Q:

Select records which not created yet for certain column value

There are two tables:
content_term
    cid | tid
    1 | 1
    1 | 2
    1 | 3
    2 | 4
    2 | 5
    2 | 6

term_group
    tid | gid
    1 | 1
    2 | 2
    3 | 3
    4 | 1
    5 | 2
    6 | 3

How is possible to select "cid" not linked to certain "gid" through "tid"?
UPD
I've tried Adrian's way with grouping by cid:
SELECT cid 
FROM   content_term ct 
WHERE  NOT EXISTS (SELECT tid 
               FROM   term_group tg 
               WHERE  tg.tid = ct.tid 
                      AND tg.gid = 4) GROUP BY cid;

And also tried Adrian's way with grouping by cid:
SELECT cid FROM content_term
    LEFT JOIN term_group ON (content_term.tid = term_group.tid AND gid = 4)
    WHERE gid IS NULL GROUP BY cid;

On "gid" = 3 or any other value - both of them returned:
cid
1
2

A:

This:
SELECT DISTINCT cid FROM content_term
    JOIN term_group ON (content_term.tid = term_group.tid AND gid = 4);

will map all cid's with gid 4. We now require all cids in content_term that are not in this list. So we use a LEFT JOIN between the unique cids and the attached cids we have just retrieved:
SELECT unique_cids.cid FROM ( SELECT DISTINCT cid FROM content_term ) AS unique_cids
    LEFT JOIN ( SELECT DISTINCT cid FROM content_term
       JOIN term_group ON (content_term.tid = term_group.tid AND gid = 4) )
           AS attached_cids ON (unique_cids.cid = attached_cids.cid)
    WHERE attached_cids.cid IS NULL;

Testing:
CREATE TABLE content_term ( cid integer, tid integer );
CREATE TABLE term_group ( tid integer, gid integer );
INSERT INTO content_term VALUES (1,1),(1,2),(1,3),(2,4),(2,5),(2,6);
INSERT INTO term_group VALUES (1,1),(2,2),(3,3),(4,1),(5,2),(6,3);

First try with gid 4:
SELECT unique_cids.cid FROM ( SELECT DISTINCT cid FROM content_term )
   AS unique_cids
LEFT JOIN ( SELECT DISTINCT cid FROM content_term        
JOIN term_group ON (content_term.tid = term_group.tid AND gid = 4)) 
          AS attached_cids ON (unique_cids.cid = attached_cids.cid)
     WHERE attached_cids.cid IS NULL;

+------+
| cid  |
+------+
|    1 |
|    2 |
+------+
2 rows in set (0.00 sec)

Retry with gid 3:
SELECT unique_cids.cid FROM ( SELECT DISTINCT cid FROM content_term ) 
    AS unique_cids
     LEFT JOIN ( SELECT DISTINCT cid FROM content_term
        JOIN term_group ON (content_term.tid = term_group.tid AND gid = 3))
            AS attached_cids ON (unique_cids.cid = attached_cids.cid)
     WHERE attached_cids.cid IS NULL;

Empty set (0.00 sec)

