Sql: How to connect tables by just using the ID - mysql

Question

so I have this problem: I have 3 different tables that need to be connected to one another. It's rather complex and my english skills aren't that good. Let me show you:
T1 T2 T3
ID_T1 |Name | |ID_T1|ID_T2| |ID_T2|Spent|
1 |James| | 1| 4| | 4 | 200 |
2 |Mike | | 2| 5| | 5 | 300 |
3 |Alex | | 3| 6| | 6 | 400 |
Basically I want to connect in Table T3 the amount spent with the names in T1 by using the ID I have in T2.
So I should "read" that James spent 200 dollars, Mike spent 300 and Alex 400.
Another thing to know is that the IDs are generated automatically, so I'm not supposed to be able to see them.
Do I have to pay attention on something when I create my Tables, or do I have to focus more on the INSERT INTO command (I work with Oracle sql Developer)?
Thank you very much! :)

Answer 1

#Burkinaboy might wanna try this
SELECT
T1.Name,
T3.SPent
FROM T1 t0 RIGHT JOIN T2 t1 ON T1.ID_T1 = T2.ID_T1 RIGHT JOIN T3 t2 ON T3.ID_T2=T2.ID_T2

Answer 2

This should give you an expected output.
Select a.Name, c.spend
From T1 a, T2 b, T3 c
Where a.Id_T1 = b.Id_T1
AND b.Id_T2 = c.Id_T2
Regarding your second question, you should know the relation between the tables meaning how they are connected.
Updating the answer as per your comments . Assuming that amount spend column you are going to hard code..
Select b.Id_T2 , "300"
From T1 a ,T2 b
Where a.Id_T1 = b.Id_T1
But in above case , all the values for column spend would be 300.

Answer 3

SELECT T1.name, T3.spent FROM T1, T3 WHERE T1.ID_T1=T2.ID_T1 AND T2.ID_T2=T3.ID_T2

Answer 4

select t1.name as name, t3.spent as spent
from t1
join t2 on t2.id_t1 = t1.id_t1
join t3 on t3.id_t2 = t2.id_t2
Use join instead of putting all tables in FROM clause.

Answer 5

Assuming you are inserting every spending in T3 table with a new ID, you can join the three tables and do a group by on it:
select
T1.ID_T1 ID,
T1.NAME,
SUM(T3.SPENT) SPENT
from T1 inner join T2 on T1.ID_T1 = T2.ID_T1
inner join T3 on T2.ID_T2 = T3.ID_T2
GROUP BY T1.ID_T1, T1.NAME;
As of now, It's not clear which database you're using exactly. (I assumed that it's Oracle as you're using Oracle SQL Developer. Similar solution should be available in other DBMS as well in case it's not.).
First, Let's create the tables:
create table t1 (
ID_T1 integer primary key,
Name varchar2(100)
);
create table t2 (
ID_T1 integer,
ID_T2 integer,
constraint pk primary key(ID_T1,ID_T2)
);
create table t3 (
ID_T2 integer primary key,
spent number(10,2)
);
Then create two sequences that'll be used while inserting:
create sequence t1_seq start with 1 increment by 1;
create sequence t2_seq start with 1 increment by 1;
Then a view which will display spendings per person:
create or replace view spendings
as
select
T1.ID_T1 ID,
T1.NAME,
SUM(T3.SPENT) SPENT
from T1 inner join T2 on T1.ID_T1 = T2.ID_T1
inner join T3 on T2.ID_T2 = T3.ID_T2
GROUP BY T1.ID_T1, T1.NAME;
Then create an INSTEAD OF trigger on the above view to make the proper inserts in underlying tables:
create or replace trigger spendings_trig
instead of INSERT or UPDATE or DELETE on SPENDINGS
for each row
declare
v_ID_T1 integer;
v_ID_T2 integer;
v_name t1.name%type;
begin
if not inserting then
raise_application_error(-20001,'Not supported');
end if;
begin
if :new.ID is null then
v_ID_T1 := T1_SEQ.NEXTVAL;
insert into T1 (ID_T1, NAME) values (v_ID_T1, :new.NAME);
end if;
select ID_T1,NAME into v_ID_T1,v_name from T1
where ID_T1 = :new.ID;
if :new.name is not null and v_name <> :new.name then
raise_application_error(-20002,'Incorrect name entered');
end if;
exception
when no_data_found then
v_ID_T1 := :new.ID;
insert into T1 (ID_T1, NAME) values (v_ID_T1, :new.NAME);
end;
v_ID_T2 := T2_SEQ.NEXTVAL;
insert into T2 values (v_ID_T1, v_ID_T2);
insert into T3 values (v_ID_T2, :new.spent);
end;
/
Now if you insert into the view like this (assuming there is no data in any of the tables):
insert into spendings (name, spent) values ('James',100);
T1 will get a row
ID_T1 NAME
------------
1 James
T2 will get a row
ID_T1 ID_T2
------------
1 1
T3 will get a row
ID_T2 SPENT
-------------
1 100
Now if you do:
insert into spendings (id,spent) values (1,100);
T1 will not be inserted as there is already row present for ID = 1
T2 will get a row:
ID_T1 ID_T2
------------
1 1
1 2
T3 will get a row
ID_T2 SPENT
-------------
1 100
2 100
And
select * from spendings;
will show:
ID NAME SPENT
------------------
1 James 200