The company is performing an analysis of their inventory. They are considering purging books that are not popular with their customers. To do this they need a list of books that have never been purchased. Write a query using a join that provides this information. Your results should include all the book details and the order number column. Sort your results by the book title.
SELECT o.order_nbr, b.*
FROM orders o JOIN books
WHERE
ORDER BY book_title
This is all I could come up with, I'm still learning Joins and struggling to figure out what the correct statement should be. Wasn't sure what to put in the WHERE clause and don't really know how to properly join these tables.
You need an ON clause to specify what you are joining on. Also, your WHERE clause is empty, and you are not specifying the type of JOIN you are using. Looking at the way the tables are set up, the expectation is you are going to join the BOOKS table on ORDER_ITEMS, which also contains ORDER_NBR.
In the question, it's asking to find books with no orders, so correct join would be a LEFT JOIN between BOOKS and ORDER_ITEMS, as that will include every book, even those without orders, which will have an ORDER_NBR of NULL
The SQL would look like
SELECT o.order_nbr, b.*
FROM books b
LEFT JOIN order_items o on b.book_id = o.book_id
WHERE o.order_nbr is null
ORDER BY book_title
This would return only the books with no orders.
Related
I need help in understanding how left join is working in below query.
There are total three tables and two left joins.
So my question is, the second left join is between customer and books table or between result of first join and books table?
SELECT c.id, c.first_name, c.last_name, s.date AS sale,
b.name AS book, b.genre
FROM customers c
LEFT JOIN sales s
ON c.id = s.customer_id
LEFT JOIN books b
ON s.book_id = b.id;
Good question.
When it comes to outer-joined tables, it depends on the predicates in the ON clause. The engine is free to reorder the fetch and scans on indexes or tables as long as the predicates are respected.
In this particular case there are three tables:
customers (c)
sales (s)
books (b)
customers is inner joined so it becomes the driving table; there are other considerations, but for simplicity you can consider that this is the table that is read first. Now, which one is second? sales or books?
The first join predicate c.id = s.customer_id doesn't establish any relationship between the secondary tables; therefore it doesn't affect which table is joined first.
The second join predicate s.book_id = b.id makes books dependent on sales. Therefore, it decides sales is the second table, and books is the last one.
A final note: if you understand the concept of dependency there are several dirty tricks you can use to force the engine to walk the tables in the order you want. I would not recommend to do this to a novice, but if at some point you realise the engine is not doing what you want, you can tweak the queries.
The second join statement specifies to join on s.book_id = b.id where s is sales and b is books. However, a record in the books table will not be returned unless it has a corresponding record in the sales AND customers tables, which is what a left join does by definition https://www.w3schools.com/sql/sql_join_left.asp. put another way, this query will return all books that have been purchased by at least one customer (and books that have been purchased multiple times will appear in the results multiple times).
We have to select authors that havent written a book but there are 3 different tables which makes me confused about how to write the join expression.
We have tables:
authors: author_id
authorships: author_id, book_id
books: book_id.
Obviously I selected the names from authors and tried inner join but it wont work for me. Help would be appreciated!
Since this sounds like a school assignment I won't give the full answer.
Try using an outer join between authors and authorship. Make sure you retrieve the book I'd from the authorship.
Try to work out what an author who has not published looks like the. You can use this to formulate the query for the answer you are looking for with an appropriate where clause.
This is a good spot to use the LEFT JOIN antipattern:
SELECT a.*
FROM authors a
LEFT JOIN authorships s ON s.author_id = a.author_id
WHERE s.author_id IS NULL
Rationale: when the LEFT JOIN comes up empty, it means that the author has no corresponding record in the authorships table. The WHERE clause filters out on unmatched authors records only (ie authors that have no books). This is called an antipattern because the purpose of a JOIN is usually to match records, whereas here we use it to detect unmatched records.
Its really easy, just check which column seems to be having common value between all this three tables if something is common atleast within two tables then put inner join on those two and an outer join on the uncommon data table.
Remember your Aliases will always matter when you join between different tables, also the ON and WHERE should be properly mentioned.
I'm trying to do this query to display the names of the stores and the quantity of each book sold with only using joins but I tried to use
SELECT DISTINCT x.stor_name, t.title, s.qty
FROM stores x
INNER JOIN discounts d
ON x.stor_id=d.stor_id
INNER JOIN salesdetail s
ON d.stor_id=s.stor_id
INNER JOIN titles t
ON s.title_id=t.title_id
ORDER BY s.qty desc;
but that only displayed one of the stores results set for 'Bookbeat'.
I tried to use Left, Right & Full Outer joins to no avail so I'm wondering how I would go about doing that query to display the names for the other stores that are not displaying their result set. As there is 7 stores and only 1 is displaying it's results.
The link is a pastebin to the database.
And this is the schema.
It's hard to say without more information about your schema - it strikes me as wrong-ish that you're joining to discounts only on stor_id. I'd expect discounts to be applied to different titles, not store-wide... and I wouldn't expect discounts to be always-enabled. Try running it without the discounts inner join. Futzing around with "Distinct" and outer joins is almost always the wrong approach with things like this
I see from your profile you're a first-year. Is this schoolwork? How do I ask and answer homework questions?
I'm learning joins in my class, but I'm not fully grasping some of the concepts. Can somebody explain how a statement with multiple joins works?
SELECT B.TITLE, O.ORDER#, C.STATE FROM BOOKS B
LEFT OUTER JOIN ORDERITEMS OI ON B.ISBN = OI.ISBN
LEFT OUTER JOIN ORDERS O ON O.ORDER# = OI.ORDER#
LEFT OUTER JOIN CUSTOMERS C ON C.CUSTOMER# = O.CUSTOMER#;
I believe I understand that the BOOKS table is the left table in the first outer join connecting BOOKS and ORDERITEMS. All BOOKS will be shown, even if there is not an ORDERITEM for a book. After the first join, I'm not sure what is really happening.
When ORDERS is joined, which is the left table and which is the right table? The same for Customers. This is where I get lost.
First thing what executor will perform — take a first pair of tables that are eligible to be joined and perform the join. On the following steps, the result of the previous join is treated as a virtual relation, therefore you again have a construct similar to ... FROM virt_tab LEFT JOIN real_tab .... This behavior is based on the closure concept used in Relational Algebra, which means that any operation on the relation produces relation, i.e. operations can be nested. And RDBMS stands for Relational DBMS, take a look at the linked wikipedia article.
So far I find PostgreSQL's docs being most definitive in this matter, take a look at them. In the linked article a generic overview on how joins are performed by the databases is given with some PostrgeSQL-specific stuff, which is expected.
One of my favorite online resources is : http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
As to your question.
All books will be displayed and only those orderitems which match a book
all only those orders which have a related record in orderitems which relate to a book will be displayed
Only customers who have orders with items in the books table will be listed.
So customers who don't have orders would not be listed
Customers who have orders but for items that are not books will NOT be listed
Fun stuff. Hope you enjoy it.
As to your second question: Right/left only matter because of the ORDER of the tables in your from statement. You could make every join a left one if you re-arrange a table order. All right/left do is specify the table from which you want ALL records.
Consider: you could just as easily right your select statement as:
SELECT B.TITLE, O.ORDER#, C.STATE
FROM CUSTOMERS C
RIGHT OUTER JOIN ORDERS O ON C.CUSTOMER# = O.CUSTOMER#
RIGHT OUTER JOIN ORDERITEMS OI ON O.ORDER# = OI.ORDER#
RIGHT OUTER JOIN BOOKS B ON B.ISBN = OI.ISBN
In this case right is saying that I want all the records from the table on the right since books is last in the list you'll get all books and only those ordereditems related to a book, only those orders for which the ordered item was a book and only those customers with orders for ordered items which were books. Thus the left / right are the same except for order. I avoid right joins for readability. I find it easier to go top down when thinking about whats included and what will not be.
Those records which are excluded will have NULL values in these types of joins.
Hope this helps.
I have a table B with cids and cities. I also have a table C that has these cids with extra information. I want to list all the cids in table C that are associated with ALL appearances of a given city in Table B.
My current solution relies on counting the number of times the given city appears in Table B and selecting only the cids that appear that many times. I don't know all the SQL syntax yet, but is there a way to select for this kind of pattern?
My current solution:
SELECT Agents.aid
FROM Agents, Customers, Orders
WHERE (Customers.city='Duluth')
AND (Agents.aid = Orders.aid)
AND (Customers.cid = Orders.cid)
GROUP BY Agents.aid
HAVING count(Agents.aid) > 1
It only works because I know right now with the HAVING statement.
Thanks for the help. I wasn't sure how to google this problem, since it's pretty specific.
EDIT: I'm pinpointing my problem a bit. I need to know how to determine if EVERY row in a table has a certain value for a field. Declaring a variable and counting the rows in a sub-selection and filtering out my results by IDs that appear that many times works, but It's really ugly.
There HAS to be a way to do this without explicitly count()ing rows. I hope.
Not an answer to your question, but a general improvement.
I'd recommend using JOIN syntax to join your tables together.
This would change your query to be:
SELECT Agents.aid
FROM Agents
INNER JOIN Orders
ON Agents.aid = Orders.aid
INNER JOIN Customers
ON Customers.cid = Orders.cid
WHERE Customers.city='Duluth'
GROUP BY Agents.aid
HAVING count(Agents.aid) > 1
What variant of SQL are you using?
To start with, you can (and should) use JOIN instead of doing it in the WHERE clause, e.g.,
select Agents.aid
from Agents
join Orders on Agents.aid = Orders.aid
join Customers on Customers.cid = Orders.cid
where Customers.city = 'Duluth'
group by Agents.aid
having count(Agents.aid) > 1
After that, I'm afraid I might be a little lost. Using the table names in your example query, what (in English, not pseudocode) are you trying to retrieve? For example, I think your sample query is retrieving the PK for all Agents that have been involved in at least 2 Orders involving Customers in Duluth.
Also, some table definitions for Agents, Orders, and Customers might help (then again, they might be irrelevant).
I'm not sure if I understood you problem, but I think the following query is what you want:
SELECT *
FROM customers b
INNER JOIN orders c USING (cid)
WHERE b.city = 'Duluth'
AND NOT EXISTS (SELECT 1
FROM customers b2
WHERE b2.city = b.city
AND b2.cid <> cid);
Probably you will need some indexes on these columns.