DATABASE()

Returns the default (current) database name as a string in the utf8 character set. If there is no default database, DATABASE() returns NULL.Within a stored routine, the default database is the database that the routine is associated with, which is not necessarily the same as the database that is the default in the calling context.

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test0913   |
+------------+

Within a stored routine, the default database is the database that the routine is associated with.

mysql> DELIMITER $$
mysql> CREATE PROCEDURE `test0912`.`test_select`()
    -> BEGIN
    ->   SELECT DATABASE();
    -> END;
    -> 
    -> CALL `test0912`.`test_select`;
    -> $$
Query OK, 0 rows affected (0.00 sec)

+------------+
| DATABASE() |
+------------+
| test0912   |
+------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

if there is no database, DATABASE() returns NULL.

mysql> select schema();
+----------+
| schema() |
+----------+
| NULL     |
+----------+