Nested case with multiple sub conditions - sql

Question

I'm having trouble to understand how to nest case statements properly.
(MSSQL Server 2012)
Let's have the following table given.
The Column StatusMissing is what I want to create
+------+--+------+--+------+--+------+--+------+--+------+--+---------------+
| a1 | | a2 | | a3 | | b1 | | c1 | | d2 | | StatusMissing |
+------+--+------+--+------+--+------+--+------+--+------+--+---------------+
| OK | | OK | | OK | | OK | | OK | | OK | | AllOK |
| NULL | | NULL | | OK | | OK | | OK | | OK | | As |
| OK | | NULL | | OK | | OK | | OK | | OK | | As |
| OK | | OK | | NULL | | OK | | OK | | OK | | As |
| OK | | OK | | OK | | NULL | | OK | | OK | | B |
| OK | | OK | | OK | | OK | | NULL | | OK | | C |
| OK | | OK | | OK | | OK | | OK | | NULL | | D |
| NULL | | OK | | OK | | NULL | | NULL | | OK | | ABC |
| NULL | | OK | | OK | | OK | | NULL | | NULL | | ACD |
| NULL | | OK | | OK | | NULL | | OK | | NULL | | ABD |
| NULL | | OK | | OK | | NULL | | NULL | | NULL | | ABCD |
| NULL | | OK | | OK | | OK | | NULL | | NULL | | ACD |
| OK | | OK | | OK | | NULL | | NULL | | OK | | BC |
| OK | | OK | | OK | | OK | | OK | | OK | | AllOK |
| OK | | NULL | | OK | | OK | | NULL | | OK | | AC |
| OK | | OK | | OK | | NULL | | OK | | NULL | | BD |
| OK | | OK | | OK | | OK | | NULL | | NULL | | CD |
+------+--+------+--+------+--+------+--+------+--+------+--+---------------+
First, to understand the concept of nesting I simplified the table:
+------+--+------+--+------+
| a1 | | a2 | | b1 |
+------+--+------+--+------+
| OK | | OK | | OK |
| OK | | OK | | NULL |
| OK | | NULL | | OK |
| NULL | | OK | | OK |
| NULL | | NULL | | OK |
| NULL | | OK | | NULL |
| OK | | NULL | | NULL |
+------+--+------+--+------+
These attempts lead to these failures.
Query1
SELECT a1, a2, b1 'StatusMissing' =
CASE
WHEN a1 IS NULL
THEN
CASE
WHEN a1 IS NULL
THEN
CASE
WHEN b1 IS NULL
THEN 'AB'
END
ELSE 'A'
END
WHEN b1 IS NULL
THEN 'B'
ELSE 'AllOK'
END
FROM Table;
Result1:
+------+--+------+--+------+--+---------------+
| a1 | | a2 | | b1 | | StatusMissing |
+------+--+------+--+------+--+---------------+
| OK | | OK | | OK | | AllOK |
| OK | | OK | | NULL | | B |
| OK | | NULL | | OK | | AllOK |
| NULL | | OK | | OK | | NULL |
| NULL | | NULL | | OK | | NULL |
| NULL | | OK | | NULL | | AB |
| OK | | NULL | | NULL | | B |
+------+--+------+--+------+--+---------------+
Query2 (Else as main)
SELECT a1, a2, b1, 'Status' =
CASE
WHEN a1 IS NOT NULL AND a2 IS NOT NULL AND b1 IS NOT NULL
THEN 'AllOK!'
ELSE
CASE
WHEN a2 IS NOT NULL OR a2 IS NOT NULL
THEN
CASE
WHEN b1 IS NULL
THEN 'AB'
END
WHEN b1 IS NULL
THEN 'B'
ELSE 'A'
END
END
FROM Table;
Result2
+------+--+------+--+------+--+---------------+
| a1 | | a2 | | b1 | | StatusMissing |
+------+--+------+--+------+--+---------------+
| OK | | OK | | OK | | AllOK |
| OK | | OK | | NULL | | AB |
| OK | | NULL | | OK | | A |
| NULL | | OK | | OK | | NULL |
| NULL | | NULL | | OK | | A |
| NULL | | OK | | NULL | | AB |
| OK | | NULL | | NULL | | B |
+------+--+------+--+------+--+---------------+
What the hell am I doing wrong?
I'm quite new to SQL, so if there is a proper function to do this I would appreciate the info!
EDIT:
If something like this would be possible in SQL i mean:
Column StatusMissing = ' missing'
If(a1 == NULL) { StatusMissing += 'A'}
EDIT2:
The column StatusMissing IS NOT THERE!
I want to create it using the SQL statements like below.
SELECT .... Status =
So basically I only have A1,A2,B1 (in the simple table). Please don't get confused with the first Table. It's only there to SHOW HOW IT SHOULD look like.

Answer 1

For the simplified table, assuming data type to be nvarchar.
Try using UPDATE-
UPDATE [dbo].[StatusMissing]
SET result='';
UPDATE [dbo].[StatusMissing]
SET result= CONCAT(result , 'A')
WHERE a1 is null or a2 is null;
UPDATE [dbo].[StatusMissing]
SET result= CONCAT(result , 'B')
WHERE b1 is null ;
UPDATE [dbo].[StatusMissing]
SET result= 'AllOK'
WHERE result ='';
This can be done in one step as well.

Answer 2

I might suggest that you make two small modifications to your output:
Instead of "As", just say "A".
Instead of "AllOK", just leave the field blank.
With these modifications, the rules are pretty easy:
select t.*,
((case when a1 is null or a2 is null or a3 is null then 'A' else '' end) +
(case when b1 is null then 'B' else '' end) +
(case when c1 is null then 'C' else '' end) +
(case when d1 is null then 'D' else '' end)
) as StatusMissing
from table t;
If you do want your version, a subquery is perhaps the easiest way:
select t. . . .,
(case when StatusMissing = '' then 'AllOK'
when StatusMissing = 'A' then 'As'
else StatusMissing
end) as StatusMissing
from (select t.*,
((case when a1 is null or a2 is null or a3 is null then 'A' else '' end) +
(case when b1 is null then 'B' else '' end) +
(case when c1 is null then 'C' else '' end) +
(case when d1 is null then 'D' else '' end)
) as StatusMissing
from table t
) t

Answer 3

You can play with COALESCE and a couple of CASE conditions
SELECT a1,
a2,
a3,
b1,
c1,
d2,
COALESCE(
CASE WHEN
b1 = 'OK'
AND c1 = 'OK'
AND d2 = 'OK'
AND (a1 IS NULL OR a2 IS NULL OR a3 is NULL)
THEN 'As'
ELSE ''
END,
CASE WHEN
(a1 IS NULL OR a2 IS NULL or a3 is NULL)
THEN 'A'
END
+ CASE WHEN
b1 IS NULL
THEN 'B'
ELSE ''
END
+ CASE WHEN
c1 IS NULL
THEN 'C'
ELSE ''
END
+ CASE WHEN
d2 IS NULL
THEN 'D'
ELSE ''
END,
'AllOK') AS 'StatusMissing'
FROM Table;