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.

Similar codes

0 vote293 views1 answer
QUERY JPQL
SQL
0 vote302 views1 answer
Oracle list partitions
SQL
0 vote289 views1 answer
Get all jobs if salary more than 5500 less than 10000 sql
SQL
-1 vote251 views1 answer
Plsql while
SQL
-1 vote261 views2 answers
Sql server date format yyyy-MM-ddThh:mm:ss
SQL
1 vote293 views1 answer
Sometimes i cant edit sql developer
SQL
1 vote287 views3 answers
Error 1451 (23000): cannot delete or update a parent row: a foreign key constraint fails
SQL
0 vote292 views1 answer
Mysql sql.gz
SQL
0 vote339 views1 answer
Pgadmin postgres ERROR: database is being accessed by other users
SQL
2 votes269 views2 answers
Oracle multiple insert
SQL