> A relation is an unordered set of tuples. SQL opts for an ordered list of tuples (a table) instead.
SQL the language does not require or describe ordering in tables — that happens as a side-effect of the physical implementation. No one should rely on ordering of table rows.
> A relation has no concept of NULL. A table does.
Null values occur in relations as soon as you write outer joins. All modern RDBMSs also allow defining nullable columns in base tables, and SQL allows/supports NULLs in that context, but with a little work one can eliminate nullable columns in base tables (we’ve already left pure relational land talking about base tables). But you cannot eliminate nulls from query results if you use outer joins.
SQL the language does not require or describe ordering in tables — that happens as a side-effect of the physical implementation. No one should rely on ordering of table rows.
> A relation has no concept of NULL. A table does.
Null values occur in relations as soon as you write outer joins. All modern RDBMSs also allow defining nullable columns in base tables, and SQL allows/supports NULLs in that context, but with a little work one can eliminate nullable columns in base tables (we’ve already left pure relational land talking about base tables). But you cannot eliminate nulls from query results if you use outer joins.