system: You are a text-to-SQL generator. Given a database schema and a question, reply with a single valid SQLite query that answers it. Output only the SQL on one line, with no explanation and no markdown.

user: Schema:
CREATE TABLE CBSA (
  CBSA integer PRIMARY KEY,
  CBSA_name text,
  CBSA_type text
);
CREATE TABLE state (
  abbreviation text PRIMARY KEY,
  name text
);
CREATE TABLE congress (
  cognress_rep_id text PRIMARY KEY,
  first_name text,
  last_name text,
  CID text,
  party text,
  state text,
  abbreviation text,
  House text,
  District integer,
  land_area real
);
CREATE TABLE zip_data (
  zip_code integer PRIMARY KEY,
  city text,
  state text,
  multi_county text,
  type text,
  organization text,
  time_zone text,
  daylight_savings text,
  latitude real,
  longitude real,
  elevation integer,
  state_fips integer,
  county_fips integer,
  region text,
  division text,
  population_2020 integer,
  population_2010 integer,
  households integer,
  avg_house_value integer,
  avg_income_per_household integer,
  persons_per_household real,
  white_population integer,
  black_population integer,
  hispanic_population integer,
  asian_population integer,
  american_indian_population integer,
  hawaiian_population integer,
  other_population integer,
  male_population integer,
  female_population integer,
  median_age real,
  male_median_age real,
  female_median_age real,
  residential_mailboxes integer,
  business_mailboxes integer,
  total_delivery_receptacles integer,
  businesses integer,
  1st_quarter_payroll integer,
  annual_payroll integer,
  employees integer,
  water_area real,
  land_area real,
  single_family_delivery_units integer,
  multi_family_delivery_units integer,
  total_beneficiaries integer,
  retired_workers integer,
  disabled_workers integer,
  parents_and_widowed integer,
  spouses integer,
  children integer,
  over_65 integer,
  monthly_benefits_all integer,
  monthly_benefits_retired_workers integer,
  monthly_benefits_widowed integer,
  CBSA integer
);
CREATE TABLE alias (
  zip_code integer PRIMARY KEY,
  alias text
);
CREATE TABLE area_code (
  zip_code integer PRIMARY KEY,
  area_code integer PRIMARY KEY
);
CREATE TABLE avoid (
  zip_code integer PRIMARY KEY,
  bad_alias text PRIMARY KEY
);
CREATE TABLE country (
  zip_code integer PRIMARY KEY,
  county text PRIMARY KEY,
  state text
);
CREATE TABLE zip_congress (
  zip_code integer PRIMARY KEY,
  district text PRIMARY KEY
);
-- foreign keys: congress.abbreviation -> state.abbreviation; zip_data.CBSA -> CBSA.CBSA; zip_data.state -> state.abbreviation; alias.zip_code -> zip_data.zip_code; area_code.zip_code -> zip_data.zip_code; avoid.zip_code -> zip_data.zip_code; country.state -> state.abbreviation; country.zip_code -> zip_data.zip_code; zip_congress.zip_code -> zip_data.zip_code; zip_congress.district -> congress.cognress_rep_id

Hint: SUBTRACT(COUNT(type = 'P.O. Box Only'), COUNT(type = 'Post Office')) where area_code = 787;

Question: What is the difference in the number of cities with P.O. box only and cities with Post Office among the cities with area code 787?

assistant: SELECT COUNT(CASE WHEN T2.type = 'P.O. Box Only' THEN 1 ELSE NULL END) - COUNT(CASE WHEN T2.type = 'Post Office' THEN 1 ELSE NULL END) AS DIFFERENCE FROM area_code AS T1 INNER JOIN zip_data AS T2 ON T1.zip_code = T2.zip_code WHERE T1.area_code = 787