SQL语句计算字段

Question

This is my question, I need help with the last bullet point. I can't figure out how to create a calculated field that calculates what the field name suggests.

From the City table select ID, name, Country and Population and from the Country table select Life Expectancy, Name, SurfaceArea, GNP. Restrict your result set using the following criteria( all of the following criteria must be met for records to be returned):

• Country SurfaceArea between 3,000,000 and 40,000,000 (using the
  between operator)

• The length of the City.District field is greater than 4

• Create the calculated field'CityPopulationPercentageOfCountryPopulation' that calculates what the field name suggests.

This is what I have so far. Thanks in advance.

SELECT City.ID
     , City.name
     , City.Country
     , City.Population
     , Country.LifeExpectancy
     , Country.Name
     , Country.SurfaceArea
     , Country.GNP
     , ROUND(City.Population * 100.0 / Country.Population, 1) AS 'CityPopulationPercentageOfCountryPopulation'
  FROM City
     , Country
 WHERE Country.SurfaceArea BETWEEN 3000000 AND 40000000 
   AND LENGTH(City.District) > 4

Country table

City table

Answer 1

Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax. Your query should look like this:

 SELECT . . .
 FROM City JOIN
      Country
      ON City.CountryCode = Country.Code  -- or whatever columns are used for the `JOIN`
 WHERE Country.SurfaceArea BETWEEN 3000000 AND 40000000 AND
     LENGTH(City.District) > 4

Answer 2

Seems like a semi-cross join here. That is, every row returned from City is matched with every row returned from Country .

I recommend ditching the old school comma syntax for the join operation, and use the JOIN keyword instead. If we're intending a semi-Cartesian product, I'd recommend including the CROSS keyword to alert future readers that the omission of any join predicates (conditions to match on) in the ON clause is intentional, and not an oversight.

Also, the LENGTH function returns number of bytes; use CHAR_LENGTH to return a count of characters.

---

I suspect we want to "match" each City to a particular Country. Maybe the City table contains a Country_ID column foreign key referencing ID column in the Country table. Or maybe the Country column is the foreign key. (That's just a guess; there's no conclusive indication this is the case in the question.)

SELECT ci.ID                   AS city_id
     , ci.name                 AS city_name
     , ci.Country              AS city_country
     , ci.Population           AS city_population
     , co.LifeExpectancy       AS country_lifeexpectancy   
     , co.Name                 AS country_name
     , co.SurfaceArea          AS country_surfacearea
     , co.GNP                  AS country_gnp
     , ROUND(100 * ci.Population / co.Population, 1) AS `CityPopulationPercentageOfCountryPopulation`
  FROM City ci
  JOIN Country co
    ON co.ID = ci.Country_ID
 WHERE co.SurfaceArea BETWEEN 3000000 AND 40000000
   AND CHAR_LENGTH(ci.District) > 4