case statement in where condition - sql-server-2012

Question

In below script,it is showing 'Incorrect syntax near the keyword 'LIKE''.i am using CASE statement for checking length of F_TEXT_CODE>10. IF it is length>10 then i am using like operator if it is less than 10 i am using '=' operator.
how to achieve this logic in where condition?.
DECLARE
#MANU NVARCHAR(4000)='MANU0071'
BEGIN
SELECT TOP 1000 TP.F_PRODUCT AS ID,
TP.F_PRODUCT_NAME AS NAME
FROM PDF_DETAILS TP
LEFT JOIN V_PROD_ALIAS_MANU MAN ON MAN.F_PRODUCT = TP.F_PRODUCT
WHERE TP.F_PRODUCT<>''AND
(CASE WHEN LEN(MAN.F_TEXT_CODE)>10 THEN
(MAN.F_TEXT_CODE) LIKE #MANU
ELSE MAN.F_TEXT_CODE = #MANU END) AND
(TP.F_CUSTOM5 IS NULL OR TP.F_CUSTOM5 = '')
END

Answer 1

You can't use case like that.
case is an expression that returns a scalar value based on condition(s).
It can't be used as a flow-control element.
The logic you are describing can be accomplished using a combination of or and and:
DECLARE #MANU NVARCHAR(4000)='MANU0071'
BEGIN
SELECT TOP 1000 TP.F_PRODUCT AS ID,
TP.F_PRODUCT_NAME AS NAME
FROM PDF_DETAILS TP
LEFT JOIN V_PROD_ALIAS_MANU MAN ON MAN.F_PRODUCT = TP.F_PRODUCT
WHERE TP.F_PRODUCT<>''
AND
(
(LEN(MAN.F_TEXT_CODE)>10 AND MAN.F_TEXT_CODE LIKE #MANU)
OR
(LEN(MAN.F_TEXT_CODE)<=10 AND MAN.F_TEXT_CODE = #MANU)
)
AND
(TP.F_CUSTOM5 IS NULL OR TP.F_CUSTOM5 = '')
END
However, please note that using like without any wildcards will return the same result as using =, so the entire thing can be written like this
DECLARE #MANU NVARCHAR(4000)='MANU0071'
BEGIN
SELECT TOP 1000 TP.F_PRODUCT AS ID,
TP.F_PRODUCT_NAME AS NAME
FROM PDF_DETAILS TP
LEFT JOIN V_PROD_ALIAS_MANU MAN ON MAN.F_PRODUCT = TP.F_PRODUCT
WHERE TP.F_PRODUCT<>''
AND
AND MAN.F_TEXT_CODE = #MANU
AND
(TP.F_CUSTOM5 IS NULL OR TP.F_CUSTOM5 = '')
END: