keys
The values of attributes of an entity must be such that they can uniquely identify
the entity.
• No two entities in an entity set are allowed to have exactly the same value for
all attributes; a key for an entity is a set of attributes that suffice to distinguish
entities from each other.
• Superkey, set of columns that determine ALL columns; candidate key, min set
of columns; PK, a single chosen candidate key.
Choice of primary keys depends on cardinality of binary r/s
• M-M, union of PK; O-M and M-O, PK of M side; O-O, PK of either side
• Weak entity set: File—saved in – folder: file is the weak entity set
Specialization/Generalization
• If the generalization is disjoint and complete, may remove the person relation (ie. employee
entity below)
• Attribute inheritance – a lower-level entity set inherits all the attributes and relationship
participation of the higher level entity set to which it is linked
• Top-down: Form a schema for each lower-level entity set, include primary key of higher-level
entity set and local attributes; Drawback: getting information about, an employee requires
accessing two relations, the one corresponding to the low-level schema and the one
corresponding to the high-level schema
ER Design Decisions
• The use of an attribute or entity set to represent an object.
• Whether a real-world concept is best expressed by an entity set or a relationship set.
• The use of a ternary relationship versus a pair of binary relationships.
• The use of a strong or weak entity set.
• The use of specialization/generalization – contributes to modularity in the design.
• The use of aggregation – can treat the aggregate entity set as a single unit without concern for the
• details of its internal structure.
(c) is better than (d)
as it allows
recording other
information about
the assignment
A decomposition of R consists of
replacing R by two or more relations
such that
• Each new relation scheme contains a
subset of the attributes of R, and
• Every attribute of R appears as an
attribute of at least one of the new
relations
3 problems with decomposition
• Lossiness, dependency checking may
require join, some queries become
more expensive
Always lossless, but may not be dependencies preserving (no)