如何进行此查询:一个字段无论如何更新,另一个字段仅在某些情况下更新?

使用CASE将字段更新为新值,或根据条件的值再次设置相同的值。

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

对于不允许低于零的数字,可以使用 GREATEST :

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

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

CREATE PROCEDURE UpdateTable(IN VALUE1 INT, IN VALUE2 INT)
BEGIN 
 IF (VALUE2 > 0) 
  THEN 
    UPDATE SOME_TABLE SET ONE_FIELD=VALUE1, ANOTHER_FIELD=VALUE2;

  ELSE 
    UPDATE SOME_TABLE SET ONE_FIELD=VALUE1; 
 END IF; 
END|