About SELECT .... For update of [fields] usage

If the SELECT statement uses Alias, DISTINCT operator, CURSORExpression, SET (union, union all, minus, intersect) operators, Group_by_clause, Or aggregate (count, max, mix, avg) fungate, the query return value cannot be updated; eg: TABLE: emp id name salary ==================== 10 aaaa 123413 abcd 231315 cdue 1245... ...... case 1> use an alias; select id emp_id, name emp_name, salary emp_salaryfrom empwhere id = 10for update; -- this means that the update cannot be achieved; you can change it to select id emp_id, name emp_name, salary from empwhere id = 10for update of salary -- update the salary field;