Slide 8.5: Database terminologies (cont.)

Database Terminologies (Cont.)

Tables and Relations

The domain of an attribute A is denoted by Domain(A):

  CP = Domain(cid)×Domain(cname)×Domain(city)×Domain(discnt)
     =     CID    ×    CNAME    ×    CITY    ×   DISCNT

CP consists of the set of all possible 4-tuples, t=(c,n,t,d); e.g.,

  t  = ( c003, Allied, Dallas, 8.00 ), and
  t' = ( c001, TipTop, Duluth, 10.00 ).

customer
cid	cname	city	discnt
c001	TipTop	Duluth	10.00
c002	Basics	Dallas	12.00
c003	Allied	Dallas	8.00
c004	ACME	Duluth	8.00
c005	ACME	Kyoto	0.00
...	...	...	...

A relation is a subset of a Cartesian product. That is, if we define a table T that has a heading given by
```
   Head(T) = A₁ ... A_n
```
then T is a subset of the Cartesian product of the sets: Domain(A₁), ..., Domain(A_n). For example, the table customer is a subset of CP. Executing a SELECT command is basically done in three steps:
1. Derive a Cartesian product of the operand tables indicated in the FROM clause.
2. Based on this intermediate result, evaluate the conditions stated in the WHERE clause.
3. Project the result from Step 2 onto the attributes indicated in the SELECT clause; DISTINCT may be added to avoid duplicate tuples in the result.