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.
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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.
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.
-- 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.
-- 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.
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)
Ɗakin karatu na tambayoyin hira na IT tare da amsoshi cikakke — daga Junior zuwa Senior.
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