Get date even if it doesn't exist in table from SQL SELECT statement

问题

I have a table that stores the amount of errors according to what alarm-id it is. The table looks something like this:

|----DATE----|---ALARM_ID---|---COUNTER---|
| 2012-01-01 |      1       |      32     |
| 2012-01-01 |      2       |      28     |
| 2012-01-02 |      1       |      12     |
| 2012-01-02 |      2       |      23     |
| 2012-01-03 |      1       |      3      |
| 2012-01-03 |      2       |      9      |
| 2012-01-05 |      1       |      8      |
| 2012-01-05 |      2       |      1      |
| 2012-01-07 |      1       |      102    |
| 2012-01-07 |      2       |      78     |

Notice the gap between date (2012-01-03 - 2012-01-05) and (2012-01-05 - 2012-01-07). On these dates there isn't any data because the system, that my program is monitoring, haven't reported any errors at that date. What I'm looking for is a SQL SELECT query that returns the total amount of errors on each date, for example:

|----DATE----|---COUNTER---|
| 2012-01-01 |      60     |
| 2012-01-02 |      35     |
| 2012-01-03 |      12     |
| 2012-01-04 |      0      |
| 2012-01-05 |      9      |
| 2012-01-06 |      0      |
| 2012-01-07 |      180    |

I have a query that returns ID's even if they doesn't exist in the table, and if the ID doesn't exist, return the ID anyway with the COUNTER value 0. As such:

        BEFORE                                     AFTER

|---ID---|---COUNTER---|                  |---ID---|---COUNTER---|
|   1    |      2      |                  |   1    |      2      |
|   2    |      6      |                  |   2    |      6      |
|   3    |      1      |       -->        |   3    |      1      |
|   5    |      9      |                  |   4    |      0      |
|   6    |      10     |                  |   5    |      9      |
                                          |   6    |      10     |
                                          |   7    |      0      |
                                          |   8    |      0      |

The query goes like this:

select t.num as ID, coalesce(yt.COUNTER, 0)
from all_stats yt right join 
( select t1.num + t2.num * 10 + t3.num * 100 + t4.num * 1000 as num 
from ( select 1 as num union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9 union select 0 ) t1 cross join 
( select 1 as num union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9 union select 0 ) t2 cross join 
( select 1 as num union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9 union select 0 ) t3 cross join 
( select 1 as num union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9 union select 0 ) t4 ) 
t on yt.ID = t.num 
where (t.num between (select min(ID) from all_stats) and (select max(ID) from all_stats)) order by ID

I can't figure out how I can change this query when it's regarding dates. Can someone please help me on this issue?

I'm using MySQL

Thanks in advance, Steve-O

回答1:

The exact details will depend on the DBMS, and on the nature of the database (e.g., OLAP-oriented vs. OLTP-oriented), but one common general approach is to create an auxiliary calendar table that represents dates as a dimension. Then you can use regular JOINs, rather than having to use complex logic to generate missing dates.

The answers to this StackOverflow question describe how to apply this approach on MySQL.

You can use a similar approach for numbers, by the way, by having a numbers tables; I've never done that myself for numbers, but it seems to be a popular idea; see this dba.stackexchange.com question.

回答2:

If you're using SQL Server 2005 or above you can use a CTE (if not, a loop or other sql technique to populate a table with the dates in the range). Note also there is a limit to the levels of recursion within a CTE.

declare @dateRange table
(
  dateBegin datetime,
  dateEnd datetime
)

insert into @dateRange (dateBegin, dateEnd) 
values ('2012-01-01', '2012-01-07')

;with cte (d)
as (select dateBegin as d
    from @dateRange tbl
    where datediff(day, tbl.dateBegin, tbl.dateEnd) <= 100
    union all
    select dateadd(day, 1, cte.d) as d
    from cte
      inner join @dateRange tbl on cte.d < tbl.dateEnd)

Then get the full results either using the CTE or a temporary table that contains the set of dates in the range:

select cte.d, sum(isnull(e.errorCounter, 0))
from cte
  left outer join @errors e on e.errorDate = cte.d
group by cte.d
order by cte.d

回答3:

You really should handle this at the application layer (ie iterate over the known date range and pull the non-zero vals from the resultset) or fix your table to always include the dates needed if you MUST have a database-centered solution. There's no really good way to generate, on the fly, a set of dates to use in building a continuous date range query.

You can see this for some examples of DB scripting solutions:

Return temp table of continuous dates

But I think you're posing the wrong question. Fix the database to include what you need, or fix how you're generating your report. Databases aren't meant to do interpolation and data generation.

来源：https://stackoverflow.com/questions/9228262/get-date-even-if-it-doesnt-exist-in-table-from-sql-select-statement

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