set @a = 1;
select * 
from Mans /* force index (primary) */
where eManID = (select 1 from dual where @a)

EXPLAIN Result
id select_type           table  type   possible_keys key    key_len ref    rows    Extra
1  PRIMARY               Mans   ALL    (NULL)        (NULL) (NULL)  (NULL) 1814845 Using where
2  UNCACHEABLE SUBQUERY  (NULL) (NULL) (NULL)        (NULL) (NULL)  (NULL) (NULL)  No tables used

select '' REGEXP concat('(',field) from table

select '' REGEXP concat('(','1') from table

mysql> SELECT CONCAT('a', 'b', NULL, 'c');
    -> NULL

mysql> SELECT CONCAT_WS('', 'a', 'b', NULL, 'c');
    -> abc