How to make this query: one field update anyway, another field only update in some condition?

Question

updaten some_table set one_field=xxx, another_field=(only update in condition, ex. large then zero then minus one).

Answer 1

Use a CASE to either update the field to a new value or to set the same value again depending on the value of the condition.

UPDATE some_table
SET one_field = 'xxx',
    another_field = CASE WHEN another_field > 0
                         THEN another_field - 1
                         ELSE another_field
                    END

For decrementing a number that is not allowed to go below zero you could use GREATEST:

UPDATE some_table
SET one_field = 'xxx',
    another_field = GREATEST(0, another_field - 1)

Answer 2

update sone_table set one_field=xxx, another_field=IF(another_field > 0, value ,  another_field)