There are 3 tables for a small trading system; USERS, ITEMS and TRADES, the trades themselves are held in table TRADES (logically enough), as:
column type
========== =====
seller_id int
buyer_id int
item_id int
quantity int
cost int
currency int
I want to display pending trades, using a 3-way join to map IDs to names in tables USER and ITEM. So far I've got:
SELECT
users.name AS seller,
items.id AS item_id,
items.name AS item,
trades.item_id AS trade_id,
trades.quantity AS quantity,
trades.cost AS cost,
trades.currency AS currency
FROM
(trades INNER JOIN users ON trades.seller_id = users.id)
INNER JOIN items ON trades.item_id = items.id
WHERE trades.buyer_id = xxx
The first join works fine, but as soon as I pass the result to the second it fails; returns zero data. I'm braced for it being a real noobie problem (in fact I'm hoping it is, easy to fix = good) but I can't see it (probably because I am in fact a noob at these things).
SELECT
users.name AS seller,
items.id AS item_id,
items.name AS item,
trades.item_id AS trade_id,
trades.quantity AS quantity,
trades.cost AS cost,
trades.currency AS currency
FROM
trades
LEFT JOIN
users
ON
trades.seller_id = users.id
LEFT JOIN
items
ON
trades.item_id = items.id
WHERE trades.buyer_id = xxx
I think trades_id needs to be trades.item_id
I also advise you to use LEFT JOIN instead of INNER JOIN, only use INNER JOIN when you want to trigger a error if there's no match. When you use LEFT JOIN it always returns data, also when there's no match in the second table. See: http://www.w3schools.com/Sql/sql_join.asp
Related
This is similar to How to find missing data rows using SQL? and How to find missing rows (dates) in a mysql table? but a bit more complex, so I'm hitting a wall.
I have a data table with the noted Primary key:
country_id (PK)
product_id (PK)
history_date (PK)
amount
I have a products table with all products, a countries table, and a calendar table with all valid dates.
I'd like to find all countries, dates and products for which there are missing products, with this wrinkle:
I only care about dates for which there are entries for a country for at least one product (i.e. if the country has NOTHING on that day, I don't need to find it) - so, by definition, there is an entry in the history table for every country and date I care about.
I know it's going to involve some joins maybe a cross join, but I'm hitting a real wall in finding missing data.
I tried this (pretty sure it wouldn't work):
SELECT h.history_date, h.product_id, h.country_id, h.amount
FROM products p
LEFT JOIN history h ON (p.product_id = h.product_id)
WHERE h.product_id IS NULL
No Joy.
I tried this too:
WITH allData AS (SELECT h1.country_id, p.product_id, h1.history_date
FROM products p
CROSS JOIN (SELECT DISTINCT country_id, history_date FROM history) h1)
SELECT f.history_date, f.product_id, f.country_id
FROM allData f
LEFT OUTER JOIN history h ON (f.country_id = h.country_id AND f.history_date = h.history_date AND f.product_id = h.product_id)
WHERE h.product_id IS NULL
AND h.country_id IS NOT NULL
AND h.history_date IS NOT null
also no luck. The CTE does get me every product on every date that there is also data, but the rest returns nothing.
I only care about dates for which there are entries for a country for
at least one product (i.e. if the country has NOTHING on that day, I
don't need to find it)
So we care about this combination:
from (select distinct country_id, history_date from history) country_date
cross join products p
Then it's just a matter of checking for existence:
select *
from (select distinct country_id, history_date from history) country_date
cross join products p
where not exists (select null
from history h
where country_date.country_id = h.country_id
and country_date.history_date = h.history_date
and p.product_id = h.product_id
)
I'm taking my first steps in terms of practical SQL use in real life.
I have a few tables with contractual and financial information and the query works exactly as I need - to a certain point. It looks more or less like that:
SELECT /some columns/ from CONTRACTS
Linked 3 extra tables with INNER JOIN to add things like department names, product information etc. This all works but they all have simplish one-to-one relationship (one contract related to single department in Department table, one product information entry in the corresponding table etc).
Now this is my challenge:
I also need to add contract invoicing information doing something like:
inner join INVOICES on CONTRACTS.contnoC = INVOICES.contnoI
(and selecting also the Invoice number linked to the Contract number, although that's partly optional)
The problem I'm facing is that unlike with other tables where there's always one-to-one relationship when joining tables, INVOICES table can have multiple (or none at all) entries that correspond to a single contract no. The result is that I will get multiple query results for a single contract no (with different invoice numbers presented), needlessly crowding the query results.
Essentially I'm looking to add INVOICES table to a query to just identify if the contract no is present in the INVOICES table (contract has been invoiced or not). Invoice number itself could be presented (it is with INNER JOIN), however it's not critical as long it's somehow marked. Invoice number fields remains blank in the result with the INNER JOIN function, which is also necessary (i.e. to have the row presented even if the match is not found in INVOICES table).
SELECT DISTINCT would look to do what I need, but I seemed to face the problem that I need to levy DISTINCT criteria only for column representing contract numbers, NOT any other column (there can be same values presented, but all those should be presented).
Unfortunately I'm not totally aware of what database system I am using.
Seems like the question is still getting some attention and in an effort to provide some explanation here are a few techniques.
If you just want any contract with details from the 1 to 1 tables you can do it similarily to what you have described. the key being NOT to include any column from Invoices table in the column list.
SELECT
DISTINCT Contract, Department, ProductId .....(nothing from Invoices Table!!!)
FROM
Contracts c
INNER JOIN Departments D
ON c.departmentId = d.Department
INNER JOIN Product p
ON c.ProductId = p.ProductId
INNER JOIN Invoices i
ON c.contnoC = i.contnoI
Perhaps a Little cleaner would be to use IN or EXISTS like so:
SELECT
Contract, Department, ProductId .....(nothing from Invoices Table!!!)
FROM
Contracts c
INNER JOIN Departments D
ON c.departmentId = d.Department
INNER JOIN Product p
ON c.ProductId = p.ProductId
WHERE
EXISTS (SELECT 1 FROM Invoices i WHERE i.contnoI = c.contnoC )
SELECT
Contract, Department, ProductId .....(nothing from Invoices Table!!!)
FROM
Contracts c
INNER JOIN Departments D
ON c.departmentId = d.Department
INNER JOIN Product p
ON c.ProductId = p.ProductId
WHERE
contnoC IN (SELECT contnoI FROM Invoices)
Don't use IN if the SELECT ... list can return a NULL!!!
If you Actually want all of the contracts and just know if a contract has been invoiced you can use aggregation and a case expression:
SELECT
Contract, Department, ProductId, CASE WHEN COUNT(i.contnoI) = 0 THEN 0 ELSE 1 END as Invoiced
FROM
Contracts c
INNER JOIN Departments D
ON c.departmentId = d.Department
INNER JOIN Product p
ON c.ProductId = p.ProductId
LEFT JOIN Invoices i
ON c.contnoC = i.contnoI
GROUP BY
Contract, Department, ProductId
Then if you actually want to return details about a particular invoice you can use a technique similar to that of cybercentic87 if your RDBMS supports or you could use a calculated column with TOP or LIMIT depending on your system.
SELECT
Contract, Department, ProductId, (SELECT TOP 1 InvoiceNo FROM invoices i WHERE c.contnoC = i.contnoI ORDER BY CreateDate DESC) as LastestInvoiceNo
FROM
Contracts c
INNER JOIN Departments D
ON c.departmentId = d.Department
INNER JOIN Product p
ON c.ProductId = p.ProductId
GROUP BY
Contract, Department, ProductId
I would do it this way:
with mainquery as(
<<here goes you main query>>
),
invoices_rn as(
select *,
ROW_NUMBER() OVER (PARTITION BY contnoI order by
<<some column to decide which invoice you want to take eg. date>>) as rn
)
invoices as (
select * from invoices_rn where rn = 1
)
select * from mainquery
left join invoices i on contnoC = i.contnoI
This gives you an ability to get all of the invoice details to your query, also it gives you full control of which invoice you want see in your main query. Please read more about CTEs; they are pretty handy and much easier to understand / read than nested selects.
I still don't know what database you are using. If ROW_NUMBER is not available, I will figure out something else :)
Also with a left join you should use COALESCE function for example:
COALESCE(i.invoice_number,'0')
Of course this gives you some more possibilities, you could for example in your main select do:
CASE WHEN i.invoicenumber is null then 'NOT INVOICED'
else 'INVOICED'
END as isInvoiced
You can use
SELECT ..., invoiced = 'YES' ... where exists ...
union
SELECT ..., invoiced = 'NO' ... where not exists ...
or you can use a column like "invoiced" with a subquery into invoices to set it's value depending on whether you get a hit or not
I have an e-commerce website (using VirtueMart) and I sell products that consist child products. When a product is a parent, it doesn't have ParentID, while it's children refer to it. I know, not the best logic but I didn't create it.
My SQL is very basic and I believe I ask for something quite easy to achieve
Select products that have children.
Sort results by prices (ASC/DSC).
SELECT * FROM Products INNER JOIN Prices ON Products.ProductID = Prices.ProductID ORDER BY Products.Price [ASC/DSC]
Explanation:
SELECT - Select (Get/Retrieve)
* - ALL
FROM Products - Get them from a DB Table named "Products".
INNER JOIN Prices - Selects all rows from both tables as long as there is a match between the columns in both tables. Rather, JOIN DB Table "Products" with DB Table "Prices".
ON - Like WHERE, this defines which rows will be checked for matches.
Products.ProductID = Prices.ProductID - Your match criteria. Get the rows where "ProductID" exists in both DB Tables "Products" and "Prices".
ORDER BY Products.Price [ASC/DSC] - Sorting. Use ASC for Ascending, DSC for Descending.
This table design is subpar for a number of reasons. First, it appears that the value 0 is being used to indicate lack of a parent (as there's no 0 ID for products). Typically this will be a NULL value instead.
If it were a NULL value, the SQL statement to get everything without a parent would be as simple as this:
SELECT * FROM Products WHERE ParentID IS NULL
However, we can't do that. If we make the assumption that 0 = no parent, we can do this:
SELECT * FROM Products WHERE ParentID = 0
However, that's a dangerous assumption to make. Thus, the correct way to do this (given your schema above), would be to compare the two tables and ensure that the parentID exists as a ProductID:
SELECT a.*
FROM Products AS a
WHERE EXISTS (SELECT * FROM Products AS b WHERE a.ID = b.ParentID)
Next, to get the pricing, we have to join those two tables together on a common ID. As the Prices table seems to reference a ProductID, we can use that like so:
SELECT p.ProductID, p.ProductName, pr.Price
FROM Products AS p INNER JOIN Prices AS pr ON p.ProductID = pr.ProductID
WHERE EXISTS (SELECT * FROM Products AS b WHERE p.ID = b.ParentID)
ORDER BY pr.Price
That might be sufficient per the data you've shown, but usually that type of table structure indicates that it's possible to have more than one price associated with a product (we're unable to tell whether this is true based on the quick snapshot).
That should get you close... if you need something more, we'll need more detail.
use the below script if you are using ssms.
SELECT pd.ProductId,ProductName,Price
FROM product pd
LEFT JOIN price pr ON pd.ProductId=pr.ProductID
WHERE EXISTS (SELECT 1 FROM product pd1 WHERE pd.productID=pd1.ParentID)
ORDER BY pr.Price ASC
Note :neither of your parent product have price in price table. If you want the sum of price of their child product use the below script.
SELECT pd.ProductId,pd.ProductName,SUM(ISNULL(pr.Price,0)) SUM_ChildPrice
FROM product pd
LEFT JOIN product pd1 ON pd.productID=pd1.ParentID
LEFT JOIN price pr ON pd1.ProductId=pr.ProductID
GROUP BY pd.ProductId,pd.ProductName
ORDER BY pr.Price ASC
You will have to use self-join:
For example:
SELECT * FROM products parent
JOIN products children ON parent.id = children.parent_id
JOIN prices ON prices.product_id = children.id
ORDER BY prices.price
Because we are using JOIN it will filter out all entries that don't have any children.
I haven't tested it, I hope it would work.
I am constructing a simple database for an inventory system, with a barcode reader. I have one form to add inventory, and one form to remove inventory. Each adjust add a column to the add or remove table.
I made queries for my add and remove that return the total value of each item, associated with the id for each item. I want to join my add and remove tables so I can calculate the difference for a current quantity.
I am running into trouble with joining my add subquery and my remove subquery.
Add:
SELECT Items.description, Count(Trans_Add.s) AS Quantity
FROM Items INNER JOIN Trans_Add ON Items.ID = Trans_Add.s
GROUP BY Items.description;
Remove:
SELECT Items.description, Count(Trans_Remove.s) AS Quantity
FROM Items INNER JOIN Trans_Remove ON Items.ID = Trans_Remove.s
GROUP BY Items.description;
Whenever I try to assign alias for either in the from clause of my new query and join them it does not work.
Try:
SELECT Items.ID, Items.Description, (Items.TotalCount + A.Quantity - B.Quantity) as OnHandQty
FROM Items INNER JOIN
(SELECT Items.ID, Count(Trans_Add.s) AS Quantity
FROM Items INNER JOIN Trans_Add ON Items.ID = Trans_Add.s
GROUP BY Items.description
) A
ON Items.ID = A.ID
INNER JOIN (
SELECT Items.description, Count(Trans_Remove.s) AS Quantity
FROM Items INNER JOIN Trans_Remove ON Items.ID = Trans_Remove.s
GROUP BY Items.description
) B
ON Items.ID = B.ID
I made up a Field named TotalCount because you did not mentioned what is the Item Quantity in Items table. So, change it accordingly, that is, according to the
right field name for Item Quantity from Items table.
I have two tables, items and bids.
create table items (
id serial primary key,
store_id int,
min_bid int
);
create table bids (
item_id int,
amount int
);
I want to select items and include information about the max bid.
select items.*, max(bids.amount) from items
join bids on bids.item_id = items.id
where items.store_id = $store_id
group by items.id
However, when there are no bids for a particular item, the item just doesn't get selected. How can I make it so that when there are no bids, the item still gets selected and fills in the max(bids.amount) column with items.min_bid? (Or 0 is fine, too.)
I tried this:
select items.*, coalesce(max(bids.amount), items.min_bid) from items
join bids on bids.item_id = items.id
where items.store_id = $store_id
group by items.id
which doesn't work. I'm assuming it's because of the join that the items aren't getting selected in the first place.
What should I do?
The two crucial elements are LEFT JOIN and COALESCE().
#Adrian already commented on LEFT [OUTER ]JOIN. It preserves all rows at the left hand of the join and fills missing columns to the right with NULL values. The manual has more on the basics.
COALESCE() replaces NULL values with the provided alternative - 0 in this case.
SELECT i.*, COALESCE(max(b.amount), 0)
FROM items i
LEFT JOIN bids b ON b.item_id = i.id
WHERE i.store_id = $store_id
GROUP BY i.id
This alternative form is often faster when large parts of the sub-table are used: Aggregate in a subquery first, join later. This way you don't need an aggregation in the outer query. The second query also demonstrates how you can supply items.min_bid as replacement for NULL values.
SELECT i.*, COALESCE(b.max_amount, i.min_bid)
FROM items i
LEFT JOIN (
SELECT item_id, max(amount) AS max_amount
FROM bids
) b ON b.item_id = i.id
WHERE i.store_id = $store_id;
First, to display the items with no bids, you have to make use of a LEFT JOIN:
select items.*, max(bids.amount)
from items
left join bids on bids.item_id = items.id
where items.store_id = $store_id
group by items.id