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.
Related
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.
After looking at example after example of both inner joins and subqueries as fields, I'm apparently not getting some aspect, and I would appreciate help please. I am trying to write one query that must, alas, run in MS Access 2007 to talk to an Oracle database. I have to get values from several different places for various bits of data. One of those bits of data is GROUP_CODE (e.g., faculty, staff, student, alum, etc.). Getting that is non-trivial. I am trying to use two inner joins to get the specific value. The value of borrower category must be the value for my main row in the outer query. Here is what this looks like:
Patron table Patron_Barcode table Patron_Group table
Patron_id Barcode Patron_Group_iD
Barcode Patron_Group_id PATRON_Group_Code
I want to get the PATRON_GROUP.PATRON_GROUP_CODE. This is only one of 35 fields I need to get in my query. (Yes, that's terrible, but wearing my librarian hat, i can't write the Java program I'd like to write to do this in a snap.)
So as a test, I wrote this query:
select PATRON.PATRON_ID As thePatron,
(SELECT PATRON_GROUP.PATRON_GROUP_CODE As borrowwerCategory
FROM (PATRON_GROUP
INNER JOIN PATRON_BARCODE ON PATRON_GROUP.PATRON_GROUP_ID = PATRON_BARCODE.PATRON_GROUP_ID
) INNER JOIN PATRON ON PATRON_BARCODE.PATRON_ID = thePatron.PATRON_ID
));
I don't know what I'm doing wrong, but this doesn't work. I've written a fair amount of SQL in my time, but never anything quite like this. What am I doing wrong?
PATRON.BARCODE is the foreign key for the BARCODE table.
PATRON_BARCODE.PATRON_GROUP_ID is the foreign key for the PATRON_GROUP table. PATRON_GROUP_CODE in PATRON_GROUP is he column value that I need.
PATRON.BARCODE -> BARCODE.PATRON_GROUP_ID -> PATRON_GROUP.PATRON_GROUP_CODR>
The main table, PATRON, will have lots of other things, like inner and outer join to PATRON_ADDRESS, etc., and I can't just do an inner join directly to what I want in my main query. This has to happen in a subquery as a field. Thanks.
Ken
I'm creating an archive for Academic Papers. Each paper may have one author, or multiple authors. I've created the tables in the following manner:
Table 1: PaperInfo - Each row contains information on the paper
Table 2: PaperAuthor - Only Two Columns: contains PaperID, and AuthorID
Table 3: AuthorList - Contains Author Information.
There is also a Table 4 which is linked to Table 4, which contains a list of Universities which the author belongs to, but I'm going to leave it out for now in case it gets too complicated.
I wish to have a Query which will link all three tables together, and display Paper Information of the recordset in a table, with columns such as these:
Paper Title
Paper Authors
The column "Paper Authors" is going to contain more than one authors in some cases.
I've wrote the following query:
SELECT a.*,b.*,c.*
FROM PaperInfo a, PaperAuthor b, AuthorList c
WHERE a.PaperID = b.PaperID AND b.AuthorID = c.AuthorID
So far, the results I've been getting for each row is one author per row. I wish to contain more authors in one column. Can this be done in anyway?
Note: I'm using Access 2010 as my database.
In straight SQL the answer unfortunately is that it isn't possible. You would need to use a processing language in order to get the result you are after.
Since you mention you are using Access 2010 please refer to this question: is there a group_concat function in ms-access?
Particularly, read the post which points to http://www.rogersaccesslibrary.com/forum/generic-function-to-concatenate-child-records_topic16&SID=453fabc6-b3z9-34z6zb14-a78f832z-19z89a2c.html
You probably need to implement a custom function but the 2nd url does what you are looking for.
This functionality is not part of the SQL standard, but different vendors have solutions for it, see for instance Pivot Table with many to many table, MySQL pivot table.
If you know the maximum number of authors per paper (for example 3 or 4), you could get away with a triple or quadruple left join.
What you are after is an inner join.
An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.
The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN return all rows from multiple tables where the join
condition is met.
http://www.w3schools.com/sql/sql_join.asp
You may want to combine the inner join with a group to give you 1 paper to many authors in your results.
The GROUP BY statement is used in conjunction with the aggregate
functions to group the result-set by one or more columns.
http://www.w3schools.com/sql/sql_groupby.asp
Say you have these tables:
PHARMACY(**___id_pharmacy___**, name, addr, tel)
PHARMACIST(**___Insurance_number___**, name, surname, qualification, **id_pharmacy**)
SELLS(**___id_pharmacy___**, **___name___**, price)
DRUG(**___Name___**, chem_formula, **id_druggistshop**)
DRUGGISTSHOP(**___id_druggistshop___**, name, address)
I think this will be more specific.
So, I'm trying to construct an SQL statement, in which I will fetch the data from id_pharmacy and name FROM PHARMACY, the insurance_number, name, and surname columns from PHARMACIST, for all the pharmacies that sell the drug called Kronol.
And that's basically it. I know I'm missing the relationships in the code I wrote previously.
Note: Those column names which have underscores left and right to them are underlined(Primary keys).
The query you've written won't work in any DBMS that I know of.
You'll most likely want to use some combination of JOINs.
Since the exact schema isn't provided, consider this pseudo code, but hopefully it will get you on the right track.
SELECT PH.Ph_Number, PH.Name, PHCL.Ins_Number, PHCL.Name, PHCL.Surname
FROM PH
INNER JOIN PHCL ON PHCL.PH_Number = PH.Ph_Number
INNER JOIN MLIST ON MLIST.PH_Number = PH.PH_Number
WHERE MLIST.Name = "Andy"
I've obviously assumed some relationships between tables that may or may not exist, but hopefully this will be pretty close. The UNION operator won't work because you're selecting different columns and a different number of columns from the various tables. This is the wrong approach all together for what you're trying to do. It's also worth mentioning that a LEFT JOIN may or may not be a better option for you, depending on the exact requirements you're trying to meet.
Ok, try this query:
SELECT A.id_pharmacy, A.name AS PharmacyName, B.Insurance_number,
B.name AS PharmacistName, B.surname AS PharmacistSurname
FROM PHARMACY A
LEFT JOIN PHARMACIST B
ON A.id_pharmacy = B.id_pharmacy
WHERE A.id_pharmacy IN (SELECT id_pharmacy FROM SELLS WHERE name = 'Kronol')
Is an outer join only used for analysis by the developer? I'm having trouble finding a use case for why you would want to include data in two or more tables that is unrelated or does not "match" your select criteria.
An example use case would be to produce a report that shows ALL customers and their purchases. That is, show even customers who have not purchased anything. If you do an ordinary join of customers and purchases, the report would show only those customers with at least one purchase.
Here is a very good list of examples by none other than Jeff Atwood - A Visual Explanation of SQL Joins he shows a new representation visually of the SQL joins.
The "doesn't match" case is useful for optional data. Something which is present for some rows, but not for others.
The OUTER JOIN is intended to address the cases where you wish to select a set of records from a primary table, which may or may not have related records contained in a secondary table.
An INNER join would omit from the list any primary records not also represented in the secondary table. Any OUTER JOIN guarantees the visiblity of all qualified primary records.
The difference between outer and inner joins is primarily that the outer joins retain each record from both the joined tables, whether the join predicate is matched or not. So a use case would need to revolve around knowing about "missing stuff", eg. customers who haven't made purchases, or members who have missing personal details.
Perhaps you have a users table and an address table related to it, since your users may choose not to enter an address, or they may have several (shipping, street, postal, etc). If you wanted to list all your users, but also display addresses for those that have them, you'd have to use an outer join for the address table.
Say you had two tables one for users and one for something like qualifications, you want to do a query that gets all users with their qualifications if they have any. In this case you would use an outer join to get all the users back and those with qualifications. An inner join would only give you the users who had a qualification.
A great article about outer joins:
Meet the experts: Terry Purcell on coding predicates in outer joins: A comparison of simple outer join constructs