What is unique key in sql

How to find unique key 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);

What is unique key in sql

Unique Key:
Only unique value and also can contain NULL

Unique key

ALTER TABLE students  
ADD UNIQUE (S_Id)

Similar codes

1 vote168 views1 answer
Drop function
SQL
35 votes175 views6 answers
Postgres
SQL
3 votes177 views2 answers
Oracle synonym
SQL
1 vote162 views1 answer
Mysql show data from table
SQL
0 vote182 views1 answer
#1231 - Variable 'sql_mode' can't be set to the value of 'NULL'
SQL
1 vote181 views2 answers
Oracle list service names
SQL
1 vote174 views1 answer
Oracle sql listagg
SQL
3 votes172 views3 answers
Django.db.utils.OperationalError: no such table:
SQL
0 vote180 views1 answer
Convert multiple columns to ROws in sql server
SQL
3 votes169 views1 answer
Oracle APEX elapsed time
SQL