ORA-00905: missing keyword error in case after where clause - sql

Question

I have the following query which is giving error ORA-00905: missing keyword. I've not been able to find the syntax despite continuous efforts for last few hours. Please help.
SELECT a.DOCUMENT_CATEGORY,
a.template_id,
a.category_id,
a.REVIEW_CATEGORY,
a.WITH_BIDS,
a.WITH_FINAL_DOCUMENTS,
b.divn_id,
b.deptt_id,
a.vdr_id,
C.DEPARTMENT,
a.TEMPLATE_TITLE
FROM DCTM_VDR_REF_DTLS a, DCTM_VDR_REF_MASTER b, VW_DIVN_DIR c
WHERE b.DIVN_ID = c.DIVN_CODE
AND b.DEPTT_ID = c.SECTN_CODE
AND a.vdr_id = b.vdr_id
AND (b.REFERENCE_NUMBER, b.APPROVED_ON) IN
( SELECT MAX (REFERENCE_NUMBER), MAX (APPROVED_ON)
FROM DCTM_VDR_REF_MASTER
WHERE REFERENCE_NUMBER =
(SELECT DISTINCT
NVL (TRIM (MR_NUMBER), TRIM (TENDER_NO))
FROM EILEDMS.EIL_DOCUMENT_SV#EDMS_DBLINK
WHERE object_name =
'A307-0IC-JA-MR-7960-1030-157-FOA'
AND r_object_type =
'eil_foa_order_pr_doc'
AND ( title = 'FOA'
OR title = 'DRAFT FOA'))
AND APPROVED_ON IS NOT NULL
GROUP BY DIVN_ID, DEPTT_ID)
AND REVIEW_CATEGORY <> 'Delete Category'
AND (CASE (SELECT IS_SCHEDULE_LOCKED
FROM DCTM_VENDOR_SCHEDULE
WHERE SCH_ID = 359)
WHEN 0
THEN
1
WHEN 1
THEN
(a.template_id || '-' || a.category_id) IN
(SELECT template_id || '-' || category_id
FROM DCTM_VENDOR_SCH_UNLOCK_DTLS
WHERE APPROVAL = 'Y'
AND APPROVAL_UPTO >= SYSDATE
AND CONSUMED = 0
AND sch_ID = 359)
END) = 1
ORDER BY c.DEPARTMENT ASC,
a.TEMPLATE_ID,
a.SORT_ORDER,
a.DOCUMENT_CATEGORY ASC
Can't we use IN clause inside a THEN statement?

Answer 1

Now that you've edited your question, it looks like you are simply trying to look up category_id and template_id in DCTM_VENDOR_SCH_UNLOCK_DTLS. Does the following work for you?
then
(
SELECT COUNT(*) -- 1 if found, 0 otherwise
FROM DCTM_VENDOR_SCH_UNLOCK_DTLS
WHERE APPROVAL = 'Y'
AND APPROVAL_UPTO >= SYSDATE
AND CONSUMED = 0
AND sch_ID = 359
AND template_id = a.template_id
AND category_id = a.category_id
AND rownum = 1
)

Answer 2

This is not actually about an IN clause after WHERE being allowed or not. The expression
a.category_id IN (SELECT ...)
evaluates to TRUE or FALSE. Your statement
a.template_id || '-' || a.category_id IN (SELECT ...)
tries to concatenate that TRUE or FALSE with a.template_id and a minus sign. This is not possible, as there is no boolean type in Oracle SQL. Think it over what you actually want to concatenate.
EDIT: Now that you set parentheses, you compare a string with another string resulting from a select statement. Fine so far. But still: All this evaluates to a boolean, not a number. Your first then results in a number (1), your second in a boolean (TRUE or FALSE). Oracle SQL has no boolean type, so your expression makes no sense to the parser and you get a syntax error.