SQL

constraint unique sql

ALTER TABLE table_name ADD CONSTRAINT cst_name UNIQUE (col_name); 
ALTER TABLE table_name ADD CONSTRAINT cst_name UNIQUE (col1, col2); -- Multiple
ALTER TABLE table_name ADD col_name NUMBER UNIQUE;				  	-- New field

unique in sql server

CREATE TABLE Persons (
    ID int NOT NULL UNIQUE,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int
);

sql unique select

SELECT DISTINCT col1, col2, ....
FROM table_name;

SELECT col1, MIN(col2)
FROM table_name
GROUP BY col1;

unique 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.

sql unique

This constraint ensures all values in a column are unique.
Example 1 (MySQL): Adds a unique constraint to the id column when
creating a new users table.
CREATE TABLE users (
id int NOT NULL,
name varchar(255) NOT NULL,
UNIQUE (id)
);
Example 2 (MySQL): Alters an existing column to add a UNIQUE
constraint.
ALTER TABLE users
ADD UNIQUE (id);

unique sql

CREATE TABLE order_details
( order_detail_id integer CONSTRAINT order_details_pk PRIMARY KEY,
  order_id integer NOT NULL,
  order_date date,
  quantity integer,
  notes varchar(200),
  CONSTRAINT order_unique UNIQUE (order_id)
);

SQL UNIQUE Constraint

CREATE TABLE Colleges (
  college_id INT NOT NULL UNIQUE,
  college_code VARCHAR(20) UNIQUE,
  college_name VARCHAR(50)
);