Get single row depending of conditional - sql

Question

I have a simple select query with some joins like:
SELECT
[c].[column1]
, [c].[column2]
FROM [Customer] AS [c]
INNER JOIN ...
So I do a left join with my principal table as:
LEFT JOIN [Communication] AS [com] ON [c].[CustomerGuid] = [com].[ComGuid]
this relatioship its 1 to *, one customer can have multiple communications
So in my select I want to get value 1 or 2 depending of condition:
Condition:
if ComTypeKey (from communication) table have a row with value 3 and have another row with vale 4 return 1 then 0
So I try something like:
SELECT
[c].[column1]
, [c].[column2]
, IIF([com].[ComTypeKey] = 3 AND [com].[ComTypeKey] = 4,1,0)
FROM [Customer] AS [c]
INNER JOIN ...
LEFT JOIN [Communication] AS [com] ON [c].[CustomerGuid] = [com].[ComGuid]
But it throws me two rows, beacause there are 2 rows on communication. My desire value is to get only one row with value 1 if my condition is true

Answer 1

If you have multiple rows you need GROUP BY, then count the relevant keys and subtract 1 to get (1, 0)
SELECT
[c].[column1]
, [c].[column2]
, COUNT(CASE WHEN [ComTypeKey] IN (3,4) THEN 1 END) - 1 as FLAG_CONDITION
FROM [Customer] AS [c]
INNER JOIN ...
LEFT JOIN [Communication] AS [com]
ON [c].[CustomerGuid] = [com].[ComGuid]
GROUP BY
[c].[column1]
, [c].[column2]

Answer 2

I'm not really sure I understand.
This will literally find if both values 3 and 4 exist for that CustomerGuid, and only select one of them in that case - not filtering out any record otherwise.
If this is not what you want, providing sample data with the expected result would remove the ambiguity.
SELECT Field1,
Field2,
...
FieldN
FROM (SELECT TMP.*,
CASE WHEN hasBothValues = 1 THEN
ROW_NUMBER() OVER ( PARTITION BY CustomerGuid ORDER BY 1 )
ELSE 1
END AS iterim_rn
FROM (SELECT TD.*,
MAX(CASE WHEN Value1 = '3' THEN 1 ELSE 0 END) OVER
( PARTITION BY CustomerGuid ) *
MAX(CASE WHEN Value1 = '4' THEN 1 ELSE 0 END) OVER
( PARTITION BY CustomerGuid ) AS hasBothValues
FROM TEST_DATA TD
) TMP
) TMP2
WHERE interim_rn = 1