This page from Markus Winand shows that numeric literals with undercore are SQL standard and partially work in some DBMSs:

https://modern-sql.com/caniuse/T662

Some people (including me) write 1 * 1000 * 1000 to make big values readable, but I can't say it's elegant. Underscores seem reasonably easy to implement, so please consider them.

Examples of numbers with underscores compliant with T662 feature of the SQL:2023 standard:

1_000_000

0.000_001

1_2.3_4e5_6

0x_FFFF_FFFF