support oracle syntax: sample

It is recommended to implement oracle's sample clause in MariaDB of sql_mode=oracle

SAMPLE [ BLOCK ]
(sample_percent)
[ SEED (seed_value) ]

The sample_clause lets you instruct the database to select from a random sample of data from the table, rather than from the entire table.

• BLOCK instructs the database to attempt to perform random block sampling instead of random row sampling.
• For sample_percent, specify the percentage of the total row or block count to be included in the sample.
• SEED seed_value Specify this to instruct the database to attempt to return the same sample from one execution to the next.

oracle sample clause and example: https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6