I WANT TO USE CASE WHEN COMMAND WITH THREEN CONTIATION PLZ GIVE U R SUGGESTIONTHIS IS MY QUERY BUT SHOW ING ERROR : 

SELECT DISTINCT A.PHONE, OTHER,STARTTIME,duration,imeinumber,imsinumber,CALL_TYPE,a.provider_key, A.CELLTOWERID, SITEADDRESS,
LAT, LONG, AZIMUTH FROM cdat A
LEFT JOIN CDATDUPL.DBO.CDATCELLTOWERAREANEW B ON case when a.provider_key!='9' then a.tower_key=b.TOWER_KEY else 
A.celltowerid = B.CELLTOWERID
AND A.PROVIDER_key=B.PROVIDER_KEY and a.state_key end=b.state_key 