How to drop all tables from a database with one SQL query?

Use the INFORMATION_SCHEMA.TABLES view to get the list of tables. Generate Drop scripts in the select statement and drop it using Dynamic SQL:

DECLARE @sql NVARCHAR(max)=''

SELECT @sql += ' Drop table ' + QUOTENAME(TABLE_SCHEMA) + '.'+ QUOTENAME(TABLE_NAME) + '; '
FROM   INFORMATION_SCHEMA.TABLES
WHERE  TABLE_TYPE = 'BASE TABLE'

Exec Sp_executesql @sql

Sys.Tables Version

DECLARE @sql NVARCHAR(max)=''

SELECT @sql += ' Drop table ' + QUOTENAME(s.NAME) + '.' + QUOTENAME(t.NAME) + '; '
FROM   sys.tables t
       JOIN sys.schemas s
         ON t.[schema_id] = s.[schema_id]
WHERE  t.type = 'U'

Exec sp_executesql @sql

Note: If you have any foreign Keys defined between tables then first run the below query to disable all foreign keys present in your database.

EXEC sp_msforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"

For more information, check here.