Data base One To Many Relationship Query - mysql

Question

I have 3 tables in my DB; Transactions, transaction_details, and accounts - basically as below.
transactions :
id
details
by_user
created_at
trans_details :
id
trans_id (foreign key)
account_id
account_type (Enum -[c,d])
amount
Accounts :
id
sub_name
In each transaction each account may be creditor or debtor. What I'm trying to get is an account statement (ex : bank account movements) so I need to query each movement when the account is type = c (creditor) or the account type is = d (debtor)
trans_id, amount, created_at, creditor_account, debtor_account
Update : I tried the following query but i get the debtor column values all Null!
SELECT transactions.created_at,trans_details.amount,(case WHEN trans_details.type = 'c' THEN sub_account.sub_name END) as creditor,
(case WHEN trans_details.type = 'd' THEN sub_account.sub_name END) as debtor from transactions
JOIN trans_details on transactions.id = trans_details.trans_id
JOIN sub_account on trans_details.account_id = sub_account.id
GROUP by transactions.id
After the help of #Jalos I had to convert the query to Laravel which also toke me 2 more hours to convert and get the correct result :) below is the Laravel code in case some one needs to perform such query
I also added between 2 dates functionality
public function accountStatement($from_date,$to_date)
{
$statemnt = DB::table('transactions')
->Join('trans_details as credit_d',function($join) {
$join->on('credit_d.trans_id','=','transactions.id');
$join->where('credit_d.type','c');
})
->Join('sub_account as credit_a','credit_a.id','=','credit_d.account_id')
->Join('trans_details as debt_d',function($join) {
$join->on('debt_d.trans_id','=','transactions.id');
$join->where('debt_d.type','d');
})
->Join('sub_account as debt_a','debt_a.id','=','debt_d.account_id')
->whereBetween('transactions.created_at',[$from_date,$to_date])
->select('transactions.id','credit_d.amount','transactions.created_at','credit_a.sub_name as creditor','debt_a.sub_name as debtor')
->get();
return response()->json(['status_code'=>2000,'data'=>$statemnt , 'message'=>''],200);
}

Answer 1

Your transactions table denotes transaction records, while your accounts table denotes account records. Your trans_details table denotes links between transactions and accounts. So, since in a transaction there is a creditor and a debtor, I assume that trans_details has exactly two records for each transaction:
select transactions.id, creditor_details.amount, transactions.created_at, creditor.sub_name, debtor.sub_name
from transactions
join trans_details creditor_details
on transactions.id = creditor_details.trans_id and creditor_details.account_type = 'c'
join accounts creditor
on creditor_details.account_id = creditor.id
join trans_details debtor_details
on transactions.id = debtor_details.trans_id and debtor_details.account_type = 'd'
join accounts debtor
on debtor_details.account_id = debtor.id;
EDIT
As promised, I am looking into the query you have written. It looks like this:
SELECT transactions.id,trans_details.amount,(case WHEN trans_details.type = 'c' THEN account.name END) as creditor,
(case WHEN trans_details.type = 'd' THEN account.name END) as debtor from transactions
JOIN trans_details on transactions.id = trans_details.trans_id
JOIN account on trans_details.account_id = account.id
GROUP by transactions.id
and it is almost correct. The problem is that due to the group-by MySQL can only show a single value for each record for creditor and debtor. However, we know that there are exactly two values for both: there is a null value for creditor when you match with debtor and a proper creditor value when you match with creditor. The case for debtor is similar. My expectation for this query would have been that MySQL would throw an error because you did not group by these computed case-when fields, yet, there are several values, but it seems MySQL can surprise me after so many years :)
From the result we see that MySQL probably found the first value and used that both for creditor and debtor. Since it met with a creditor match as a first match, it had a proper creditor value and a null debtor value. However, if you write bullet-proof code, you will never meet these strange behavior. In our case, doing some minimalistic improvements on your code transforms it into a bullet-proof version of it and provides correct results:
SELECT transactions.id,trans_details.amount,max((case WHEN trans_details.type = 'c' THEN account.name END)) as creditor,
max((case WHEN trans_details.type = 'd' THEN account.name END)) as debtor from transactions
JOIN trans_details on transactions.id = trans_details.trans_id
JOIN account on trans_details.account_id = account.id
group by transactions.id
Note, that the only change I did with your code is to wrap a max() function call around the case-when definitions, so we avoid the null values, so your approach was VERY close to a bullet-proof solution.
Fiddle: http://sqlfiddle.com/#!9/d468dc/10/0
However, even though your thought process was theoretically correct (theoretically there is no difference between theory and practice, but in practice they are usually different) and some slight changes are transforming it into a well-working code, I still prefer my query, because it avoids group by clauses, which can be useful, if necessary, but here it's unnecessary to do group by, which is probably better in terms of performance, memory usage, it's easier to read and keeps more options open for you for your future customisations. Yet, your try was very close to a solution.
As about my query, the trick I used was to do several joins with the same tables, aliasing them and from that point differentiating them as if they were different tables. This is a very useful trick that you will need a lot in the future.