A primary key yayi wajen gano kowane jeri a cikin tebur; a foreign key yana nuna primary key na wani tebur, yana kafa dangantaka tsakanin teburai da kuma yana tilasta referential integrity (ba za ka iya nuna jeri da babu jiya ba).
users (
id ,
name (),
email ()
);
A primary key shine na musammu da babu ciutaka — yana gano kowane jeri. Ana takaita shi ta atomatik (hangen jiya a sauri) kuma shine abin da foreign keys suka nuna.
CREATE TABLE orders (
id INT PRIMARY KEY,
user_id INT,
total DECIMAL(10,2),
FOREIGN KEY (user_id) REFERENCES users(id) -- user_id must match a users.id
);
A foreign key (user_id) yana nuna primary key na wani tebur (users.id), yana kafa dangantaka "ayi na jerin abu ne na mai amfani." Teburyin data yana tilasta haka: ba za ka iya saka ayi tare da user_id da babu jiya a cikin users ba.
INSERT INTO orders (user_id, total) VALUES (999, 50); -- ❌ ERROR if user 999 doesn't exist
-- the foreign key PREVENTS orphaned records (orders referencing non-existent users)
Referential integrity yana nufin teburyin data yana garantowa dangantaka suna da kyau — ba za ka iya samun ayi wanda yake nuna mai amfani da babu jiya. Wannan yana katewa alajihe daga waje/marhamare.
FOREIGN KEY (user_id) REFERENCES users(id)
ON DELETE CASCADE -- delete the user → delete their orders too
-- ON DELETE SET NULL -- set user_id to NULL
-- ON DELETE RESTRICT -- prevent deleting a user who has orders (default-ish)
ON DELETE/ON UPDATE suna bayyana me ya sa ya faru ga jeran da suka dogara lokacin da aka goge jeri ko ya canja — CASCADE (watsa), SET NULL, ko RESTRICT (katawa). Zabar daidai yana muhimmu ne don katawa rashin son saurin sharewar data ko jeran da ba su da mai samun su.
Primary keys da foreign keys sune asali na teburyin database ido na inganta — sune yadda tsarin dangantaka yake kafa gani da dangantaka tsakanin teburai, saboda haka fahimtar su yana da mahimmanci don teburin tsari da aiki tare da kowane teburyin dangantaka. Primary keys (gano kowane jeri musammu, an takaita atomatik) sune tushen gani na tebur da abin da dangantaka suka nuna. Foreign keys suna kafa dangantaka tsakanin teburai (ajiyoyi na jerin abin mahaukata, jne.) wanda yake bayyana tsarin dangantaka na tsarin aikace-aikace.
Muhimmanci, foreign keys suna tilasta referential integrity — teburyin data yana garantowa dangantaka suna inganta (ba za ka iya nuna jeri da babu jiya ba), wanda yana katewa alajihe dada daga jeran da ba su da mai samun su ko marhamare, fa'idar amfani babbar gida wanda yake jiya da kyau atomatik.
Fahimtar ON DELETE/ON UPDATE hali (CASCADE, SET NULL, RESTRICT) yana muhimmu ne saboda suna bayyana me ya sa ya faru ga jererran da suka dogara lokacin da jeri da aka nuna ya canja ko an goge — zabar daidai yana katewa rashin son saurin sharewar data (misali CASCADE goge fiye da yadda aka sa) ko jeran da ba su da mai samun su, yanke wajen gida wanda yana da mahimmanci.
Tun da dangantaka tsakanin teburai shine ma'anar teburyin data na dangantaka, kuma tun da primary/foreign keys (da referential integrity) sune yadda dangantaka aka kafa da kare, fahimtar su — mangaji ne don gani da dangantaka, referential integrity ne don jiya da kyau data, da kuma on-delete hali ne don sarrafa jererran da suka dogara — shine asali, tushen ilimi don teburin tsari da gida tambaya da yawa na tambayoyi wanda yana da mahimmanci don aiki daidai tare da jererran data na dangantaka.