Mavzu: Ma’lumotlarni qayta ishlashda mantiqiy ifodalardan foydalanish Reja

--- IMYA STOLBSA --------- LIKE (shablon) -----------------------------------
NOT ESCAPE (o‘tkazish nomi)
Sodda so‘rov bajaramiz: "Apelьsin" kompaniyasi uchun kredit limitini ko‘rsatish:
SELECT COMPANY, CREDIT_LIMIT
FROM CUSTOMERS
WHERE COMPANY = 'Apelьsin'
Quyidagicha '%' shablonli LIKE operatorini qo‘llaymiz:
SELECT COMPANY, CREDIT_LIMIT
FROM CUSTOMERS
WHERE COMPANY LIKE '%n'
Bu xolda LIKE '%n' operatori 'n' xarfiga tugaydigan xamma yozuvlarni ko‘rsatadi, agar '%' shabloni birinchi kelsa:
SELECT COMPANY, CREDIT_LIMIT
FROM CUSTOMERS
WHERE COMPANY LIKE '%gan'
Ba’zida '%' shabloni o‘rniga '*' belgisi qo‘llanadi znak, masalan v MS SQL uchun, c:\>dir *.exe! 
Agar faqat bitta simvol ixtiyoriy bo‘lsa '_'! shabloni qo‘llanadi. Masalan:
SELECT COMPANY, CREDIT_LIMIT
FROM CUSTOMERS
WHERE COMPANY LIKE 'Ap_lьsin'
Operator IS NULL
SELECT operatori uchun NULL qiymati bilan ishlash qoidalarini ko‘ramiz. Konkret misol ko‘ramiz:
Xali ofisga biriktirilmagan xizmatchini topish:
SELECT NAME FROM SALESREPS
WHERE REP_OFFICE = NULL
SQL quyidagi satrni uchratganda:
REP_OFFICE = NULL
Quyidagi shartni tekshiradi:
NULL = NULL
Bunday tekshirish yana NULL qaytaradi!!! Qiymat tekshiruvchi operator uchun agar natija TRUE bo‘lmasa, satr natijaviy to‘plamga kirmaydi!!! Lekin bunday satrlar aslida mavjuddir!!! Bu holda NULL qiymatiga tekshirish to‘g‘ri operatorini qo‘llash lozim:
------------ ustunning nomi IS ---------- NULL ------------------------------
NOT
Qo‘llaymiz:Ofisga biriktirilmagan xizmatchini toping.
SELECT NAME FROM SALESREPS
WHERE REP_OFFICE IS NULL
NOT shartini qo‘llash mumkin: Ofisga biriktirilgan xamma xizmatchilarni toping.
SELECT NAME FROM SALESREPS