Question

这是我的问题，我需要最后一个要点的帮助。我无法弄清楚如何创建一个计算字段来计算字段名称的含义。

从City表中选择ID，名称，国家和人口，并从Country表中选择Life Expectancy，Name，SurfaceArea，GNP。使用以下条件限制结果集（必须满足以下所有条件才能返回记录）：

Country SurfaceArea介于3,000,000和40,000,000之间（使用
在运营商之间）
City.District字段的长度大于4
创建计算结果 field'CityPopulationPercentageOfCountryPopulation'计算字段名称的含义。

这是我到目前为止所拥有的。提前致谢。

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

SQL语句计算字段

2 个答案: