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SQL> wbconnect h2;
Connection to "User=sa, Catalog=H2TEST, URL=jdbc:h2:c:/Daten/db/h2/h2test" successful
Execution time: 1.55s
SA@H2TEST/PUBLIC> create table foo (id integer, some_time timestamp);
Table 'foo' created
Execution time: 0.02s

SA@H2TEST/PUBLIC> insert into foo (id) values (1);
INSERT executed successfully
1 row(s) affected.
Execution time: 0s

SA@H2TEST/PUBLIC> ALTER TABLE FOO ALTER COLUMN some_time SET DEFAULT current_timestamp;
ALTER executed successfully
Execution time: 0s

SA@H2TEST/PUBLIC> insert into foo (id) values (2);
INSERT executed successfully
1 row(s) affected.
Execution time: 0s

SA@H2TEST/PUBLIC> select * from foo;
ID | SOME_TIME
---+------------------------
1  | (null)
2  | 2013-01-08 21:49:33

(2 Rows)
SELECT executed successfully
Execution time: 0.09s
SA@H2TEST/PUBLIC>