Table Key (Cont.)
Every Table T Has at Least One Key
Given a table T with Head(T)=A1…An, consider the attribute set S1 with all these attributes:
NULL Values
A null value is no data entry at all.
It is not the same thing as a zero or a blank.
The null value should be interpreted as unknown or as not yet defined.
SQL> CREATE TABLE product (
2> pid CHAR(5) PRIMARY KEY,
3> pname VARCHAR(32) NOT NULL,
4> city VARCHAR(32),
5> quantity INTEGER CHECK( quantity >= 0 ),
6> price REAL NOT NULL CHECK( price >= 0.0 ) );
...
SQL> INSERT INTO product VALUES( 'p07', 'stapler', null, null, 3.50 );
...
SQL> SELECT * FROM product WHERE city IS NULL;
|
⇓
| pid |
pname |
city |
quantity |
price |
| p07 |
stapler |
null |
null |
3.50 |
For example, when query the average quantity on products, the quantity for the stapler product is left out of the average.
Take your time (don’t hurry) on the exam.
You don’t get a bonus for finishing quickly.
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