How to make increment if same name occurs using triggers? - sql

Question

I have a table:
id
name
amount
If the same name was inserted, then amount should be incremented.
Else, insert with 0 amount.
How to create a trigger with a given condition?

Answer 1

You shouldn't really be doing that. AMOUNT column effectively counts number of NAME appearances in that table. You can always count it, can't you? So, what's the purpose of doing that?
If you want to know which name was inserted prior (or after) some other name (equal to the previous one), sort them by ID (if it is incremental, such as an identity column or if it gets its value from a sequence).
If ID isn't incremental, add DATE_INSERTED column (and sort by it; or apply ROW_NUMBER analytic function which orders values by DATE_INSERTED).
Also, what happens if you delete one of those duplicate names? Will you retroactively decrement AMOUNT column for all previous instances of that NAME?
But, if you insist, here's one option. As you can't just select from a table you're inserting into (because of the mutating table error), I'm using a compound trigger. The part of "insert amount 0" is done by setting the column's default value (you don't need any code for that).
SQL> create table test
2 (id number primary key,
3 name varchar2(20),
4 amount number default 0);
Table created.
SQL> create or replace trigger trg_ai_test
2 for insert on test
3 compound trigger
4
5 type test_rt is record (id test.id%type,
6 name test.name%type);
7 type rli_t is table of test_rt index by pls_integer;
8 g_rli rli_t;
9
10 after each row is
11 begin
12 g_rli(g_rli.count + 1).id := :new.id;
13 g_rli(g_rli.count).name := :new.name;
14 end after each row;
15
16 after statement is
17 l_cnt number;
18 begin
19 for i in 1 .. g_rli.count loop
20 dbms_output.put_Line('x');
21 dbms_output.put_line(i ||' '|| g_rli(i).id ||' '||g_rli(i).name);
22 select count(*) into l_cnt
23 from test
24 where name = g_rli(i).name;
25
26 update test set
27 amount = l_cnt
28 where id = g_rli(i).id;
29 end loop;
30
31 end after statement;
32 end trg_ai_test;
33 /
Trigger created.
SQL>
Testing:
SQL> insert into test (id, name) values (1, 'little');
1 row created.
SQL> insert into test (id, name) values (2, 'foot');
1 row created.
SQL> insert into test (id, name) values (3, 'little');
1 row created.
SQL> insert into test (id, name) values (9, 'little');
1 row created.
SQL> select * from test;
ID NAME AMOUNT
---------- -------------------- ----------
1 little 1
2 foot 1
3 little 2
9 little 3
SQL>
Or, a simpler solution (described above), without using that much code:
SQL> select id, name,
2 row_number() over (partition by name order by id) as amount
3 from test;
ID NAME AMOUNT
---------- -------------------- ----------
2 foot 1
1 little 1
3 little 2
9 little 3
SQL>