db2联合，如果为null，则返回零

Question

此联合查询工作完美，但是在某些情况下，这三个条件之一没有记录。

我要做的就是使它至少在找不到记录的情况下至少返回值“ 0”。

换句话说，如果没有找到PRIOR，则返回

123 | Current | 1
123 | Full    | 2

我会得到

123 | PRIOR   | 0
123 | Current | 1
123 | Full    | 2

查询：

select ID, 'PRIOR' as Range, count(*) as count
    from table1
where ID = 123
AND date between '2017-01-01' and '2017-04-13'
group by ID
union all
select ID, 'CURRENT' as Range, count(*) as count
    from table1
where ID = 123
AND date between '2018-01-01' and '2018-04-13'
group by ID
union ALL
select ID, 'FULL' as Range, count(*) as count
    from table1
where ID = 123
AND date between '2017-01-01' and '2017-12-31'
group by ID;

Answer 1

您可以重新定义查询以使用不带WHERE子句的条件聚合：

SELECT
    ID, 'PRIOR' AS Range,
    COUNT(CASE WHEN date BETWEEN '2017-01-01' AND '2017-04-13' AND ID = 123
               THEN 1 END) AS count
FROM table1
UNION ALL
SELECT
    ID, 'CURRENT' AS Range,
    COUNT(CASE WHEN date BETWEEN '2018-01-01' AND '2018-04-13' AND ID = 123
               THEN 1 END) AS count
FROM table1
UNION ALL
SELECT
    ID, 'FULL' AS Range,
    COUNT(CASE WHEN date BETWEEN '2017-01-01' AND '2017-12-31' AND ID = 123
               THEN 1 END) AS count
FROM table1;

上面的版本应该解决您的问题的原因是，它保证了联合中的每个子查询都会生成计数报告，因为每个子查询都覆盖了整个表。 现在唯一的要求是table1存在。 在您的原始版本中，至少需要有一条ID为123记录，否则将不返回任何记录。

Answer 2

如何将值放在一行中？

select ID,
       sum(case when date between '2017-01-01' and '2017-04-13' then 1 else 0 end) as prior,
       sum(case when date between '2018-01-01' and '2018-04-13' then 1 else 0 end) as current,
       sum(case when date between '2017-01-01' and '2017-12-31' then 1 else 0 end) as full
from table1
where ID = 123
group by ID;

就个人而言，我会发现单行中的数据更易于使用。