9_weather_observation_station_4.sql

/*
Weather Observation Station 4

Let N be the number of CITY entries in STATION, and let N' be the number of 
distinct CITY names in STATION; query the value of N - N' from STATION. 
In other words, find the difference between the total number of CITY entries in 
the table and the number of distinct CITY entries in the table.

- Input Format
The STATION table is described as follows:
    +-------------+--------------+
    | Field       |  Type        |
    +-------------+--------------+
    | ID          | NUMBER       |
    | CITY        | VARCHAR2(21) |
    | STATE       | VARCHAR2(2)  |
    | LAT_N       | NUMBER       |
    | LONG_W      | NUMBER       |
    +-------------+--------------+

where LAT_N is the northern latitude and LONG_W is the western longitude.
*/

SELECT 
    COUNT(city) - COUNT(DISTINCT city) 
FROM 
    station
;