db2联合，如果为null，则返回零

Question

This unioned query works perfectly but there are some instances where there are no records for one of the three conditions.

All I would like to do is make it so that this at least returns a value of '0' if no records are found.

In other words, if nothing is found for PRIOR, instead of returning

123 | Current | 1
123 | Full    | 2

I would get

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

QUERY:

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

You may rephrase your query to use conditional aggregation with no WHERE clause:

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;

The reason the above version should get around your problem is that it guarantees that a count report will happen for each subquery in the union, as each subquery covers the entire table. The only requirement now is that table1 exists. In your original version, there would need to be at least one record with an ID of 123 , otherwise no records would be returned.

Answer 2

How about putting the values in a single row?

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;

Personally, I would find the data on a single row easier to work with.