Native Relational Operations
| Name |
Symbol |
Keyboard Form |
Example |
| PROJECT |
R[ ] |
R[ ] |
R[ Ai1 ... Aik ] |
| SELECT |
R where C |
R where C |
R where A1 = 5 |
| JOIN |
∞ |
JOIN |
R∞S, or R JOIN S |
| DIVISION |
÷ |
DIVIDEBY |
R÷S, or R DIVIDEBY S |
Projection
Assume
Head(R)=A1…An.
The projection of
R on attributes
Ai1,…,Aik, where
{Ai1,…,Aik}⊆{A1,…,An} is a table
T whose heading is
Head(T)=Ai1…Aik, with the following content.
For every row
r in the table
R there will be single row
t in the table
T such that
r[Aij]=t[Aij]
for every
Aij contained in
{Ai1,…,Aik}.
- The project of
R on {Ai1,…,Aik} is denoted by R[Ai1,…,Aik].
- Duplicate rows will be deleted until only one copy in the result of the projection.
| customer |
| cid |
cname |
city |
discnt |
| c001 |
TipTop |
Duluth |
10.00 |
| c002 |
Basics |
Dallas |
12.00 |
| c003 |
Applied |
Dallas |
8.00 |
| c004 |
ACME |
Duluth |
8.00 |
| c006 |
ACME |
Kyoto |
0.00 |
|
⇒
CN := CUSTOMER[cname]
|
| CN |
| cname |
| TipTop |
| Basics |
| Applied |
| ACME |
|
|
|
My girlfriend’s dog died, so I tried to
cheer her up by getting her an identical one.
It just made her more upset.
She screamed at me,
“What am I supposed to do with two dead dogs?”
|