Question

根据任务编写查询。这个问题问：“列出所有从事初学者或中级活动的非美国地点”。

这是我的查询

SELECT DISTINCT Location.LCountry, Activity.ALevel
FROM Location, Activity
WHERE LCountry != 'USA'
AND ALevel = 'Beginner' OR ALEVEL' = 'Intermediate';

运行此查询时，我仍然会得到一个包含美国值的列表。当我不包含OR ALevel = 'Intermediate"子句时，它会打印出正确的查询。

Answer 1

在括号中使用OR condition

SELECT DISTINCT Location.LCountry, Activity.ALevel
FROM Location inner join Activity on Location.LCountry=Activity.Country
WHERE LCountry != 'USA'
AND (ALevel = 'Beginner' OR ALEVEL = 'Intermediate');

或者您可以使用如下所示的IN运算符

SELECT DISTINCT Location.LCountry, Activity.ALevel
FROM Location inner join Activity on Location.LCountry=Activity.Country
WHERE LCountry != 'USA'
AND ALevel in( 'Beginner' , 'Intermediate');

Answer 2

您需要使用ANSI-92类型的JOIN并使用正确的引号作为：

SELECT DISTINCT l.LCountry, a.ALevel
  FROM Location l
  LEFT JOIN Activity a
    ON ( l.location = a.location )
 WHERE l.LCountry != 'USA'
   AND ( a.ALevel = 'Beginner' OR a.ALEVEL = 'Intermediate');

顺便说一句，别名使查询更易于阅读。

或者使用分组来获得如下不同的结果：

SELECT l.LCountry, a.ALevel
  FROM Location l
  LEFT JOIN Activity a
     ON ( l.location = a.location )
 WHERE l.LCountry != 'USA'
   AND ( a.ALevel = 'Beginner' OR a.ALEVEL = 'Intermediate')
 GROUP BY l.LCountry, a.ALevel;

Answer 3

不同位置的单引号不会相应放置或放错位置。

因此请尝试以下查询

SELECT DISTINCT Location.LCountry, Activity.ALevel
FROM Location, Activity
WHERE Location.LCountry <> 'USA'
AND Activity.ALevel IN ('Beginner','Intermediate')

Answer 4

使用更新后的查询：

SELECT DISTINCT Location.LCountry, Activity.ALevel
FROM Location, Activity
WHERE LCountry != 'USA'
AND (ALevel = 'Beginner' OR ALEVEL' = 'Intermediate');

SQL WHERE子句不使用！= / <>过滤掉指定的值

4 个答案: