Kipengele cha WHERE huchuja safu, kikirudisha tu zile zinazokidhi sharti. Ni cha msingi kwa kupata data maalum badala ya jedwali zima, na huunga mkono seti tajiri ya operators na masharti.
users age ;
users country ;
users age active ;
users country country ;
users active;
Masharti hutumia operators za ulinganisho na huunganishwa na AND/OR/NOT (tumia mabano kudhibiti precedence katika masharti changamano).
-- BETWEEN — a range (inclusive)
WHERE age BETWEEN 18 AND 65
-- IN — match any value in a list
WHERE country IN ('US', 'CA', 'UK')
-- LIKE — pattern matching (% = any chars, _ = one char)
WHERE name LIKE 'A%' -- starts with A
WHERE email LIKE '%@gmail.com' -- ends with @gmail.com
WHERE code LIKE '_X_' -- exactly 3 chars with X in the middle
BETWEEN (mipaka), IN (orodha za thamani), na LIKE (ulinganishaji wa muundo kwa wildcards za %/_) hushughulikia mahitaji ya kawaida ya uchujaji kwa ufupi.
-- ❌ WRONG — you CANNOT compare to NULL with = (NULL is "unknown")
WHERE email = NULL -- returns NOTHING (never true)
-- ✅ use IS NULL / IS NOT NULL
WHERE email IS NULL
WHERE email IS NOT NULL
Muhimu: NULL huwakilisha "isiyojulikana," na huwezi kuilinganisha kwa = (email = NULL haifai kuwa kweli kamwe). Lazima utumie IS NULL/IS NOT NULL — hitilafu ya kawaida.
WHERE conditions on INDEXED columns are fast; on un-indexed columns the database
must scan every row. For frequently-filtered columns, add an index.
Also: functions on a column (WHERE UPPER(name) = 'X') can prevent index use.
Kipengele cha WHERE ni cha msingi kwa kuuliza — ndiyo jinsi unavyopata data maalum badala ya jedwali zima, jambo linalohitajika kwa karibu kila swali halisi, hivyo kukielewa ni maarifa muhimu ya kila siku.
Kujua operators (ulinganisho, AND/OR/NOT zenye precedence sahihi) na operators maalum zenye manufaa (BETWEEN kwa mipaka, IN kwa orodha za thamani, LIKE kwa ulinganishaji wa muundo) hufunika wigo wa mahitaji ya uchujaji.
Hoja muhimu hasa na inayokoseshwa mara kwa mara ni ushughulikiaji wa NULL: kwa sababu NULL humaanisha "isiyojulikana," huwezi kuilinganisha kwa = (hitilafu ya kawaida ambapo WHERE col = NULL hurudisha kitu kimyakimya), na lazima utumie IS NULL/IS NOT NULL badala yake — kuelewa hili huzuia hitilafu ya kawaida na inayochanganya.
Ni cha thamani pia kujua uhusiano wa utendaji: kuchuja kwenye safuwima zenye index ni haraka wakati safuwima zisizo na index zinahitaji kuchanganua kila safu (hivyo safuwima zinazochujwa mara kwa mara hunufaika na index), na kwamba kutumia functions kwenye safuwima katika WHERE kunaweza kuzuia matumizi ya index.
Kwa kuwa kuchuja data kwa WHERE ni mojawapo ya operesheni za kawaida zaidi za hifadhidata, na kwa kuwa kuelewa operators, ushughulikiaji wa NULL (mtego wa kawaida), na athari za index/utendaji ni msingi wa kuandika maswali sahihi na yenye ufanisi, kufahamu vyema kipengele cha WHERE ni maarifa ya msingi, ya lazima kujua kwa kufanya kazi na SQL.