Execute the SQL query with if statement in Where clause - sql

Question

Need to execute the SQL query with IF statement in Where clause.
SELECT
CASE
WHEN Isnull(CustGuarFlag ,'') = '' THEN DocName
ELSE DocName + ' - '+
CASE
WHEN CustGuarFlag= 'C' THEN 'Customer'
ELSE 'Guarantor'
END
END DocName,
ISNULL(A.DOCSTAGE, '') DOCSTAGE,
ISNULL(A.DOCREFNO,'') DOCREFNO,
ISNULL(CONVERT(VARCHAR, A.DOCREFDT, 103), '') DOCREFDT,
ISNULL(CONVERT(VARCHAR, A.RECDDT, 103), '') RECDDT ,
ISNULL(A.DOCVALUE, 0) DOCVALUE ,
ISNULL(CONVERT(VARCHAR, A.DOCVLDUPTO, 103), '') DOCVLDUPTO,
a.pk_id AS AppDocpk,
b.pk_id AS Docpk,
CASE
WHEN Isnull(a.DocVerifiedStatus, '') = 'null'
THEN ''
ELSE Isnull(a.DocVerifiedStatus,'')
END AS DocVerifiedStatus,
ISNULL(RejRemarks, '') AS RejRemarks,
ISNULL(KYCDocHdr_Fk, 0) AS KYCPk,
ISNULL(DocShrtDescr, '') AS DocShrtDescr
FROM
loln_apprvldoc A WITH(NOLOCK)
JOIN
LGEN_DOCUMENTS B WITH(NOLOCK) ON A.Doc_FK = B.PK_ID
LEFT OUTER JOIN
Lgen_KYCApprvlDoc c ON a.KYCDocHdr_Fk = c.PK_Id
WHERE
A.PRPSLNO = 'KATHU1602080002'
AND ISNULL(A.DocStat, '') = 'R'
AND ISNULL(A.DocVerifiedStatus, '') IN ('Rejection', '', 'null')
AND CASE
WHEN Isnull(c.DocVerifiedStatus, '') = 'A'
THEN c.Pk_id IS NOT NULL
ELSE c.Pk_id IS NULL
END

Answer 1

CASE doesn't belong in a WHERE clause. Replace that CASE clause with
(
(Isnull(c.DocVerifiedStatus,'')='A' AND c.Pk_id IS NOT NULL)
OR
(Isnull(c.DocVerifiedStatus,'')<>'A' AND c.Pk_id IS NULL)
)