如何处理空值-JPA / SQL / JPQL

Question

应该很简单，但是我现在真的很困惑

我有一个数据库查询：

   public double markingAvg(Marking id) {
        System.out.println("In MarkingAvg");
        System.out.println("id = " + id);
        System.out.println("id = " + id.getId());
        Query m = em.createQuery("SELECT m.markSectionOne, m.markSectionTwo, m.markSectionThree, m.markSectionFour, m.markSectionFive, m.markSectionSix, m.markSectionSeven, m.markSectionEight, m.markSectionNine, m.markSectionTen, m.markSectionEleven, m.markSectionTwelve, m.markSectionThirteen FROM MARKING m WHERE m.id = :id", Double.class);
        m.setParameter("id", id.getId()); // Note the getId()
        System.out.println(m);


        List<Object[]> allMarks = m.getResultList();
        double total = 0.0;
        int count = 0;

        for (Object[] marks : allMarks) {
            for (Object mark : marks) {
                total += Double.parseDouble((String) mark);
                ++count;
            }
        }
        return total / (double) count;
    }

它只是从13列中获取值并对其取平均值，但是当它们中有空值时会中断，有没有办法说出任何一列中是否有空值以将总计返回到为0？ 谢谢

Answer 1

您可以为此使用内联条件检查：

    total += Double.parseDouble((String) ((mark==null) ? 0 : mark)));

Answer 2

您也可以在查询中处理此问题：例如：

ij> CREATE TABLE T(I INT);
ij> INSERT INTO T VALUES 1, 2, 3, NULL;
4 rows inserted/updated/deleted
ij> SELECT * FROM T;
I          
-----------
1          
2          
3          
NULL       

4 rows selected
ij> SELECT (CASE WHEN I IS NULL THEN 0 ELSE I END) FROM T;
1          
-----------
1          
2          
3          
0          

4 rows selected

在您的情况下，用SQL计算平均值可能更好

ij> SELECT AVG(CAST(I AS DOUBLE)) AS AVERAGE FROM T;
AVERAGE                 
------------------------
2.0                     
WARNING 01003: Null values were eliminated from the argument of a column function.

1 row selected

注意警告。 如果您实际上希望NULL在平均值中计数为0，则必须使用上述CASE表达式。

Answer 3

将查询更改为以下内容：

SELECT NVL(m.markSectionOne,0), 
       NVL(m.markSectionTwo,0), 
       NVL(m.markSectionThree,0), 
       NVL(m.markSectionFour,0), 
       NVL(m.markSectionFive,0), 
       NVL(m.markSectionSix,0), 
       NVL(m.markSectionSeven,0),
       NVL(m.markSectionEight,0),
       NVL(m.markSectionNine,0), 
       NVL(m.markSectionTen,0),
       NVL(m.markSectionEleven,0), 
       NVL(m.markSectionTwelve,0),
       NVL(m.markSectionThirteen,0) 
FROM MARKING m WHERE m.id = :id;