NULL ina amfani ta yaya a cikin SQL?

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**`NULL`** yana wakiltar **babu ko ba a sanin ba – ba sifili, ba buɗe string, amma **rashin** ƙima. NULL yana da halin musamman, wasu lokuta da ba taba jira ba a cikin kwatantawa, tarin kuma mantiki – wanda zai sa ya zama tushen kuskure mai yawa. ## NULL shine "ba a sanin" — kwatantawa tare da NULL ```sql -- ❌ you CANNOT compare to NULL with = or <> (the result is "unknown," never true) WHERE email = NULL -- returns NOTHING (never true!) WHERE email <> NULL -- also returns nothing -- ✅ use IS NULL / IS NOT NULL WHERE email IS NULL WHERE email IS NOT NULL ``` Domin NULL yana nufin "ba a sanin ba," kowane kwatantawa *tare da* NULL yana haifar da "ba a sanin ba" (ba gaskiya ba), don haka `= NULL` ba kuwa zai dace ba. Dole ku yi amfani da `IS NULL`/`IS NOT NULL` — mafi gida kuskure na NULL. ## NULL a cikin mantiki mai aba uku ```text SQL uses THREE-valued logic: TRUE, FALSE, and UNKNOWN (NULL). NULL = NULL → UNKNOWN (not true!) NULL AND true → UNKNOWN NULL OR true → TRUE NOT NULL → UNKNOWN → Conditions evaluating to UNKNOWN are treated as not-matching by WHERE. ``` ## NULL a cikin tarin da ilmin ilzamari ```sql -- aggregates IGNORE NULLs (except COUNT(*)) SELECT AVG(salary) FROM employees; -- averages only non-NULL salaries SELECT COUNT(salary) FROM employees; -- counts only non-NULL (vs COUNT(*) = all rows) -- arithmetic with NULL → NULL SELECT price + NULL; -- NULL (any operation with NULL is NULL) ``` Tarin suna tsallake NULLs, kuma kowane ilmin ilzamari da ya shiga NULL yana haifar da NULL. ## Hadewa da NULL: COALESCE da NULLIF ```sql -- COALESCE — return the first non-NULL value (provide a default) SELECT COALESCE(phone, 'No phone') FROM users; -- 'No phone' if phone is NULL SELECT COALESCE(nickname, name, 'Unknown') FROM users; -- first non-NULL -- NULLIF — return NULL if two values are equal SELECT NULLIF(value, 0); -- NULL if value is 0 (e.g. avoid divide-by-zero) ``` `COALESCE` yana ba da wa'u don NULL (yazo a yau da kullum don taunawa), kuma `NULLIF` yana canza ƙima zuwa NULL cikin sharato. ## Dalilin da zai sa ya zama mahimmanci Ganin halin NULL yana da mahimmanci don rubuta SQL daidai, domin kasancewar NULL na musamman - halin ƙa'ida yana zama **tushen kuskure mai yawa" — kuma NULLs suna bayyana a koyaushe a cikin ainihin bayanai (filayen da ake iya ayewa, babu ƙimomi, haɗe-haɗen da ba ta yi)