Question

我对SQL很新，我在Khan Academy上自学了SQL。我按如下方式创建了一个数据库表：

CREATE TABLE Data_Exp (ID INTEGER PRIMARY KEY AUTOINCREMENT, Subject TEXT, 
                       Label TEXT, Gender TEXT, X1 INTEGER, X2 INTEGER, 
                       X3 INTEGER, X4 INTEGER, X5 INTEGER);

INSERT INTO Data_Exp(Subject, Label, Gender, X1, X2, X3, X4, X5) VALUES("S01", "A", "F", 5, 7, 6, 5, 4);
INSERT INTO Data_Exp(Subject, Label, Gender, X1, X2, X3, X4, X5) VALUES("S02", "B", "M", 8, 8, 6, 4, 6);
INSERT INTO Data_Exp(Subject, Label, Gender, X1, X2, X3, X4, X5) VALUES("S03", "A", "M", 6, 1, 4, 3, 3);
INSERT INTO Data_Exp(Subject, Label, Gender, X1, X2, X3, X4, X5) VALUES("S04", "C", "F", 3, 3, 7, 2, 5);
INSERT INTO Data_Exp(Subject, Label, Gender, X1, X2, X3, X4, X5) VALUES("S05", "B", "F", 9, 2, 5, 3, 7);

上述代码的输出为：

然后我继续添加此代码：

SELECT Subject, Gender, Label, ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1) AS Y FROM Data_Exp;

并获得以下结果（如预期的那样）：

然后我使用CASE语句来评估列级别：

SELECT Subject, Gender, Label, (SELECT CASE WHEN Y > 6 THEN "Level 6"
    WHEN Y > 5 THEN "Level 5" 
    WHEN Y > 4 THEN "Level 4"
    WHEN Y > 3 THEN "Level 3"
    WHEN Y > 2 THEN "Level 2"
    WHEN Y > 1 THEN "Level 1"
    END AS "Level" FROM (SELECT ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1) AS Y FROM Data_Exp)) AS Level FROM Data_Exp;

但不是得到这个结果：

我在列级别

中得到了这个奇怪的输出

我哪里出错？

Answer 1

您的查询无效，因为您正在进行某种笛卡尔式加入

您正在寻找的是类似下面的解决方案

 SELECT Subject, 
 Gender, 
 Label, 
CASE WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 6 THEN "Level 6"
     WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 5 THEN "Level 5" 
     WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 4 THEN "Level 4"
     WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 3 THEN "Level 3"
     WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 2 THEN "Level 2"
     WHEN ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1)  > 1 THEN "Level 1"
     END AS "Level" 
  FROM Data_Exp;

否则您可以使用下面的模板。

下面的查询需要修补，对于你想要实现的目标而言可能很乏味

SELECT Subject, 
Gender, 
Label, 
(
  SELECT CASE WHEN Y > 6 THEN "Level 6"
    WHEN Y > 5 THEN "Level 5" 
    WHEN Y > 4 THEN "Level 4"
    WHEN Y > 3 THEN "Level 3"
    WHEN Y > 2 THEN "Level 2"
    WHEN Y > 1 THEN "Level 1"
    END AS "Level" 
 FROM 
 (SELECT ROUND(0.4*X1 + 0.15*(X2+X3+X4+X5),1) AS "Y" FROM Data_Exp) AS "lol"
INNER JOIN Data_Exp ON  Data_Exp.Gender = lol.Gender AND Data_Exp.Label = lol.Label
) AS "Level" 
FROM
 FROM Data_Exp;

最后但并非最不重要的是，了解您的数据库（Postgres，MySql或其他内容）将有所帮助，因为您可以使用其他解决方案（如声明变量）来存储公式，这肯定更容易。< / p>

在可汗学院的挑战中使用别名和案例陈述时奇怪的表输出

1 个答案: