set column as unique in sql server
ALTER TABLE Persons
ADD UNIQUE (ID);set column as unique in sql server
ALTER TABLE Persons
ADD UNIQUE (ID);sql make sure every user is unqiuew
create table user(
user_id INT auto_increment PRIMARY KEY,
username varchar(50) not null,
avator varchar(50),
gender boolean not null,
phone varchar(20),
unique(username)
);
create table journey(
id INT auto_increment PRIMARY KEY,
start varchar(100) not null,
dest varchar(100) not null,
time date not null,
person INT,
user_id INT not null,
foreign key(user_id) references user(user_id)
);unique constraint mssql
USE AdventureWorks2012;
GO
CREATE TABLE Production.TransactionHistoryArchive4
(
TransactionID int NOT NULL,
CONSTRAINT AK_TransactionID UNIQUE(TransactionID)
);
GOunique element in sql
DISTINCT
- select distinct * from employees; ==> retrieves any row if it has at
least a single unique column.
- select distinct first_name from employees; ==> retrieves unique names
from table. (removes duplicates)
- select distinct count(*) from employees; retrieve number of unique rows
if any row has at least a single unique data.unique key in sql
Unique Key:
Only unique value and also can contain NULLUnique Key in sql
CREATE TABLE Students ( /* Create table with a single field as unique */
ID INT NOT NULL UNIQUE
Name VARCHAR(255)
);
CREATE TABLE Students ( /* Create table with multiple fields as unique */
ID INT NOT NULL
LastName VARCHAR(255)
FirstName VARCHAR(255) NOT NULL
CONSTRAINT PK_Student
UNIQUE (ID, FirstName)
);
ALTER TABLE Students /* Set a column as unique */
ADD UNIQUE (ID);
ALTER TABLE Students /* Set multiple columns as unique */
ADD CONSTRAINT PK_Student /* Naming a unique constraint */
UNIQUE (ID, FirstName);Copyright © 2021 Codeinu
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