如何在SQL的WHERE中使用COALESCE？

Question

In the scenario where I have a dataset like so, and I need a Coalesce as to speak, within the WHERE clause.

The requirement is to filter where if there is a row with Type Actual, filter out the type of Expected. Please see dataset below which should make things clearer

Table Parent Record (GUID), Type (text), Date (datetime)

Actual Data

1. 100, ABC Active, 01/01/2010
2. 100, ABC Draft, 01/01/2010
3. 100, DEF Draft, 01/01/2010
4. 100, GHI Active, 01/01/2010
5. 100, GHI Draft, 01/01/2010

Expected Results

1. 100, ABC Active, 01/01/2010
2. 100, DEF Draft, 01/01/2010
3. 100, GHI Active, 01/01/2010

As you can see, where there is a row with type Active , for a certain type, then the Draft is removed. If there is no type with an Active row, then just show the Draft .

could someone please help with the WHERE clause for this

Answer 1

I think you need conditional ordering :

select top (1) with ties t.*
from table t
order by row_number() over (partition by ParentRecord, col
                            order by (case when Type = 'Active'
                                           then 1 
                                           else 2
                                      end)
                           );

This assumes type column doesn't have a other data like you have posted.

Answer 2

You want filtering . . . but I don't see where coalesce() comes in. One way to do what you want is:

select t.*
from t
where t.type = 'Active'
union all
select t.*
from t
where t.type = 'Draft' and
      not exists (select 1 from t t2 where t2.guid = t.guid);

You can express this in a single query as:

select t.*
from t
where (t.type = 'Active') or
      (t.type = 'Draft' and
       not exists (select 1 from t t2 where t2.guid = t.guid)
      );

Answer 3

It would be simpler if the 'Active' and 'Draft' status were stored in a separate column since this would avoid taking sub-strings in the Type column. There's actually not much in the WHERE clause.

Using a temp table for example:

DECLARE @temp TABLE (GUID int, Type varchar(100), Date datetime)
INSERT INTO @temp VALUES
 (100, 'ABC Active', '01/01/2010'), (100, 'ABC Draft', '01/01/2010')
,(100, 'DEF Draft', '01/01/2010'), (100, 'GHI Active', '01/01/2010')
,(100, 'GHI Draft', '01/01/2010'), (100, 'JKL Active', '01/01/2010')

SELECT T.GUID
      ,COALESCE( (SELECT T2.Type
                    FROM @temp T2
                   WHERE T2.GUID = T.GUID
                     AND T2.Type LIKE '%Active'
                     AND LEFT(T.Type, 3) = LEFT(T2.Type, 3)
                  ) 
                   , T.Type
                ) AS [Type]
      ,T.Date
  FROM @temp T
 WHERE T.Type LIKE '%Draft'

Produces the expected output:

GUID    Type         Date
100     ABC Active   2010-01-01 00:00:00.000
100     DEF Draft    2010-01-01 00:00:00.000
100     GHI Active   2010-01-01 00:00:00.000