About SELECT .... FOR UPDATE OF [fields] instructions
CURSOR expression, SET (union, union all, minus, intersect) operators, group_by_clause, or aggregate (count, max, mix, avg) functions, you can not update query return value ; eg:
TABLE: emp
id name salary
==========================
10 aaaa 1234
13 abcd 2313
15 cdue 1245
... ... ...
case 1> use an alias;
select id emp_id, name emp_name, salary emp_salary
from emp
where
for update; - This is not to achieve the purpose of updating;
Can be modified to
select id emp_id, name emp_name, salary
from emp
where
for update of salary - on the salary field is updated;
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