You can use it, but as a result of your subquery
SELECT * FROM
(
SELECT [column1] as 'K1' , ISNULL([column2], 'else') + ' ' + ISNULL([column3], 'else') as 'K2'
FROM [table] full join [table] on A.Name = C.AccountIdName where A.Name IS NOT NULL
)
res WHERE res.K1 IS NOT NULL
UPDATED
SELECT * FROM
(
SELECT [column1] as 'K1' , ISNULL([column2], 'else') + ' ' + ISNULL([column3], 'else') as 'K2'
FROM [table] full join [table] on A.Name = C.AccountIdName where A.Name IS NOT NULL
)
res1 WHERE res1.K1 IS NOT NULL AND res1.K3 IS NOT NULL
UNION ALL
SELECT * FROM
(
SELECT [column1] as 'K1' , ISNULL([column2], 'else') + ' ' + ISNULL([column3], 'else') as 'K2'
FROM [table] full join [table] on A.Name = C.AccountIdName where A.Name IS NOT NULL
)
res2 WHERE res2.K2 IS NOT NULL AND res2.K4 IS NOT NULL //BUT DOES IT HAS K4???