The Easy City 2 [NCTU Floor 25]

#16 first 3 character of nth smallest/largest country and its cities
SELECT SUBSTR(country, 1, 3), COUNT(city) Cities
    FROM citytable
        GROUP BY country
        ORDER BY Cities ASC LIMIT 1, 1;

#17 Delete data where name of city starts or ends with specified character or lat/lon between some values
DELETE FROM citytable
    WHERE city RLIKE "[nugi]$" OR city RLIKE "^[^wx]" OR
          lat BETWEEN 121.5 AND 125.5 AND lon BETWEEN 25.3 AND 25.5;

#18 Swap(lat, lon) if last number of ID is m
UPDATE citytable
    SET lat = lat + lon, lon = lat-lon, lat = lat - lon WHERE id % 10 = 4;

#19 Euclidean distance between smallest lat, lon and largest lat, lon
SELECT ROUND(SQRT(POW(MAX(lat) - MIN(lat), 2) + POW(MAX(lon) - MIN(lon), 2)), 4)
    FROM citytable;