Oracle plsql 触发器

Question

How do I only allow a user to insert a row into a relational table only once a day?

CREATE OR REPLACE TRIGGER trigger_name
BEFORE INSERT ON Table_name
declare date[type?]
count int := 0;
BEGIN
if(00:00 to sysdate)
END IF
END

Answer 1

Why use a trigger when you can use a constraint?

create table t (
  c1       int,
  ins_date date default sysdate,
  ins_day  date as ( trunc ( ins_date ) ),
  constraint one_per_day unique ( ins_day )
);

insert into t ( c1 )
  values ( 1 );

insert into t ( c1 )
  values ( 2 );

ORA-00001: unique constraint (CHRIS.ONE_PER_DAY) violated

select * from t;

C1    INS_DATE              INS_DAY                
    1 07-NOV-2019 09:31:14  07-NOV-2019 00:00:00 

Answer 2

@ChrisSaxon has the correct idea of creating a constraint instead of trying a trigger. Not only is it cleaner it'll work, whereas a trigger will NOT work (cleanly anyway). Any trigger attempting to enforce this would of necessity need to select from the same table resulting in a mutating table exception (ora-04091) or would need to be a:( compound trigger:(. But we can take Chris' suggestion a step further. Make the column ins_day a virtual column. Also as you indicated "allow a user to insert a row into a relational table only once a day" implies you can have multiple entries per day, just not by the same user. Incorporating these then we arrive at:

create table test_1per_user_day (
  t1pd_id  integer,
  user_id  integer,
  ins_date date default sysdate,
  vir_1per_day  date GENERATED ALWAYS AS (trunc(ins_date)), 
  constraint t1d_one_per_day unique (user_id, vir_1per_day )
);

-- test run individually and stlect * from test_1per_day after each 
alter session set nls_date_format = 'yyyy-mm-dd hh24:mi:ss';
insert into test_1day(t1pd_id, user_id) values (1,1);
insert into test_1day(t1pd_id, user_id, ins_date) values (1,1, sysdate-2);
insert into test_1day(t1pd_id, user_id, ins_date,vir_1per_day) values (1,1, sysdate-1,sysdate);
insert into test_1day(t1pd_id, user_id) values (2,2);
insert into test_1day(t1pd_id, user_id) values (2,2);
update test_1day set vir_1per_day = sysdate;