Îïåðàöèè ñðàâíåíèÿ
Ðàññìîòðèì îïåðàöèè ñðàâíåíèÿ. Ðåëÿöèîííûå îïåðàòîðû ìîãóò èñïîëüçîâàòüñÿ ñ ðàçëè÷íûìè ýëåìåíòàìè. Ïðè ýòîì âàæíî ñîáëþäàòü ñëåäóþùåå ïðàâèëî: ýëåìåíòû äîëæíû èìåòü ñðàâíèìûå òèïû. Åñëè â áàçå äàííûõ îïðåäåëåíû äîìåíû, òî ñðàâíèâàåìûå ýëåìåíòû äîëæíû îòíîñèòüñÿ ê îäíîìó äîìåíó.
×òî æå ìîæåò áûòü ýëåìåíòîì ñðàâíåíèÿ? Ýëåìåíòîì ñðàâíåíèÿ ìîæåò âûñòóïàòü:
· çíà÷åíèå ïîëÿ
· ëèòåðàë
· àðèôìåòè÷åñêîå âûðàæåíèå
· àãðåãèðóþùàÿ ôóíêöèÿ
· äðóãàÿ âñòðîåííàÿ ôóíêöèÿ
· çíà÷åíèå (çíà÷åíèÿ), âîçâðàùàåìûå ïîäçàïðîñîì.
Ïðè ñðàâíåíèè ëèòåðàëîâ êîíå÷íûå ïðîáåëû èãíîðèðóþòñÿ. Òàê, ïðåäëîæåíèå WHERE first_name = ‘Ïåòð ‘ áóäåò èìåòü òîò æå ðåçóëüòàò, ÷òî è ïðåäëîæåíèå WHERE first_name = ‘Ïåòð’.
SELECT first_name, last_name, dept_no
FROM employee
WHERE job_code = "Admin" ïîëó÷èòü ñïèñîê ñîòðóäíèêîâ
(è íîìåðà èõ îòäåëîâ),
çàíèìàþùèõ äîëæíîñòü
àäìèíèñòðàòîðîâ
FIRST_NAME LAST_NAME DEPT_NO
=============== ==================== =======
Terri Lee 000
Ann Bennet 120
Sue Anne O'Brien 670
Kelly Brown 600
SELECT first_name, last_name, dept_no,
job_country
FROM employee
WHERE job_country <> "USA" ïîëó÷èòü ñïèñîê ñîòðóäíèêîâ
(à òàêæå íîìåðà èõ îòäåëîâ
è ñòðàíó),
ðàáîòàþùèõ âíå ÑØÀ
FIRST_NAME LAST_NAME DEPT_NO JOB_COUNTRY
=============== ================ ======= ==============
Ann Bennet 120 England
Roger Reeves 120 England
Willie Stansbury 120 England
Claudia Sutherland 140 Canada
Yuki Ichida 115 Japan
Takashi Yamamoto 115 Japan
Roberto Ferrari 125 Italy
Jacques Glon 123 France
Pierre Osborne 121 Switzerland
