Sql haqida tushuncha

SELECT NAME, QUOTA, SALES

FROM SALESREPS

WHERE SALES < QUOTA OR SALES < 300.0

Sotuvlari xaqiqiy xajmi rejadagidan va $300.00 dan kam bo‘lgan xizmatchilarni aniqlash

 SELECT NAME, QUOTA, SALES

FROM SALESREPS

WHERE SALES < QUOTA AND SALES < 300.0

Sotuvlari xaqiqiy xajmi rejadagidan kam, lekin $150.00 dan ko‘p bo‘lgan xizmatchilarni aniqlash

SELECT NAME, QUOTA, SALES

FROM SALESREPS

WHERE (SALES < QUOTA) AND (NOT SALES > 150.000)

AND ifodasi algebrasi.

   Qiymat                       Natija

--------------  ----------       

TRUE AND TRUE   -> TRUE

FALSE AND TRUE  -> FALSE

TRUE AND FALSE  -> FALSE

FALSE AND FALSE -> FALSE

NULL AND TRUE   -> NULL

TRUE AND NULL   -> NULL

FALSE AND NULL  -> FALSE

NULL AND FALSE  -> FALSE

NULL AND NULL   -> NULL

OR ifodasi algebrasi.

   Qiymat                    Natija

--------------  ----------       

TRUE OR TRUE    -> TRUE

FALSE OR TRUE   -> TRUE

TRUE OR FALSE   -> TRUE

FALSE OR FALSE  -> FALSE

NULL OR TRUE    -> TRUE

TRUE OR NULL    -> TRUE

FALSE OR NULL   -> NULL

NULL OR FALSE   -> NULL

NULL OR NULL    -> NULL

 NOT ifodasi algebrasi.

   Qiymat               Natija

--------------  ----------       

NOT TRUE        -> FALSE

NOT FALSE       -> TRUE

NOT NULL        -> NULL

Qo‘shma izlash operatorlarining xar biri o‘z ustivorligiga ega. Eng yuqori ustivorlik NOT ga tegishli, undan so‘ng AND va oxirida OR!

SQL92 standartida IS operatori yordamida ifoda rost, yolg‘on yoki aniqlanmaganligini tekshirish mumkin. Uning sintaksisi quyidagicha:

IS operatori sintaksisi

--------- Solishtirish ---------- IS (------ TRUE ---------------)

                                  (------ FALSE --------------)

--- Mantiqiy ifoda ---      (------ UNKNOWN ------------)

Masalan quyidagicha yozish  mumkin: ((SALES - QUOTA) > 100.000) IS UNKNOWN . Bunday shart SALES yoki QUOTA ustunlari NULL  qiymatga ega satrlarni izlashga imkon beradi.