There are multiple ways to drop a database in postgres .

DEMO:

In this article, we will drop the database named: DBACLASS.

postgres=# \list
                             List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   
-----------+----------+----------+---------+-------+-----------------------
 DBACLASS  | postgres | UTF8     | C       | C     |                       --- >>>>>>> THIS ONE 
 postgres  | postgres | UTF8     | C       | C     | 
 template0 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
(4 rows)

Method 1: ( using drop database command).

 
$psql -d postgres
password: 


postgres=# drop database "DBACLASS";;

ERROR:  database "DBACLASS" is being accessed by other users
DETAIL:  There is 1 other session using the database.

Drop command failed, because the some sessions are already connected to the database. Lets clear them.

postgres=# select application_name,client_hostname,pid,usename from pg_stat_activity where datname='DBACLASS';

    application_name     | client_hostname |  pid  | usename  
-------------------------+-----------------+-------+----------
 pgAdmin 4 - DB:DBACLASS |                 | 12755 | postgres
(1 row)



postgres=# select pg_terminate_backend(pid) from pg_stat_activity where pid='12755';
 pg_terminate_backend 
----------------------
 t
(1 row)

postgres=# 
postgres=# 
postgres=# select application_name,client_hostname,pid,usename from pg_stat_activity where datname='DBACLASS';
 application_name | client_hostname | pid | usename 
------------------+-----------------+-----+---------
(0 rows)

Now no sessions are present on the database, So we can proceed with drop command.

postgres=# \conninfo
You are connected to database "postgres" as user "postgres" on host "localhost" at port "5432".

postgres=# \list
                             List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   
-----------+----------+----------+---------+-------+-----------------------
 DBACLASS  | postgres | UTF8     | C       | C     | 
 postgres  | postgres | UTF8     | C       | C     | 
 template0 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
(4 rows)

postgres=# drop database "DBACLASS";
DROP DATABASE


postgres=# \list
                             List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   
-----------+----------+----------+---------+-------+-----------------------
 postgres  | postgres | UTF8     | C       | C     | 
 template0 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
(3 rows)

Method 2: (using dropdb executable command)

Go to bin location of the postgres home_location at os level and run dropdb command.

postgres$ pwd
/Library/PostgreSQL/10/bin

postgres$ ./dropdb -e  "DBACLASS"
Password: 
SELECT pg_catalog.set_config('search_path', '', false)
DROP DATABASE "DBACLASS";

Method 3 : (Using pgadmin tool)

In this method, we will drop the database using pgadmin gui tool.