I got 3 relations
customer (customer_name, city)
loan (loan_number, branch_name, amount)
borrower (customer_name, loan_number)
Find customer name, city and their loan amounts.
First query:
SELECT c.customer_name, c.city, l.amount
FROM borrower b
JOIN loan l USING (loan_number)
JOIN customer c USING (customer_name)
Second query:
SELECT c.customer_name, c.city, l.amount
FROM customer c
JOIN borrower b USING (customer_name)
JOIN loan l USING (loan_number)
Will the two queries produce same result?
In inner joins (as opposed to outer joins), the order of tables does not matter.
Short Answer: No. The order doesn't matter for INNER JOIN (and hence for theta join).
Long Answer:
INNER JOIN (or simply JOIN) between two or more tables gives you the intersection of the tables. Think of tables as sets and intersection of the tables is the common area of the sets where there in an overlap of data. No matter where you place the sets, what's common between Set A, Set B and Set C (table A, table B & table C), won't change. Hence, order of tables doesn't matter for INNER JOIN.
When you do LEFT OUTER JOIN between two tables (two sets), you are not just taking the intersection of the two sets. You are actually taking the entire of the set (table) you placed on the left hand side. Similarly, for RIGHT OUTER JOIN, you are taking the entire of set (table) you placed on the right hand side. Hence, for LEFT and RIGHT OUTER JOIN, order of tables does matter.
For FULL OUTER JOIN, you are essentially taking not just the intersection, not just the table on the left or right, but the whole of two tables (or sets). Hence, order doesn't matter again for FULL OUTER JOIN.
Related
Suppose I have three tables here one of them is a Country table second one is a city table and the third one is a customer table, and I am writing a query here like this
SELECT country.country_name_eng, city.city_name, customer.customer_name
FROM Country
LEFT JOIN city ON city.country_id = country.id
LEFT JOIN customer ON customer.city_id = city.id
I want to know here who is the left table of the customer table and who is the left table of the city table as well as who is the right table of the country table and who is the right table of the customer table
'Left' is always the table at the left of the JOIN keyword, no matter if left, right, inner or full outer join, analogously 'right'.
Left and right joins, though, have different semantics:
With a normal (inner) join a data row is only added if there's a matching entry in both tables, e.g. if you JOIN ON l.id = r.id then if l contains an id with value 7 a row for is only added if there's a matching id with value 7 in r as well (and vice versa).
A left (outer) join instead adds a row for id 7 in l even if there's no matching id in r – then the corresponding columns for table r are filled with nulls.
Analogously a right (outer) join, all entries of r are added with nulls for l's columns if no matching entry found.
A full outer join is a combination of left and right outer join, any entry of l and r is used with nulls on either side for the counter part if none exists.
In your concrete query you'd list all countries even if there don't exist any cities – and if they do exist, for each country all of them even if there don't exist any customers there. In the latter case the columns for the customer are all filled with null values, and if no city exists either, additionally all city columns.
I have 2 tables-one customers, one transactions. One customer does not have any transactions. How do I handle that? As I'm trying to join my tables, the customer with no transaction does not show up as shown in code below.
SELECT Orders.Customer_Id, Customers.AcctOpenDate, Customers.CustomerFirstName, Customers.CustomerLastName, Orders.TxnDate, Orders.Amount
FROM Orders
INNER JOIN Customers ON Orders.Customer_Id=Customers.Customer_Id;
I need to be able to account for the customer with no transaction such as querying for least transaction amount.
Use below updated query - Right Outer join is used instead of Inner join to show all customers regardless of the customer placed an order yet.
SELECT Orders.Customer_Id, Customers.AcctOpenDate,
Customers.CustomerFirstName, Customers.CustomerLastName,
Orders.TxnDate, Orders.Amount
FROM Orders
Right Outer JOIN Customers ON Orders.Customer_Id=Customers.Customer_Id;
INNER Joins show only those records that are present in BOTH tables
OUTER joins gets SQL to list all the records present in the designated table and shows NULLs for the fields in the other table that are not present
LEFT OUTER JOIN (the first table)
RIGHT OUTER JOIN (the second table)
FULL OUTER JOIN (all records for both tables)
Get up to speed on the join types and how to handle NULLS and that is 90% of writing SQL script.
Below is the same query with a left join and using ISNULL to turn the amount column into 0 if it has no records present
SELECT Orders.Customer_Id, Customers.AcctOpenDate, Customers.CustomerFirstName, Customers.CustomerLastName
, Orders.TxnDate, ISNULL(Orders.Amount,0)
FROM Customers
LEFT OUTER JOIN Orders ON Orders.Customer_Id=Customers.Customer_Id;
try this :
SELECT Orders.Customer_Id, Customers.AcctOpenDate, Customers.CustomerFirstName, Customers.CustomerLastName, Orders.TxnDate, Orders.Amount
FROM Orders
Right OUTER JOIN Customers ON Orders.Customer_Id=Customers.Customer_Id;
I strongly recommend LEFT JOIN. This keeps all rows in the first table, along with matching columns in the second. If there are no matching rows, these columns are NULL:
SELECT c.Customer_Id, c.AcctOpenDate, c.CustomerFirstName, c.CustomerLastName,
o.TxnDate, o.Amount
FROM Customers c LEFT JOIN
Orders o
ON o.Customer_Id = c.Customer_Id;
Although you could use RIGHT JOIN, I never use RIGHT JOINs, because I find them much harder to follow. The logic of "keep all rows in the first table I read" is relatively simple. The logic of "I don't know which rows I'm keeping until I read the last table" is harder to follow.
Also note that I included table aliases and change the CustomerId to come from customers -- the table where you are keeping all rows.
Using CASE will replace "null" with 0 then you can sum the values. This will count customers with no transactions.
SELECT c.Name,
SUM(CASE WHEN t.ID IS NULL THEN 0 ELSE 1 END) as TransactionsPerCustomer
FROM Customers c
LEFT JOIN Transactions t
ON c.Name = t.customerID
group by c.Name
SELECT c.Name,
SUM(CASE WHEN t.ID IS NULL THEN 0 ELSE 1 END) as numberoftransaction
FROM customers c
LEFT JOIN transactions t
ON c.Name = t.customerID
group by c.Name
I remember this rule of thumb from back in college that if you put a left join in a SQL query, then all subsequent joins in that query must also be left joins instead of inner joins, or else you'll get unexpected results. But I don't remember what those results are, so I'm wondering if maybe I'm misremembering something. Anyone able to back me up on this or refute it? Thanks! :)
For instance:
select * from customer
left join ledger on customer.id= ledger.customerid
inner join order on ledger.orderid = order.id -- this inner join might be bad mojo
Not that they have to be. They should be (or perhaps a full join at the end). It is a safer way to write queries and express logic.
Your query is:
select *
from customer c left join
ledger l
on c.id = l.customerid inner join
order o
on l.orderid = o.id
The left join says "keep all customers, even if there is no matching record in ledger. The second says, "I have to have a matching ledger record". So, the inner join converts the first to an inner join.
Because you presumably want all customers, regardless of whether there is a match in the other two tables, you would use a left join:
select *
from customer c left join
ledger l
on c.id = l.customerid left join
order o
on l.orderid = o.id
You remember correctly some parts of it!
The thing is, when you chain join tables like this
select * from customer
left join ledger on customer.id= ledger.customerid
inner join order on ledger.orderid = order.id
The JOIN is executed sequentialy, so when customer left join ledger happens, you are making sure all joined keys from customer return (because it's a left join! and you placed customers to the left).
Next,
The results of the former JOIN are joined with order (using inner join), forcing the "the first join keys" to match (1 to 1) with the keys from order so you will end up only with records that were matched in order table as well
Bad mojo? it really depends on what you are trying to accomplish.
If you want to guarantee all records from customers return, you should keep "left joining" to it.
You can, however, make this a little more intuitive to understand (not necessarily a better way of writing SQL!) by writing:
SELECT * FROM
(
(SELECT * from customer) c
LEFT JOIN
(SELECT * from ledger) l
ON
c.id= l.customerid
) c_and_l
INNER JOIN (OR PERHAPS LEFT JOIN)
(SELECT * FROM order) as o
ON c_and_l.orderid (better use c_and_l.id as you want to refer to customerid from customers table) = o.id
So now you understand that c_and_l is created first, and then joined to order (you can imagine it as 2 tables are joining again)
I have to make a list of customer who do not have any invoice but have paid an invoice … maybe twice.
But with my code (stated below) it contains everything from the left join. However I only need the lines highlighted with green.
How should I make a table with only the 2 highlights?
Select paymentsfrombank.invoicenumber,paymentsfrombank.customer,paymentsfrombank.value
FROM paymentsfrombank
LEFT OUTER JOIN debtors
ON debtors.value = paymentsfrombank.value
You only want to select columns from paymentsfrombank. So why do you even join?
select invoice_number, customer, value from paymentsfrombank
except
select invoice_number, customer, value from debtors;
(This requires exact matches as in your example, i.e. same amount for the invoice/customer).
There are two issues in your SQL. First, you need to join on Invoice number, not on value, as joining on value is pointless. Second, you need to only pick those payments where there are no corresponding debts, i.e. when you left-join, the table on the right has "null" in the joining column. The SQL would be something like this:
SELECT paymentsfrombank.invoicenumber,paymentsfrombank.customer,paymentsfrombank.value
FROM paymentsfrombank
LEFT OUTER JOIN debtors
ON debtors.InvoiceNumber = paymentsfrombank.InvoiceNumber
WHERE debtors.InvoiceNumber is NULL
in mysql we usually have this way to flip the relation and extract the rows that dosen't have relation.
Select paymentsfrombank.invoicenumber,paymentsfrombank.customer,paymentsfrombank.value
FROM paymentsfrombank
LEFT OUTER JOIN debtors
ON debtors.value = paymentsfrombank.value where debtors.value is null
You can use NOT EXISTS :
SELECT p.*
FROM paymentsfrombank p
WHERE NOT EXISTS (SELECT 1 FROM debtors d WHERE d.invoice_number = p.invoice_number);
However, the LEFT OUTER JOIN would also work if you add filtered with WHERE Clause to filtered out only missing customers that haven't any invoice information :
SELECT p.invoicenumber, p.customer, p.value
FROM paymentsfrombank P LEFT OUTER JOIN
debtors d
ON d.InvoiceNumber = p.InvoiceNumber
WHERE d.InvoiceNumber IS NULL;
Note : I have used table alias (p & d) that makes query to easier read & write.
The original query returns 160k rows. When I add the LEFT OUTER JOIN:
LEFT OUTER JOIN Table_Z Z WITH (NOLOCK) ON A.Id = Z.Id
the query returns only 150 rows. I'm not sure what I'm doing wrong.
All I need to do is add a column to the query, which will bring back a code from a different table. The code could be a number or a NULL. I still have to display NULL, hence the reason for the LEFT join. They should join on the "id" columns.
SELECT <lots of stuff> + the new column that I need (called "code").
FROM
dbo.Table_A A WITH (NOLOCK)
INNER JOIN
dbo.Table_B B WITH (NOLOCK) ON A.Id = B.Id AND A.version = B.version
--this is where I added the LEFT OUTER JOIN. with it, the query returns 150 rows, without it, 160k rows.
LEFT OUTER JOIN
Table_Z Z WITH (NOLOCK) ON A.Id = Z.Id
LEFT OUTER JOIN
Table_E E WITH (NOLOCK) ON A.agent = E.agent
LEFT OUTER JOIN
Table_D D WITH (NOLOCK) ON E.location = D.location
AND E.type = 'Organization'
AND D.af_type = 'agent_location'
INNER JOIN
(SELECT X , MAX(Version) AS MaxVersion
FROM LocalTable WITH (NOLOCK)
GROUP BY agemt) P ON E.agent = P.location AND E.Version = P.MaxVersion
Does anyone have any idea what could be causing the issue?
When you perform a LEFT OUTER JOIN between tables A and E, you are maintaining your original set of data from A. That is to say, there is no data, or lack of data, in table E that can reduce the number of rows in your query.
However, when you then perform an INNER JOIN between E and P at the bottom, you are indeed opening yourself up to the possibility of reducing the number of rows returned. This will treat your subsequent LEFT OUTER JOINs like INNER JOINs.
Now, without your exact schema and a set of data to test against, this may or may not be the exact issue you are experiencing. Still, as a general rule, always put your INNER JOINs before your OUTER JOINs. It can make writing queries like this much, much easier. Your most restrictive joins come first, and then you won't have to worry about breaking any of your outer joins later on.
As a quick fix, try changing your last join to P to a LEFT OUTER JOIN, just to see if the Z join works.
You have to be very careful once you start with LEFT JOINs.
Let's suppose this model: You have tables Products, Orders and Customers. Not all products necessarily have been ordered, but every order must have customer entered.
Task: Show all products, and if the product was ordered, list the ordering customers; i.e., product without orders will be shown as one row, product with 10 orders will have 10 rows in the resultset. This calls for a query designed around FROM Products LEFT JOIN Orders.
Now someone could think "OK, Customer is always entered into orders, so I can make inner join from orders to customers". Wrong. Since the table Customers is joined through left-joined table Orders, it has to be left-joined itself... otherwise the inner join will propagate into the previous level(s) and as a result, you will lose all products that have no orders.
That is, once you join any table using LEFT JOIN, any subsequent tables that are joined through this table, need to keep LEFT JOINs. But it does not mean that once you use LEFT JOIN, all joins have to be of that type... only those that are dependent on the first performed LEFT JOIN. It would be perfectly fine to INNER JOIN the table Products with another table Category for example, if you only want to see Products which have a category set.
(Answer is based on this answer: http://www.sqlservercentral.com/Forums/Topic247971-8-1.aspx -> last entry)