MySQL: Is there a better way to do this query?
if add indexes:

CREATE TABLE `table_b` (
  `S_ID` int(11) DEFAULT NULL,
  `C_ID` int(11) DEFAULT NULL,
  KEY `S_ID` (`S_ID`,`C_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
CREATE TABLE `table_a` (
  `id` int(11) DEFAULT NULL,
  `S_ID` int(11) DEFAULT NULL,
  `C_ID` int(11) DEFAULT NULL,
  KEY `S_ID` (`S_ID`,`C_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci


and little modify the query:

set @v_npref = 50;

SELECT ca.id, ca.S_ID, ca.C_ID
FROM TABLE_B AS clc 
JOIN TABLE_A AS ca ON (clc.S_ID = ca.S_ID AND clc.C_ID = @v_npref )
WHERE
ca.C_ID IS NOT NULL OR NOT EXISTS (SELECT id FROM TABLE_A ca2
                                 WHERE ca2.S_ID = ca.S_ID AND ca2.C_ID = @v_npref )

group by ca.S_ID;


we can compare plan of new and original queries,
original:

modified:


this is just my variant, what better 2 FULL scan (as You can see total number of scanned rows bigger) or file sort at the final possible decide only on real data (depend from size of sorted data)