disk if they're stored on the disk.  

That said, every vendor does provide some means for the administrator to define where the data will be kept, and many offer varying levels of control over physical storage aspects (e.g. ISAM, clustered indexes).  AFAIK all that stuff lies outside the SQL standard.  I've used DBMSs that understand SQL DML, but completely separate mechanisms for defining tables and storage.