This is a special sampling method, optimized for bucket tables, as shown in the following example. The SELECT clause specifies the columns to sample data from. The rand() function can also be used when sampling entire rows. If the sample column is also the CLUSTERED BY column, the sample will be more efficient:

-- Sampling based on the whole row
> SELECT name FROM employee_trans
> TABLESAMPLE(BUCKET 1 OUT OF 2 ON rand()) a;
+--------+
| name   |
+--------+
| Steven |
+--------+
1 row selected (0.129 seconds)

-- Sampling based on the bucket column, which is efficient
> SELECT name FROM employee_trans 
> TABLESAMPLE(BUCKET 1 OUT OF 2 ON emp_id) a;
+---------+
| name    |
+---------+
| Lucy    |
| Steven  |
| Michael |
+---------+
3 rows selected (0.136 seconds)