This is a really interesting question, and you'll get it done in Multiple ways::

1. By dropping and recreating the current schema

Here, In general, we have a public schema by default. So, I'm using it as an instance.

-- Recreate the schema
DROP SCHEMA public CASCADE;
CREATE SCHEMA public;

-- Restore default permissions
GRANT ALL ON SCHEMA public TO postgres;
GRANT ALL ON SCHEMA public TO public;

If you are using PostgreSQL 9.3 or greater, you may also need to restore the default grants.

Pros:

This will clean an entire Schema and re-create it as a new one.

Cons:

You'll lose other entities too like Functions, Views, Materialized views, etc.

2. By using fetching all table names from pg_tables table.

PostgreSQL stores all the tables on its record table named pg_table.

SELECT
  'DROP TABLE IF EXISTS "' || tablename || '" CASCADE;' 
from
  pg_tables WHERE schemaname = 'public';

As you can see, By the use of subquery, We can remove the entire tables from the schema.

Pros:

When the other data entities are Important and you just want to delete only tables from the schema, this approach will really helpful to you.

3. Terminal

• Login using postgres user on your shell

$ sudo -u postgres psql

• Connect your database

$ \c mydatabase

Paste these commands:

DROP SCHEMA public CASCADE;
CREATE SCHEMA public;
     
GRANT ALL ON SCHEMA public TO postgres;
GRANT ALL ON SCHEMA public TO public;

Note: This bunch of commands are similar to the first point, so the Pros and cons will remain the same.

Following steps might be helpful (For linux users):

1. At first enter the postgres command prompt by following command:

   sudo -u postgres psql
   

2. Enter the database by this command (my database name is: maoss):

   \c maoss
   

3. Now enter the command for droping all tables:

   DROP SCHEMA public CASCADE;
   CREATE SCHEMA public;
   
   GRANT ALL ON SCHEMA public TO postgres;
   GRANT ALL ON SCHEMA public TO public;
   

4. Now exit from psql by following command:

   \q
   

If you want to nuke all tables anyway, you can dispense with niceties such as CASCADE by putting all tables into a single statement. This also makes execution quicker.

SELECT 'TRUNCATE TABLE ' || string_agg('"' || tablename || '"', ', ') || ';' 
FROM pg_tables WHERE schemaname = 'public';

Executing it directly:

DO $$
DECLARE tablenames text;
BEGIN    
    tablenames := string_agg('"' || tablename || '"', ', ') 
        FROM pg_tables WHERE schemaname = 'public';
    EXECUTE 'TRUNCATE TABLE ' || tablenames;
END; $$

Replace TRUNCATE with DROP as applicable.

If you want delete data (not delete table):

-- Truncate tables and restart sequnces
SELECT 'TRUNCATE TABLE "' || table_schema || '"."' || table_name || '" RESTART IDENTITY CASCADE;' 
FROM information_schema.tables 
WHERE table_catalog = '<database>' AND table_schema = '<schema>';

Or if you want drop table your can use this sql:

-- For tables
SELECT 'DROP TABLE "' || table_schema || '"."' || table_name || '" CASCADE;' 
FROM information_schema.tables 
WHERE table_catalog = '<database>' AND table_schema = '<schema>';

-- For sequences
SELECT 'DROP SEQUENCE d_a_seq "' || sequence_schema || '"."' || sequence_name || '";' 
FROM information_schema.sequences 
WHERE sequence_catalog = '<database>' AND sequence_schema = '<schema>';

Note: my answer is about really deleting the tables and other database objects; for deleting all data in the tables, i.e. truncating all tables, Endre Both has provided a similarily well-executed (direct execution) statement a month later.

For the cases where you can’t just DROP SCHEMA public CASCADE;, DROP OWNED BY current_user; or something, here’s a stand-alone SQL script I wrote, which is transaction-safe (i.e. you can put it between BEGIN; and either ROLLBACK; to just test it out or COMMIT; to actually do the deed) and cleans up “all” database objects… well, all those used in the database our application uses or I could sensibly add, which is:

• triggers on tables
• constraints on tables (FK, PK, CHECK, UNIQUE)
• indicēs
• VIEWs (normal or materialised)
• tables
• sequences
• routines (aggregate functions, functions, procedures)
• all nōn-default (i.e. not public or DB-internal) schemata “we” own: the script is useful when run as “not a database superuser”; a superuser can drop all schemata (the really important ones are still explicitly excluded, though)
• extensions (user-contributed but I normally deliberately leave them in)

Not dropped are (some deliberate; some only because I had no example in our DB):

• the public schema (e.g. for extension-provided stuff in them)
• collations and other locale stuff
• event triggers
• text search stuff, … (see here for other stuff I might have missed)
• roles or other security settings
• composite types
• toast tables
• FDW and foreign tables

This is really useful for the cases when the dump you want to restore is of a different database schema version (e.g. with Debian dbconfig-common, Flyway or Liquibase/DB-Manul) than the database you want to restore it into.

I’ve also got a version which deletes “everything except two tables and what belongs to them” (a sequence, tested manually, sorry, I know, boring) in case someone is interested; the diff is small. Contact me or check this repo if interested.

SQL

-- Copyright © 2019, 2020
--      mirabilos <t.glaser@tarent.de>
--
-- Provided that these terms and disclaimer and all copyright notices
-- are retained or reproduced in an accompanying document, permission
-- is granted to deal in this work without restriction, including un‐
-- limited rights to use, publicly perform, distribute, sell, modify,
-- merge, give away, or sublicence.
--
-- This work is provided “AS IS” and WITHOUT WARRANTY of any kind, to
-- the utmost extent permitted by applicable law, neither express nor
-- implied; without malicious intent or gross negligence. In no event
-- may a licensor, author or contributor be held liable for indirect,
-- direct, other damage, loss, or other issues arising in any way out
-- of dealing in the work, even if advised of the possibility of such
-- damage or existence of a defect, except proven that it results out
-- of said person’s immediate fault when using the work as intended.
-- -
-- Drop everything from the PostgreSQL database.

DO $$
DECLARE
        q TEXT;
        r RECORD;
BEGIN
        -- triggers
        FOR r IN (SELECT pns.nspname, pc.relname, pt.tgname
                FROM pg_catalog.pg_trigger pt, pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace AND pc.oid=pt.tgrelid
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pt.tgisinternal=false
            ) LOOP
                EXECUTE format('DROP TRIGGER %I ON %I.%I;',
                    r.tgname, r.nspname, r.relname);
        END LOOP;
        -- constraints #1: foreign key
        FOR r IN (SELECT pns.nspname, pc.relname, pcon.conname
                FROM pg_catalog.pg_constraint pcon, pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace AND pc.oid=pcon.conrelid
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pcon.contype='f'
            ) LOOP
                EXECUTE format('ALTER TABLE ONLY %I.%I DROP CONSTRAINT %I;',
                    r.nspname, r.relname, r.conname);
        END LOOP;
        -- constraints #2: the rest
        FOR r IN (SELECT pns.nspname, pc.relname, pcon.conname
                FROM pg_catalog.pg_constraint pcon, pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace AND pc.oid=pcon.conrelid
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pcon.contype<>'f'
            ) LOOP
                EXECUTE format('ALTER TABLE ONLY %I.%I DROP CONSTRAINT %I;',
                    r.nspname, r.relname, r.conname);
        END LOOP;
        -- indicēs
        FOR r IN (SELECT pns.nspname, pc.relname
                FROM pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pc.relkind='i'
            ) LOOP
                EXECUTE format('DROP INDEX %I.%I;',
                    r.nspname, r.relname);
        END LOOP;
        -- normal and materialised views
        FOR r IN (SELECT pns.nspname, pc.relname
                FROM pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pc.relkind IN ('v', 'm')
            ) LOOP
                EXECUTE format('DROP VIEW %I.%I;',
                    r.nspname, r.relname);
        END LOOP;
        -- tables
        FOR r IN (SELECT pns.nspname, pc.relname
                FROM pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pc.relkind='r'
            ) LOOP
                EXECUTE format('DROP TABLE %I.%I;',
                    r.nspname, r.relname);
        END LOOP;
        -- sequences
        FOR r IN (SELECT pns.nspname, pc.relname
                FROM pg_catalog.pg_class pc, pg_catalog.pg_namespace pns
                WHERE pns.oid=pc.relnamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pc.relkind='S'
            ) LOOP
                EXECUTE format('DROP SEQUENCE %I.%I;',
                    r.nspname, r.relname);
        END LOOP;
        -- extensions (only if necessary; keep them normally)
        FOR r IN (SELECT pns.nspname, pe.extname
                FROM pg_catalog.pg_extension pe, pg_catalog.pg_namespace pns
                WHERE pns.oid=pe.extnamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
            ) LOOP
                EXECUTE format('DROP EXTENSION %I;', r.extname);
        END LOOP;
        -- aggregate functions first (because they depend on other functions)
        FOR r IN (SELECT pns.nspname, pp.proname, pp.oid
                FROM pg_catalog.pg_proc pp, pg_catalog.pg_namespace pns, pg_catalog.pg_aggregate pagg
                WHERE pns.oid=pp.pronamespace
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast')
                    AND pagg.aggfnoid=pp.oid
            ) LOOP
                EXECUTE format('DROP AGGREGATE %I.%I(%s);',
                    r.nspname, r.proname,
                    pg_get_function_identity_arguments(r.oid));
        END LOOP;
        -- routines (functions, aggregate functions, procedures, window functions)
        IF EXISTS (SELECT * FROM pg_catalog.pg_attribute
                WHERE attrelid='pg_catalog.pg_proc'::regclass
                    AND attname='prokind' -- PostgreSQL 11+
            ) THEN
                q := 'CASE pp.prokind
                        WHEN ''p'' THEN ''PROCEDURE''
                        WHEN ''a'' THEN ''AGGREGATE''
                        ELSE ''FUNCTION''
                    END';
        ELSIF EXISTS (SELECT * FROM pg_catalog.pg_attribute
                WHERE attrelid='pg_catalog.pg_proc'::regclass
                    AND attname='proisagg' -- PostgreSQL ≤10
            ) THEN
                q := 'CASE pp.proisagg
                        WHEN true THEN ''AGGREGATE''
                        ELSE ''FUNCTION''
                    END';
        ELSE
                q := '''FUNCTION''';
        END IF;
        FOR r IN EXECUTE 'SELECT pns.nspname, pp.proname, pp.oid, ' || q || ' AS pt
                FROM pg_catalog.pg_proc pp, pg_catalog.pg_namespace pns
                WHERE pns.oid=pp.pronamespace
                    AND pns.nspname NOT IN (''information_schema'', ''pg_catalog'', ''pg_toast'')
            ' LOOP
                EXECUTE format('DROP %s %I.%I(%s);', r.pt,
                    r.nspname, r.proname,
                    pg_get_function_identity_arguments(r.oid));
        END LOOP;
        -- nōn-default schemata we own; assume to be run by a not-superuser
        FOR r IN (SELECT pns.nspname
                FROM pg_catalog.pg_namespace pns, pg_catalog.pg_roles pr
                WHERE pr.oid=pns.nspowner
                    AND pns.nspname NOT IN ('information_schema', 'pg_catalog', 'pg_toast', 'public')
                    AND pr.rolname=current_user
            ) LOOP
                EXECUTE format('DROP SCHEMA %I;', r.nspname);
        END LOOP;
        -- voilà
        RAISE NOTICE 'Database cleared!';
END; $$;

Tested, except later additions (extensions contributed by Clément Prévost), on PostgreSQL 9.6 (jessie-backports). Aggregate removal tested on 9.6 and 12.2, procedure removal tested on 12.2 as well. Bugfixes and further improvements welcome!

Just execute the query bellow:

DO $$ DECLARE
    r RECORD;
BEGIN
    FOR r IN (SELECT tablename FROM pg_tables WHERE schemaname = current_schema()) LOOP
        EXECUTE 'DROP TABLE IF EXISTS ' || quote_ident(r.tablename) || ' CASCADE';
    END LOOP;
END $$;

Using PSQL with \gexec

This is a far more comprehensive query then the ones thus far, as it will work with special table names.

SELECT FORMAT('DROP TABLE %I.%I.%I CASCADE;', table_catalog, table_schema, table_name)
FROM information_schema.tables
WHERE table_type = 'BASE TABLE'
  AND table_schema <> 'information_schema'
  AND table_schema NOT LIKE 'pg_%';

You can preview the commands to be run, and you can execute the output of this query by typing \gexec after you run it in psql.

NOTE: The use of CASCADE will drop all things (like VIEWs) that depend on the table

The easiest way is to drop the public schema as others have suggested in previous answers. However, this is NOT a good way. You never know what has been done to the public schema that has since been forgotten and was not documented. You also don't know if this will work the same into the future. In V9, it would have been fine, but in V10 all your users would loose access to the schema, and must be granted access again otherwise your application will break. I haven't checked V11, but the point is that you never know what will break as you move from machine to machine, site to site or version to version. It also cannot be done if you are a user that has access to the database, but not to the schema.

If you need to do this programmatically then other answers above cover this, but one thing the answers above don't consider is to get Postgres to do the work for you. If you use pg_dump with the -c option as below:

sudo su postgres -c "pg_dump -U postgres WhateverDB -c -f "/home/Anyone/DBBackupWhateverDB-ServerUnscheduled.sql""

That will create a DB restore script with sql statements that will delete all the tables.

If the only purpose in asking the question was to delete the tables prior to restore, then your restore will do the work for you.

However, if you need it for something else, you can simply copy the drop statements from the sql script.

A terminal based approach using psql worked best for me. I even created a bash function because it is convenient for development:

psqlDropTables() {
    PGPASSWORD=<your password>
    PGTABLE=<your table name>
    PGUSER=<your pg user name>
    PGPASSWORD=$PGPASSWORD psql -ah 127.0.0.1 $PGTABLE $PGUSER -c "
      SELECT
'DROP TABLE IF EXISTS \"' || tablename || '\" CASCADE;' from
pg_tables WHERE schemaname = 'public';" | grep DROP | awk 'NR>1{print $0}' | sed "s/\"/'/g" | PGPASSWORD=$PGPASSWORD xargs -i  psql -ah 127.0.0.1 $PGTABLE $PGUSER -c {}
}

It creates all the required drop table statements as stated in this response, replaces the " with ' and runs them on the DB.

Maybe the simplest way is:

1. Drop database contains that tables with:

   drop database DATABASE_NAME;

2. Recreate that database:

   create database DATABASE_NAME;

Here's the ready-made query for you:

SELECT

'drop table if exists "' || tablename || '" cascade;' as pg_drop

FROM

pg_tables

WHERE

schemaname='your schema';

for macOS. If you have a terminal via PostgreSQL application, a simple command can help you:

drop table "organisations" cascade;