, SQL , . NULL. , NOT NULL .
DEFAULT ( ) CREATE TABLE , , , , DEFAULT , , , , .
, -, -. - city, , :
CREATE TABLE Salespeople (
snum integer NOT NULL UNIQUE,
sname char(10) NOT NULL UNIQUE,
city char(10) DEFAULT = 'New York',
comm decimal CHECK (comm < 1));
, - , , , ( ) . , , , , , , .
, ( ) , .
NULL. NULL () , , IS NULL, . , - . , , , (NULL). NULL , SQL , .
, . , , , , . , = NULL:
SELECT *
FROM Customers
WHERE rating <= 100;
rating = 0, . .
, , ?
, NOT NULL.
|
|
, = NULL, .
UNIQUE PRIMARY KEY . , , , . , , , . , UNIQUE PRIMARY KEY ( ) .
, . NOT NULL, NULL, UNIQUE, , PRIMARY KEY, , UNIQUE, , , , CHECK , , , , . , DEFAULT, , INSERT, NULL, DEFAULT NOT NULL.
FOREIGN KEY REFERENCES, 19, , , , , .
SQL
1. , onum, cnum snum , NULL .
2. , 10%, NULL, snum , A M (, ).
3. , , onum , cnum, cnum snum. NULL .
(. A .)