SQL Server Indexes

Clustered index
- reorders the way records in the table are physically stored
- a table can have only one clustered index
- the leaf nodes of a contain the data pages.

Nonclustered index
- the logical order of the index does not match the physical stored order of the rows on disk
- the leaf nodes of a nonclustered index do not consist of the data pages. Instead, they contain index rows.

Sources: FAQ: clustered vs nonclustered indexes

1-2-3 Normal forms

1^st Normal Form – Eliminate Repeating Groups (ERG)
all occurrences of a record type must contain the same number of fields. No sets

2^nd Normal Form – Eliminate Redundant Data (ERD)
violated when a non-key field is a fact about a subset of a key.

3^rd Normal Form – Eliminate Columns Not Dependent on Key (ENDK)
violated when a non-key field is a fact about another non-key field.

All normal forms are additive.

Source