Foreign key

Ushbu qo'llanmada siz PostgreSQL foreign keyi va foreign key cheklovlaridan foydalangan holda jadvallarning foreign keylarini qanday qo'shish mumkinligi haqida bilib olasiz.

PostgreSQL-da foreign key boshqa jadvaldagi qatorni noyob tarzda identifikatsiya qiluvchi jadvaldagi ustun yoki ustunlar guruhidir.

Foreign key primary keyga yoki havola qilingan jadvalning unique chekloviga murojaat qilish orqali ikkita jadvaldagi ma'lumotlar o'rtasida bog'lanishni o'rnatadi.

Foreign keyni o'z ichiga olgan jadval havolalar jadvali yoki bolalar jadvali deb ataladi. Aksincha, foreign key bilan havola qilingan jadval havola qilingan jadval yoki ota-jadval deb nomlanadi.

Foreign keylarning asosiy maqsadi relyatsion ma'lumotlar bazasida havolalar yaxlitligini ta'minlash, ota-ona va pastki jadvallar o'rtasidagi munosabatlarning haqiqiyligini ta'minlashdir.

Masalan, foreign key havola qilingan jadvalga mos qiymatlarga ega bo'lmagan qiymatlarni kiritishni oldini oladi.

Bundan tashqari, foreign key asosiy jadvalda o'zgarishlar sodir bo'lganda, bolalar jadvalidagi tegishli qatorlarni avtomatik ravishda yangilash yoki o'chirish orqali barqarorlikni saqlaydi.

Jadval boshqa jadvallar bilan aloqasiga qarab bir nechta foreign keylarga ega bo'lishi mumkin.

Foreign keyini aniqlash uchun siz foreign key cheklovidan foydalanishingiz mumkin.

PostgreSQL foreign keyni cheklash sintaksisi

Quyida foreign key cheklovi sintaksisi tasvirlangan:

[CONSTRAINT fk_name]
   FOREIGN KEY(fk_columns) 
   REFERENCES parent_table(parent_key_columns)
   [ON DELETE delete_action]
   [ON UPDATE update_action]

Ushbu sintaksisda:

• Birinchidan, CONSTRAINT kalit so'zidan keyin foreign key cheklovi nomini belgilang. CONSTRAINT bandi ixtiyoriy. Agar siz uni o'tkazib yuborsangiz, PostgreSQL avtomatik yaratilgan nomni tayinlaydi.

• Ikkinchidan, FOREIGN KEY kalit so'zlaridan keyin qavslar ichida bir yoki bir nechta foreign key ustunlarini belgilang.

• Uchinchidan, REFERENCES bandidagi foreign key ustunlari tomonidan havola qilingan asosiy jadval va asosiy kalit ustunlarini belgilang.

• Nihoyat, ON DELETE va ON UPDATE bandlarida kerakli o'chirish va yangilash amallarini belgilang.

Yo'q qilish va yangilash harakatlari ota-ona jadvalidagi asosiy kalit o'chirilganda va yangilanganda xatti-harakatlarni aniqlaydi.

Asosiy kalit kamdan-kam yangilanganligi sababli, ON UPDATE action amalda kamdan-kam qo'llaniladi. Biz e'tiborni ON DELETE amaliga qaratamiz.

PostgreSQL quyidagi amallarni qo'llab-quvvatlaydi:

• SET NULL

• SET DEFAULT

• RESTRICT

• NO ACTION

• CASCADE

PostgreSQL foreign key constraint examples

Quyidagi bayonotlar customers va contacts jadvallarini yaratadi:

DROP TABLE IF EXISTS customers;
DROP TABLE IF EXISTS contacts;

CREATE TABLE customers(
   customer_id INT GENERATED ALWAYS AS IDENTITY,
   customer_name VARCHAR(255) NOT NULL,
   PRIMARY KEY(customer_id)
);

CREATE TABLE contacts(
   contact_id INT GENERATED ALWAYS AS IDENTITY,
   customer_id INT,
   contact_name VARCHAR(255) NOT NULL,
   phone VARCHAR(15),
   email VARCHAR(100),
   PRIMARY KEY(contact_id),
   CONSTRAINT fk_customer
      FOREIGN KEY(customer_id) 
        REFERENCES customers(customer_id)
);

Bu misolda customers jadvali ota-ona jadvali, contacts jadvali esa bolalar jadvalidir.

Har bir mijoz nol yoki ko'p kontaktga ega va har bir kontakt nol yoki bitta mijozga tegishli.

contacts jadvalidagi customer_id ustuni customers jadvalidagi bir xil nomdagi asosiy kalit ustuniga havola qiluvchi foreign key ustunidir.

contacts jadvalidagi fk_customer foreign key cheklovi customer_id ni foreign key sifatida belgilaydi:

CONSTRAINT fk_customer
   FOREIGN KEY(customer_id) 
      REFERENCES customers(customer_id)

Foreign key cheklovida ON DELETE va ON UPDATE amallari mavjud emasligi sababli ular sukut bo'yicha NO ACTION ga o'rnatiladi.

NO ACTION

Quyidagilar customers va contacts jadvallariga ma'lumotlarni kiritadi:

INSERT INTO customers(customer_name)
VALUES('BlueBird Inc'),
      ('Dolphin LLC');	   
	   
INSERT INTO contacts(customer_id, contact_name, phone, email)
VALUES(1,'John Doe','(408)-111-1234','[email protected]'),
      (1,'Jane Doe','(408)-111-1235','[email protected]'),
      (2,'David Wright','(408)-222-1234','[email protected]');

Quyidagi bayonot customers jadvalidan mijoz identifikatori 1ni o'chiradi:

DELETE FROM customers
WHERE customer_id = 1;

ON DELETE NO ACTION tufayli PostgreSQL cheklovni buzadi, chunki mijoz identifikatori 1 ning havola qatorlari hali ham contacts jadvalida mavjud:

ERROR:  update or delete on table "customers" violates foreign key constraint "fk_customer" on table "contacts"
DETAIL:  Key (customer_id)=(1) is still referenced from table "contacts".
SQL state: 23503

RESRICT amali NO ACTIONga o'xshaydi. Farq faqat foreign key cheklovi INITIALLY DEFERRED yoki INITIALLY IMMEDIATE DEFERRABLE rejimi bilan foreign key cheklovi belgilaganingizdagina yuzaga keladi. Kelgusi darsda bu haqda ko'proq gaplashamiz.

SET NULL

SET NULL, ota-jadvaldagi havola qilingan satrlar o'chirilganda, avtomatik ravishda NULLni pastki jadvalning havola qiluvchi satrlaridagi foreign key ustunlariga o'rnatadi.

Birinchidan, namunaviy jadvallarni tashlang va ularni ON DELETE bandidagi SET NULL amalidan foydalanadigan foreign key yordamida qayta yarating:

DROP TABLE IF EXISTS contacts;
DROP TABLE IF EXISTS customers;

CREATE TABLE customers(
   customer_id INT GENERATED ALWAYS AS IDENTITY,
   customer_name VARCHAR(255) NOT NULL,
   PRIMARY KEY(customer_id)
);

CREATE TABLE contacts(
   contact_id INT GENERATED ALWAYS AS IDENTITY,
   customer_id INT,
   contact_name VARCHAR(255) NOT NULL,
   phone VARCHAR(15),
   email VARCHAR(100),
   PRIMARY KEY(contact_id),
   CONSTRAINT fk_customer
      FOREIGN KEY(customer_id) 
	  REFERENCES customers(customer_id)
	  ON DELETE SET NULL
);

Ikkinchidan, customers va contacts jadvallariga ma'lumotlarni kiriting:

INSERT INTO customers(customer_name)
VALUES('BlueBird Inc'),
      ('Dolphin LLC');	   
	   
INSERT INTO contacts(customer_id, contact_name, phone, email)
VALUES(1,'John Doe','(408)-111-1234','[email protected]'),
      (1,'Jane Doe','(408)-111-1235','[email protected]'),
      (2,'David Wright','(408)-222-1234','[email protected]');

Uchinchidan, customers jadvalidan 1 identifikatorli mijozni o'chirib tashlang:

DELETE FROM customers
WHERE customer_id = 1;

ON DELETE SET NULL amali tufayli contacts jadvalidagi havola satrlari NULLga o'rnatiladi.

Nihoyat, contacts jadvalidagi ma'lumotlarni ko'rsating:

SELECT * FROM contacts;

Chiqish:

 contact_id | customer_id | contact_name |     phone      |          email
------------+-------------+--------------+----------------+--------------------------
          3 |           2 | David Wright | (408)-222-1234 | [email protected]
          1 |        null | John Doe     | (408)-111-1234 | [email protected]
          2 |        null | Jane Doe     | (408)-111-1235 | [email protected]
(3 rows)

Chiqish shuni ko'rsatadiki, mijoz identifikatori 1 qiymatlari NULLga o'zgargan.

CASCADE

ON DELETE CASCADE ota-jadvaldagi havola qilingan satrlar o'chirilganda, bolalar jadvalidagi barcha havola qiluvchi qatorlarni avtomatik ravishda o'chiradi. Amalda, ON DELETE CASCADE eng ko'p ishlatiladigan variant hisoblanadi.

Quyidagi bayonotlar fk_customerning CASCADE ga o'zgartirishlarini o'chirish amali bilan namunaviy jadvallarni qayta yaratadi:

DROP TABLE IF EXISTS contacts;
DROP TABLE IF EXISTS customers;

CREATE TABLE customers(
   customer_id INT GENERATED ALWAYS AS IDENTITY,
   customer_name VARCHAR(255) NOT NULL,
   PRIMARY KEY(customer_id)
);

CREATE TABLE contacts(
   contact_id INT GENERATED ALWAYS AS IDENTITY,
   customer_id INT,
   contact_name VARCHAR(255) NOT NULL,
   phone VARCHAR(15),
   email VARCHAR(100),
   PRIMARY KEY(contact_id),
   CONSTRAINT fk_customer
      FOREIGN KEY(customer_id) 
	  REFERENCES customers(customer_id)
	  ON DELETE CASCADE
);

INSERT INTO customers(customer_name)
VALUES('BlueBird Inc'),
      ('Dolphin LLC');	   
	   
INSERT INTO contacts(customer_id, contact_name, phone, email)
VALUES(1,'John Doe','(408)-111-1234','[email protected]'),
      (1,'Jane Doe','(408)-111-1235','[email protected]'),
      (2,'David Wright','(408)-222-1234','[email protected]');

Quyidagi bayonot mijoz identifikatori 1ni o'chirib tashlaydi:

DELETE FROM customers
WHERE customer_id = 1;

ON DELETE CASCADE amali tufayli contacts jadvalidagi barcha havola qatorlari avtomatik ravishda oʻchiriladi:

SELECT * FROM contacts;

Chiqish:

 contact_id | customer_id | contact_name |     phone      |          email
------------+-------------+--------------+----------------+--------------------------
          3 |           2 | David Wright | (408)-222-1234 | [email protected]
(1 row)

SET DEFAULT

ON DELETE SET DEFAULT ota-jadvaldagi havola qilingan satrlar o'chirilganda, ichki jadvaldagi havola qiluvchi qatorlarning foreign key ustuniga standart qiymatni o'rnatadi.

Mavjud jadvalga foreign key cheklovini qo'shing

Mavjud jadvalga foreign key cheklovini qo'shish uchun siz ALTER TABLE bayonotining quyidagi shaklidan foydalanasiz:

ALTER TABLE child_table 
ADD CONSTRAINT constraint_name 
FOREIGN KEY (fk_columns) 
REFERENCES parent_table (parent_key_columns);

Mavjud jadvalga ON DELETE CASCADE opsiyasi bilan foreign key cheklovini qo'shganda, siz quyidagi amallarni bajarishingiz kerak:

Birinchidan, mavjud Foreign keyi cheklanishini bekor qiling:

ALTER TABLE child_table
DROP CONSTRAINT constraint_fkey;

Ikkinchidan, ON DELETE CASCADE amali bilan yangi foreign key cheklovini qoʻshing:

ALTER TABLE child_table
ADD CONSTRAINT constraint_fk
FOREIGN KEY (fk_columns)
REFERENCES parent_table(parent_key_columns)
ON DELETE CASCADE;

Xulosa

• Ikki jadval o'rtasidagi ma'lumotlarning yaxlitligi va izchilligini ta'minlash uchun tashqi kalitlardan foydalaning.

• Jadval yaratishda tashqi kalit cheklovini aniqlash uchun FOREIGN KEY cheklovidan foydalaning.

• Mavjud jadvalga tashqi kalit cheklovini qo'shish uchun ALTER TABLE ... ADD CONSTRAINT ... FOREIGN KEY dan foydalaning.

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