WHERE clause yana taɓa layuka, yana dawo da kawai waɗanda suka dace da sharati. Yana da mahimmanci ga samun takamaiman bayanai maimakon teburin gaba daya, kuma yana goyi bayan ɗimbin abokan aiki da sharatai.
users age ;
users country ;
users age active ;
users country country ;
users active;
Sharatai suna amfani da abokan aiki na kwatantawa kuma suna haɗa tare da AND/OR/NOT (yi amfani da parentheses domin sarrafa jinin cikin sharatai masu sarari).
-- 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 (jeri), IN (jerin ƙimaomi), da LIKE (daidaita pattern tare da %/_ wildcards) suna magana game da bukatu na taɓawa ta hankali.
-- ❌ 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
Mahimmanci: NULL yana wakilta "ba a sani ba," kuma ba za ku iya kwatantawa da shi tare da = (email = NULL ba ta tabbas ba). Dole ne ku yi amfani da IS NULL/IS NOT NULL — kuskure na gida.
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.
WHERE clause yana da mahimmanci ga bugunawa — haka ake samun takamaiman bayanai maimakon teburin gaba daya, wanda zai buƙaci kusan kowane buguwa na gaskiya, saboda haka fahimta shi yana da mahimmanci lokcin yau da kullun.
Sanin abokan aiki (kwatantawa, AND/OR/NOT tare da daidaitaccen jinin) da abokan aiki na musamman masu amfani (BETWEEN don jeri, IN don jerin ƙimaomi, LIKE don daidaita pattern) yana rufe kaɗai duk rin bukatu na taɓawa.
Wata alama ta mahimmanci kuma ana kuskure ta yau da kullun ita ce magani na NULL: saboda NULL yana nufin "ba a sani ba," ba za ku iya kwatantawa da shi tare da = (kuskure na yau a yau inda WHERE col = NULL ba ta dawo da komai), kuma dole ne ku yi amfani da IS NULL/IS NOT NULL - fahimta wannan yana hana kuskurewa mai ruɗi.
Hakanan yana da daraja su sani da haɗin karifi: taɓawa akan ginshikin lamba da index yana saurain yau a yau bayyana shimfidar ginshiki take buƙaci na ɓugge kowane layuka (saboda haka ginshikin da ake taɓa sau da yawa yana amfani da indexes), kuma amfani da aiki akan ginshiki a WHERE na iya hana amfani da index.
Tun da taɓawa bayanai tare da WHERE ita ce ɗaya daga cikin ayyuka da aka yi a gida na teburin bayanai, kuma tun da fahimta abokan aiki, magani na NULL (wata alama ta kuskure), da abubuwan haɗi da karifi/karifi yana da asali don rubutu daidai, mai inganci na buguwa, gudu da WHERE clause yana da mahimmanci, dole ne a sani ilmin aiki tare da SQL.