SQL DEFAULT cheklovi

DEFAULT cheklovi ishlatiladi ustun uchun avtomatik ravishda standart qiymatni kiriting, hech qanday qiymat belgilanmagan bo‘lsa.

Standart qiymat barcha yangi yozuvlarga qo‘shiladi (agar boshqa qiymat ko‘rsatilmagan bo‘lsa).

CREATE TABLE da DEFAULT cheklovi

Quyidagi SQL "Shahar" ustuni uchun DEFAULT qiymatini o‘rnatadi "Shaxslar" jadvalini yaratishda:

CREATE TABLE Persons (
    ID int PRIMARY KEY,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255) DEFAULT 'Sandnes'
);

DEFAULT cheklovi joriy sanani kiritish uchun CURRENT_DATE() kabi funksiyalar yordamida tizim qiymatlarini kiritish uchun ham ishlatilishi mumkin:

MySQL:

CREATE TABLE Orders (
    ID int PRIMARY KEY,
    OrderNumber int NOT NULL,
    OrderDate date DEFAULT CURRENT_DATE()
);

SQL Server:

SQL Serverda bir xil natijaga erishish uchun quyidagi SQL dan foydalaning (joriy sanani kiritish uchun):

CREATE TABLE Orders (
    ID int PRIMARY KEY,
    OrderNumber int NOT NULL,
    OrderDate date DEFAULT CAST(GETDATE() AS date)
);

ALTER TABLE da DEFAULT cheklovi

Jadval allaqachon yaratilganda "Shahar" ustunida DEFAULT cheklovini aniqlash uchun quyidagi SQL dan foydalaning:

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';

SQL Server:

ALTER TABLE Persons
ADD CONSTRAINT df_City DEFAULT 'Sandnes' FOR City;

MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes';

Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes';

DEFAULT cheklovini o‘chirish

DEFAULT cheklovini olib tashlash uchun quyidagi SQL dan foydalaning:

MySQL:

ALTER TABLE Persons
ALTER City DROP DEFAULT;

SQL Server:

ALTER TABLE Persons
DROP CONSTRAINT df_City;

MS Access:

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;

Oracle:

ALTER TABLE Persons
MODIFY (City DEFAULT NULL);

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