Table 「public.manage」

testdb=# \d manage
                   Table "public.manage"
  Column   |     Type     | Collation | Nullable | Default 
-----------+--------------+-----------+----------+---------
 member_id | character(5) |           |          | 
 group_id  | character(4) |           |          | 

testdb=# SELECT * FROM manage ORDER BY member_id;
 member_id | group_id 
-----------+----------
 S0001     | A010
 S0002     | B081
 S0003     | C014
 S0004     | B081
 S0005     | A025
(5 rows)

testdb=# UPDATE manage SET group_id = NULL WHERE group_id LIKE 'A%';
UPDATE 2
testdb=# SELECT * FROM manage;
 member_id | group_id 
-----------+----------
 S0001     | 
 S0002     | B081
 S0003     | C014
 S0004     | B081
 S0005     | 
(5 rows)

testdb=# INSERT INTO manage VALUES('S0006', 'A073'), ('S0007', NULL);
INSERT 0 2
testdb=# SELECT * FROM manage;
 member_id | group_id 
-----------+----------
 S0001     | 
 S0002     | B081
 S0003     | C014
 S0004     | B081
 S0005     | 
 S0006     | A073
 S0007     | 
(7 rows)

testdb=# SELECT count (group_id) FROM manage;
 count 
-------
     4
(1 row)