I am having trouble with writing a SQL query in Access with foreign keys. There are two tables, 'Customers'(ID, Name, Surname) and 'Orders'(ID, Customer, Date, Volume). ID fields are primary, and Orders.Customer is a foreign key linked to Customers.ID, so a customer can have many orders or none.
My goal is to do a search on customers based on many criteria, one of which being if customers have at least an order which volume is superior to a certain quantity. I tried joins with SELECT DISTINCT but it still gives me duplicate results, plus I had to create an empty order for every customer without orders if the query didn't use the above condition.
Does anyone have an idea about that? Maybe some special instruction on foreign keys or 2 separate queries?
Based on the information you give, i only can give you hints on what I think you're doing/understanding wrong :
SELECT DISTINCT does select you a unique record, not a unique value, so if your statement selects all fields (*), distinct won't help you much there.
My guess is you had to create an empty order for each customer because you used INNER JOIN, try LEFT OUTER JOIN instead
For example :
SELECT DISTINCT Customers.*
FROM Customers
LEFT OUTER JOIN Orders
ON (Orders.Customer = Customers.id)
WHERE Volume > put_your_value
Related
I am trying to find the correct join construction to join together the relevant customer info from the Rentals table with the Accidents table. I often run into this issue where my joining fields aren't unique but not sure what else to join on. The accidents table only has about 1500 records but when I join it to pull in more customer data, I get like 35k records. I can do some joins but frequently have joins like this at work and I feel like a dummy because I am not sure how to troubleshoot...
SELECT a.*,
r.market,
r.date_of_birth,
r.is_blocked,
r.rate_type
FROM `accidents` a
LEFT JOIN `rentals` r -- I also tried an INNER JOIN
USING (customer_id) -- Other fields I tried to match on: Full Name
ORDER BY accident_dt DESC
In this database I need to find the total amount that each customer paid for books in a category, and then sort them by their customer ID. The code appears to run correctly but I end up with approximately 20 extra rows than I should, although the sum appears to be correct in the right rows.
The customer ID is part of customer, but is not supposed to appear in the select clause, when I try and ORDER BY it, I get strange errors. The DB engine is DB2.
SELECT distinct customer.name, book.cat, sum(offer.price) AS COST
FROM offer
INNER JOIN purchase ON purchase.title=offer.title
INNER JOIN customer ON customer.cid=purchase.cid
INNER JOIN member ON member.cid=customer.cid
INNER JOIN book ON book.title=offer.title
WHERE
member.club=purchase.club
AND member.cid=purchase.cid AND purchase.club=offer.club
GROUP BY customer.name, book.cat;
You should fix your join conditions to include the ones in the where clause (between table relationships usually fit better into an on clause).
SELECT DISTINCT is almost never appropriate with a GROUP BY.
But those are not your question. You can use an aggregation function:
GROUP BY customer.name, book.cat
ORDER BY MIN(customer.id)
I have two tables: Company and Contact, with a relationship of one-to-many.
I have another table Track which identifies some of the companies as parent companies to other companies.
I want to write a SQL query that selects the parent companies from Track and the amount of contacts that each parent has.
SELECT Track.ParentId, Count(Contact.companyId)
FROM Track
INNER JOIN Contact
ON Track.ParentId = Contact.companyId
GROUP BY Track.ParentId
however The result holds less records than when I run the following query:
SELECT DISTINCT Track.ParentId
FROM Track
I tried the first query with an added DISTINCT and it returned the same results (less then what it was meant to).
You're performing an INNER JOIN with the Contact table, which means that any rows from the first table (Track in this case) with no matches to the JOINed table will not show up in your results. Try using a LEFT OUTER JOIN instead.
The COUNT with Contact.companyId will only count rows where there is a match (Contact.companyId is not NULL). Since you're counting contacts that's fine as they will count as 0. If you were trying to count some other set of data and tried to do a COUNT on a specific column (rather than COUNT(*)) then any NULL values in that column would not count towards your total, which might or might not be what you want.
I used an INNER JOIN which returns only records that are identical in both tables.
To return all records from Track table, and records that match in the Contact table, I need to use LEFT JOIN.
I had a select query, which I designed in the SQL view of the query design tool. With some of my results I found duplicates of the same records. As in there were not multiples in the table, only the query (Same Primary Key). Here is the original query.
SELECT t1.*
FROM Inventory AS t1 INNER JOIN Inventory AS t2 ON
t1.Part_ID = t2.Part_ID WHERE (t1.Inventory_ID<>t2.Inventory_ID);
I aimed to query Inventory for records with the same Part_ID (FK) but different Inventory_ID(PK). There is a composite key between part_ID (FK) and location_ID (FK), if that makes any difference.
I have since changed this query to:
SELECT DISTINCT t1.*
FROM Inventory AS t1 INNER JOIN Inventory AS t2 ON t1.Part_ID = t2.Part_ID
WHERE (t1.Inventory_ID<>t2.Inventory_ID);
This removes the duplicate records, however, I don't believe that my original query should produce replicate data results. I am worried that this suggests that there is something wrong with my tables?
My table looks like the following:
Thanks
The thing is that you might have multiple occurences of part_ID on the INNER JOIN side of your select. So if a part with the same part_ID and a different inventory_ID exists in 2 other locations, you will get duplicates.
To check that, you could do a test on a few duplicates, or rewrite your original query with a GROUP BY instruction on the INNER JOIN side of the query.
I am trying to understand the JOIN command from w3schools tutorial and there is an example.
Can you please "translate" it to me what exactly it does? I already know what do the dots do, but INNER JOIN, ON and so on messed me up?
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID=Customers.CustomerID;
And does it creates a new table in my SQL database or it just creates (lets call it..) "virtual" table which I can use it in the moment
The dot notation here signifies the column of a table.
Table.Column
So the SELECT is retrieving the columns specified from each table.
JOIN matches the tables based on the condition that appears after ON
this queries joins customers to orders based the condition of having the same CustomerID.
For more on joins check out http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
Perhaps the first thing to understand is the "dot notation." When you see a value like Orders.CustomerID SQL will read that to mean the CustomerID column within the Orders table.
The INNER JOIN is looking for an exact match for the records you specify with the ON clause...
So, in this example, SQL will first find all the records in the Orders table which have a CustomerID that matches the CustomerID found in the Customers table.
Then, it will look to the SELECT clause and show you the OrderID (from the Orders table), the CustomerName (from the Customers table) and the OrderDate from the Orders table.
If you're using SQL Server Management Studio (SSMS) or some other similar tool, it should display your results - kindof like a virtual table... but it doesn't actually create a table.
Hope that helps,
Russell
There are INNER AND OUTER JOINS. I think this post sums up those differences well.
What is the difference between "INNER JOIN" and "OUTER JOIN"?
As far as the concept itself you are 'joining' the tables by finding things in common between them based on a shared value in each. If you have ever done a vlookup with Microsoft Excel then you are familiar with this concept. In the query you've posted you are selecting values from both the Orders and Customers table. You can tell which table you are selecting values from by the prefix Orders. or Customers. You can then tell which column by the value following the dot i.e. OrderID, CustomerName, OrderDate.
The way the query knows to pull a conjoined field is if the CustomerID field matches.
So step by step
You are selecting
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
From the table Orders
FROM Orders
You wish to pull corresponding values from Customers. There are numerous join methods and logic that goes with each but you have chosen INNER JOIN explained in the link above.
INNER JOIN Customers
The criteria for joining is CustomerID
ON Orders.CustomerID=Customers.CustomerID;
To use an approximate Excel Vlookup analogy
The Lookup_value is CustomerID
The Table array is ORDERS
The range lookup is the type of join you are performing
SELECT statement returns data, it doesn't create any table not even virtual table whatsoever (if you see any table after executing SELECT statement, thats just a way the returned data presented). Create and modify table done using DDL (Data Definition Language) statements when SELECT is one of DML (Data Manipulation Language) [reference].
Join statement explained very well here, from basic use to more advance with illustration.