, - . SQL ORDER BY, . . , GROUP BY, (ASC) (DESC) . . (ORDERS), , :
SELECT * FROM Orders ORDER BY cnum DESC;
cnum , , amt:
SELECT * FROM Orders ORDER BY cnum DESC, amt DESC;
ORDER BY . , , . ANSI . :
SELECT cname,city FROM Customers ORDER BY cnum;
cnum , ORDER BY . , , , , . , ORDER BY, . ORDER BY GROUP BY . ORDER BY . ORDER BY. , ; :
SELECT snum, odate, MAX(amt) FROM Orders GROUP BY snum, odate ORDER BY snum;
, . , , . , , , . .. , SELECT, ORDER BY 1, . , , Salespeople, commission (comm):
SELECT sname, comm FROM Salespeople ORDER BY 2 DESC;
ORDER BY , ; ORDER BY . , , SELECT, ORDER BY, . , (orders) (salespeople) :
|
|
SELECT snum, COUNT (DISTINCT onum) FROM Orders GROUP BY snum ORDER BY 2 DESC;
, , . ANSI SQL, , : SELECT snum, COUNT (DISTINCT onum) FROM Orders GROUP BY snum ORDER BY COUNT (DISTINCT onum) DESC;
. ORDER BY NULL- , , NULL-, . ANSI, , .
FOREIGN KEY.
- . , FOREIGN KEY, , . REFERENCES. , , , . . .
, , , ; .
. UNIQUE , .
NULL, FOREIGN KEY . , . , .
, . , ( ), ON DELETE NO ACTION ON UPDATE NO ACTION, . ON DELETE CASCADE ON UPDATE CASCADE.