如何在SQL中获取正确的SELECT语句？

Question

我对SQL，MS Access＆amp; amp; PostgreSQL的。我很快就收到了这个查询任务。我首先阅读了基本的SQL语法，然后用逻辑编写了这个。不知怎的，我得到了这个SELECT错误：

[]

表名和列名是正确的。这是我的目标的简短描述

SELECT 
  change_incoming_area AS Bodenfläche, 
  CASE 
    WHEN change_type_id = 2 
    AND (NOT cng.city_old_id=cng.city_new_id 
    AND EXTRACT(YEAR FROM cng.date_change) = EXTRACT(YEAR FROM tb_city_annual_population.population_year)) THEN 
      ROUND((cng.change_incoming_area/SELECT pop.city_area FROM tb_city_annual_population pop WHERE pop.city_id=cng.city_old_id)*100,2)
    WHEN change_type_id = 2
    AND (cng.city_old_id=cng.city_new_id 
    AND EXTRACT(YEAR FROM cng.date_change) = EXTRACT(YEAR FROM tb_city_annual_population.population_year) 
    AND cng.change_incoming_area<SELECT pop.city_area FROM tb_city_annual_population pop WHERE pop.city_id=cng.city_old_id) THEN
      ROUND(((SELECT pop.city_area FROM tb_city_annual_population pop WHERE pop.city_id=cng.city_old_id AS origarea-cng.change_incoming_area)/origarea)*100,2)
    WHEN change_type_id = 2 
    AND (cng.city_old_id=cng.city_new_id 
    AND EXTRACT(YEAR FROM cng.date_change) = EXTRACT(YEAR FROM tb_city_annual_population.population_year) 
    AND ((cng.change_incoming_area+SELECT pop.city_area FROM tb_city_annual_population pop WHERE pop.city_id=cng.city_old_id AS origarea3)/origarea3)*100>100) THEN
      ROUND(((cng.change_incoming_area+SELECT pop.city_area FROM tb_city_annual_population pop WHERE pop.city_id=cng.city_old_id AS origarea2)/origarea2)*100,2)
    ELSE 100 
  END 
FROM tb_change cng;

Answer 1

正如jarlh立即评论的那样：子查询必须在括号中。

这一行

  ROUND( ( cng.change_incoming_area / select ... ) * 100, 2)

必须是

  ROUND( ( cng.change_incoming_area / (select ...) ) * 100, 2)

或只是

  ROUND( cng.change_incoming_area / (select ...) * 100, 2)

关于子查询本身：它当然必须只产生一个值。使用表名tb_city_annual_population，我希望每个城市和每年都有一个记录。但是在你的where子句中，你只能按城市限制：

  SELECT pop.city_area 
  FROM tb_city_annual_population pop 
  WHERE pop.city_id = cng.city_old_id

这不会给你几条记录（每年一条记录）吗？也许你想让它变成这样的东西：

  SELECT pop.city_area 
  FROM tb_city_annual_population pop 
  WHERE pop.city_id = cng.city_old_id
  AND pop.year = EXTRACT(YEAR FROM cng.date_change)

还有更多错误：

ROUND(((cng.change_incoming_area+SELECT pop.city_area FROM tb_city_annual_population pop 
        WHERE pop.city_id=cng.city_old_id AS origarea2)/origarea2)*100,2)

AS origarea2是什么？您不能在表达式的中间使用AS。它应该是

ROUND((cng.change_incoming_area + (SELECT pop.city_area FROM tb_city_annual_population pop 
       WHERE pop.city_id = cng.city_old_id)) / origarea2 * 100, 2)

AS origarea3和AS origarea相同。

然后在

AND EXTRACT(YEAR FROM cng.date_change) = 
    EXTRACT(YEAR FROM tb_city_annual_population.population_year)) THEN 

您引用tb_city_annual_population.population_year，但未从tb_city_annual_population中选择，则从tb_change中选择。

然后从population_year中提取年份。那么population_year是约会吗？这个名字表明它是一个代表一年的整数。