Is there more efficient way to write this sql query? - sql

Question

i need to write one efficient query on the 2 tables below, with the conditions follow the tables:
First Table
CityCode CustomerID AccountID
Paris 1 1
Roma 2 1
London 1 2
Paris 3 2
Roma 4 3
Berlin 5 4
Second Table
Credit_card_ind Credit AccountID
0 1000 1
1 5000 2
0 2300 3
1 30000 4
0 - doesn't have card
1 - has a card
In the query we need the following conditions:
1. all customers which their credit is above 5000
2. not showing accounts where one of their customers doesn't have credit card
3. all the accounts where their credit is below 30000
4. not showing accounts where one of their customers is from 'Roma'
5. showing accounts with more than 1 customers.
(*) - there might be no records return.
i wrote the query as follow and i wanted to confirm its the best way to do so, where the intention is to reduce the number of times we approach the tables and doing Joins:
Select c.AccountID,
c.CustomerID
From Customers as c
Join credit_cards as ca on c.AccountID = ca.AccountID
Where ca.credit > 5000 And ca.credit < 30000
And c.AccountID not in (
Select Distinct newTBL.AccountID
From (
Select c1.CustomerID,
c1.AccountID
From customers as c1
Join credit_cards as ca1 on c.AccountID = ca.AccountID
Where ca1.credit_card_ind = 0
Or c1.CityCode like ‘Roma’
) as newTBL
)
And c.AccountID in (
Select newCus.AccountID
From (
Select AccountID,
Count(CustomerID) as [Num_of_Cus]
From customers
Group by AccountID
) as newCus
Where newCus.[Num_of_Cus] > 1
)

Answer 1

First, you can simplify your query by removing the nested subqueries:
Select c.AccountID, c.CustomerID
From Customers c Join
credit_cards ca
on c.AccountID = ca.AccountID
Where ca.credit > 5000 And ca.credit < 30000 And
c.AccountID not in (Select c1.CustomerID
From customers c1 Join
credit_cards ca1
on c.AccountID = ca.AccountID
Where ca1.credit_card_ind = 0 Or c1.CityCode like 'Roma'
) And
c.AccountID in (Select AccountID
From customers
Group by AccountID
Having Count(CustomerID) > 1
);
This may help. You can also write this using window functions, which are probably more efficient. I think the following query captures your original conditions:
select c.AccountID, c.CustomerID
from (select c.*, count(*) over (partition by c.accountid) as cnt,
max(case when c.CityCode like 'Roma' then 1 else 0 end) as cnt_Roma
from customers c
) c join
credit cr
on c.accountid = cr.accountid
where ca.credit > 5000 And ca.credit < 30000 and
c.cnt > 0 and c.cnt_Roma = 0 and
ca.credit_card_ind <> 0;

Answer 2

Try this.
SELECT cs.AccountID
FROM customer cs
JOIN credit cr
ON cs.AccountID = cr.AccountID
WHERE CityCode <> 'Roma'
AND Credit_card_ind = 1
AND Credit > 5000
AND Credit < 30000
GROUP BY cs.AccountID
HAVING Count(cs.CustomerID) > 1