I have a new app that I'm trying to figure out data for. Is for recreational vehicle ads and my idea is:
Advertisement - contains AdID plus common items like price, etc.),
RV - contains motorhome specific info,
TravelTrailer - contains travel trailer info
If an ad joins with one or the other conditional on the AdID being found in one of my "secondary" tables (traveltrailer or RV) I have seen queries online like the following:
select
E.EmployeeName, coalesce(s.store,o.office) as Location
from
Employees E
left outer join
Stores S on ...
left outer join
Offices O on ...
Problem is the travel trailer and RV tables will contain some exclusive items that don't exist in the other. As a simple example the tables could be like-
Trailer - length, sleeping capacity
RV - length, sleeping capacity, motor
Using the left join approach above sometimes I might retrieve the motor column (if it's an RV) and sometimes I might not (if it's a travel trailer). I can't figure out how to do that.
I could pass in a type (RV or TravelTrailer) and run one of two different queries based on type but I'd like to just pass in an AdID and get my info with that. I could also do two queries, one to get the ad info using the AdID and then a 2nd one based on the type that comes back from the advertisement table. I'd just like to figure out how to do this in one pull if possible.
Thank you!
I would rethink the way you are storing the data to begin with.
Combine the RV table and TravelTrailer tables as follows:
products (product_id, product_type_id, product_attribute, attribute_value)
Where product_type_id references id on table product_types: (FK constraint)
product_types (id, name)
Ie. you might have RV be id #1 and motor homes id #2.
and where product_attribute_id references id on table product_attributes: (FK constraint)
product_attributes (id, name)
Ie. you might have weight be id #1, length be id #2, sleep capacity be id #3, etc.
This way you will not have some products in table X, some products in table Y, others in Z, etc. and you can write queries more normally. If your queries are constantly more complex than they need to be, it's usually design related.
One approach would be this:
select
Advertisement.AdId
,Advertisement.Price
,coalesce(Trailer.Length,RV.Length) as Length
,coalesce(Motor,"") as Motor
-- other fields as desired
from Advertisement
left join Trailer on Trailer.AdID = Advertisement.AdId
left join RV on RV.AdID = Advertisement.AdId;
Related
I currently have three tables.
master_tradesmen
trades
master_tradesmen_trades (joins the previous two together in a many-to-many relationship). The 'trade_id' and 'master_tradesman_id' are the foreign keys.
Here is what I need to happen. A user performs a search and types in a trade. I need a query that displays all of the information from the master_tradesmen table whose trade in the master_tradesmen_trade table matches the search. For example, if 'plumbing' is typed in the search bar (trade_id 1), all of the columns for Steve Albertsen (master_tradesman_id 6) and Brian Terry (master_tradesman_id 8) would be displayed from the master_tradesmen table. As a beginner to SQL, trying to grasp the logic of this is about to make my head explode. I'm hoping that someone with more advanced SQL knowledge can wrap their head around this much easier than I can. Note: the 'trades' column in master_tradesmen is for display purposes only, not for querying. Thank you so much in advance!
You have a catalog for the tradesmen, & another catalog for the trades.
The trades should only appear once in the trades catalog in order to make your DB more consistent
Then you have your many-to-many table which connects the trades & master tradesmen tables.
If we want to get the tradesmen according to the given trade in the input, we should first
know the id of that trade which has to be unique, so in your DB you would have something
like the img. below :
Now we can make a query to select the id of trade :
DECLARE #id_trade int = SELECT trade_id FROM trades WHERE trade_name LIKE '%plumbing%'
Once we know the trading id, we can redirect to the 'master_tradesmen_trades' table to know the name of the people how work that trade :
SELECT * FROM master_tradesmen_trades WHERE trade_id = #id_trade
You will get the following result :
You may say, 'But there is still something wrong with it, as i am still not able to see the tradesmen', this is the moment when we make an inner join:
SELECT * FROM master_tradesmen_trades trades_and_tradesmen
INNER JOIN master_tradesman tradesmen
ON tradesmen.id = trades_and_tradesmen.master_tradesmen_id
WHERE trade_id = #id_trade
IF you need to see specific columns, you can do :
SELECT first_name, last_name, city, state FROM master_tradesmen_trades trades_and_tradesmen
INNER JOIN master_tradesman tradesmen
ON tradesmen.id = trades_and_tradesmen.master_tradesmen_id
WHERE trade_id = #id_trade
I have a few tables set up in Access as follows (forgive the slightly redundant example content):
Table 1:
- ID
- FirstName
- SecondName
Table 2:
- ID
- Details
- PersonID -> Table 1[ID]
Table 3:
- ID
- Group
- PersonDetails -> Table 2[ID]
Table 1 is the base table containing records and retrieving no other information. For example, it could store someone's first and second names, along with an autonumber ID.
Table 2 contains records which, amongst other things, contain a field that links to Table 1 and stores the ID of one of the records held there. With the lookup wizard I can choose to utilise all fields from Table 1, store the ID of the Table 1 record in the Table 2 field and also display the first and second names in the combobox on the form to make choosing a record more intuitive.
In table 3, I need to store the ID of one of the records in Table 2. However, I would also like to again display in the form combobox the first and second names from the related record (in Table 1) whose ID is stored in Table 2. I can't choose to utilise, for example, the PersonDetails field from table 2 as this just puts ID numbers into the combobox - I'd need to do something equivalent of:
Table 2[ID]->[FirstName]
Is this possible to do with the lookup wizard in Access or would I have to look into queries or VBA?
Thanks.
Your query for your combo should look something like this:
SELECT cp.ID, cp.ReferenceName, c.Company, p.FeePerHour
FROM (ClientProfile AS cp LEFT JOIN Clients AS c ON cp.ClientID = c.ID)
LEFT JOIN Pricing AS p ON cp.PricePlanID = p.ID;
The main problem with your query is that you're missing the Parenthesis that are needed when you have multiple joins. If you had another join, you'd need a second set of parenthesis.
I took some liberty and used table aliases. It makes SQL concise and more readable.
If this query still doesn't work it might be because you're trying to join "child tables" to the "main table" without using your Foreign Key fields as the joining field. If this were my database the SQL would look something like this:
SELECT cp.ClientProfileID, cp.ReferenceName, c.Company, p.FeePerHour
FROM (ClientProfile AS cp LEFT JOIN Clients as C ON cp.ClientID = c.ClientID)
LEFT JOIN Pricing AS p ON cp.ClientProfileID = p.ClientProfileID;
Personally, I basically never use just plain ID as a field name. And when creating a foreign key I usually use the same field name as what the original table has. There are exceptions, especially in the case where you need to link back to the same table more than once. Consider the case where you are tracking real estate properties and you have a Buyer and a Seller that are both entities (but different from each other) in the same People table. You would then need to name them something like BuyerID and SellerID but ultimately they would link back to the PersonID in the Person table.
So Here's the table design I would go for. Notice I changed Group to PriceGroup. Watch out for reserved words when programming, not only in SQL but any language. If you name your field Group at some point you'll be trying to "Group on Group" which sounds and looks bad, if nothing else.
Client:
- ClientID
- FirstName
- SecondName
ClientProfile:
- ClientProfileID
- Details
- ClientID (Foreign Key to Client)
Pricing:
- PricingID
- PriceGroup
- ClientProfileID (Foreign Key to ClientProfile)
I'm curious about how I could go about getting the data I need out of a "circle" of tables.
I have 5 tables(and a few supporting ones): 3 entities joined by junction tables. So the general model is like this:
Cards have many Budgets, and Accounts have many Budgets, and Accounts have many Cards.
So my relationships make a circle, through the junction tables, form Card to Budget to Account back to Card, This structure works all fine and dandy until today when I tried to construct a query using all 5 tables, and noticed that I know of no way to avoid abiguous joins which this structure in place. I'm thinking it might have been a better idea to create AccountBudget and CardBudget tables, but since they will both define exactly the same type of data, one table seemed more efficient.
The information I'm trying to get is basically the total budget limit for all cards of a certain type, and the total budget limit for all accounts of that same type. Am I just looking at this problem wrong?
// Card Budget_Card Budget Budget_Account Account
// ------- --------- -------- -------------- ---------
// cardId------\ budgetId<---------budgetId------>budgetId -----accountId--(to Card)->
// accountId --->cardId limit accountId<------/ typeId
// (etc) typeId (etc)
// (typeId in Budget is either 1 for an account budget or 2 for a card budget.)
As you can see, it's a circle. What I'm trying to accomplish is return one row with two columns: the sum of Budget.limit for the record in Account where typeId = 1, and the sum of Budget.limit for all rows in Card belonging to Accounts of the same type.
As per suggestion, I can in fact get the data I need from a union, but it's no use to me if the data is not in two separate columns:
SELECT DISTINCTROW Sum(Budget.limit) AS SumOfLimit
FROM (Account RIGHT JOIN Card ON Account.accountId = Card.accountId)
RIGHT JOIN (Budget LEFT JOIN Budget_Card ON Budget.budgetID = Budget_Card.budgetId) ON Card.cardId = Budget_Card.cardId
GROUP BY Budget.typeId, Budget.quarterId, Account.typeId
HAVING (((Budget.typeId)=2) AND ((Budget.quarterId)=[#quarterId]) AND ((Account.typeId)=[#accountType]))
UNION SELECT DISTINCTROW Sum(Budget.limit) AS SumOfLimit
FROM Budget LEFT JOIN (Account RIGHT JOIN Budget_Account ON Account.accountId = Budget_Account.accountId) ON Budget.budgetID = Budget_Account.budgetId
GROUP BY Budget.typeId, Budget.quarterId, Account.typeId
HAVING (((Budget.typeId)=1) AND ((Budget.quarterId)=[#quarterId]) AND ((Account.typeId)=[#accountType]));
So, if I understand you correctly, you've made separate column headers with the same name, and so your data becomes skewed because the information needs to be separated? If this is the case I would suggest changing the column headers as you've proposed, or in linking two queries together. To connect the data by querying the same tagged name will combine results. If you want to designate something, it's always a good idea to create separate names for column headers.
Here is an explanation of using SQL to query multiple tables: http://www.techrepublic.com/article/sql-basics-query-multiple-tables/1050307
First make the query for the Cards, then union with the query for the Accounts
Although it would be easier to relate cards to accounts and then only have budgets for accounts, however i don't know if that would work with your schema
The database is quite simple. Below there is a part of a schema relevant to this question
ROUND (round_id, round_number)
TEAM (team_id, team_name)
MATCH (match_id, match_date, round_id)
OUTCOME (team_id, match_id, score)
I have a problem with query to retrieve data for all matches played. The simple query below gives of course two rows for every match played.
select *
from round r
inner join match m on m.round_id = r.round_id
inner join outcome o on o.match_id = m.match_id
inner join team t on t.team_id = o.team_id
How should I write a query to have the match data in one row?
Or maybe should I redesign the database - drop the OUTCOME table and modify the MATCH table to look like this:
MATCH (match_id, match_date, team_away, team_home, score_away, score_home)?
You can almost generate the suggested change from the original tables using a self join on outcome table:
select o1.team_id team_id_1,
o2.team_id team_id_2,
o1.score score_1,
o2.score score_2,
o1.match_id match_id
from outcome o1
inner join outcome o2 on o1.match_id = o2.match_id and o1.team_id < o2.team_id
Of course, the information for home and away are not possible to generate, so your suggested alternative approach might be better after all. Also, take note of the condition o1.team_id < o2.team_id, which gets rid of the redundant symmetric match data (actually it gets rid of the same outcome row being joined with itself as well, which can be seen as the more important aspect).
In any case, using this select as part of your join, you can generate one row per match.
you fetch 2 rows for every matches played but team_id and team_name are differents :
- one for team home
- one for team away
so your query is good
Using the match table as you describe captures the logic of a game simply and naturally and additionally shows home and away teams which your initial model does not.
You might want to add the round id as a foreign key to round table and perhaps a flag to indicate a match abandoned situation.
drop outcome. it shouldn't be a separate table, because you have exactly one outcome per match.
you may consider how to handle matches that are cancelled - perhaps scores are null?
Where are Cartesian Joins used in real life?
Can some one please give examples of such a Join in any SQL database.
just random example. you have a table of cities: Id, Lat, Lon, Name. You want to show user table of distances from one city to another. You will write something like
SELECT c1.Name, c2.Name, SQRT( (c1.Lat - c2.Lat) * (c1.Lat - c2.Lat) + (c1.Lon - c2.Lon)*(c1.Lon - c2.Lon))
FROM City c1, c2
Here are two examples:
To create multiple copies of an invoice or other document you can populate a temporary table with names of the copies, then cartesian join that table to the actual invoice records. The result set will contain one record for each copy of the invoice, including the "name" of the copy to print in a bar at the top or bottom of the page or as a watermark. Using this technique the program can provide the user with checkboxes letting them choose what copies to print, or even allow them to print "special copies" in which the user inputs the copy name.
CREATE TEMP TABLE tDocCopies (CopyName TEXT(20))
INSERT INTO tDocCopies (CopyName) VALUES ('Customer Copy')
INSERT INTO tDocCopies (CopyName) VALUES ('Office Copy')
...
INSERT INTO tDocCopies (CopyName) VALUES ('File Copy')
SELECT * FROM InvoiceInfo, tDocCopies WHERE InvoiceDate = TODAY()
To create a calendar matrix, with one record per person per day, cartesian join the people table to another table containing all days in a week, month, or year.
SELECT People.PeopleID, People.Name, CalDates.CalDate
FROM People, CalDates
I've noticed this being done to try to deliberately slow down the system either to perform a stress test or an excuse for missing development deliverables.
Usually, to generate a superset for the reports.
In PosgreSQL:
SELECT COALESCE(SUM(sales), 0)
FROM generate_series(1, 12) month
CROSS JOIN
department d
LEFT JOIN
sales s
ON s.department = d.id
AND s.month = month
GROUP BY
d.id, month
This is the only time in my life that I've found a legitimate use for a Cartesian product.
At the last company I worked at, there was a report that was requested on a quarterly basis to determine what FAQs were used at each geographic region for a national website we worked on.
Our database described geographic regions (markets) by a tuple (4, x), where 4 represented a level number in a hierarchy, and x represented a unique marketId.
Each FAQ is identified by an FaqId, and each association to an FAQ is defined by the composite key marketId tuple and FaqId. The associations are set through an admin application, but given that there are 1000 FAQs in the system and 120 markets, it was a hassle to set initial associations whenever a new FAQ was created. So, we created a default market selection, and overrode a marketId tuple of (-1,-1) to represent this.
Back to the report - the report needed to show every FAQ question/answer and the markets that displayed this FAQ in a 2D matrix (we used an Excel spreadsheet). I found that the easiest way to associate each FAQ to each market in the default market selection case was with this query, unioning the exploded result with all other direct FAQ-market associations.
The Faq2LevelDefault table holds all of the markets that are defined as being in the default selection (I believe it was just a list of marketIds).
SELECT FaqId, fld.LevelId, 1 [Exists]
FROM Faq2Levels fl
CROSS JOIN Faq2LevelDefault fld
WHERE fl.LevelId=-1 and fl.LevelNumber=-1 and fld.LevelNumber=4
UNION
SELECT Faqid, LevelId, 1 [Exists] from Faq2Levels WHERE LevelNumber=4
You might want to create a report using all of the possible combinations from two lookup tables, in order to create a report with a value for every possible result.
Consider bug tracking: you've got one table for severity and another for priority and you want to show the counts for each combination. You might end up with something like this:
select severity_name, priority_name, count(*)
from (select severity_id, severity_name,
priority_id, priority_name
from severity, priority) sp
left outer join
errors e
on e.severity_id = sp.severity_id
and e.priority_id = sp.priority_id
group by severity_name, priority_name
In this case, the cartesian join between severity and priority provides a master list that you can create the later outer join against.
When running a query for each date in a given range. For example, for a website, you might want to know for each day, how many users were active in the last N days. You could run a query for each day in a loop, but it's simplest to keep all the logic in the same query, and in some cases the DB can optimize the Cartesian join away.
To create a list of related words in text mining, using similarity functions, e.g. Edit Distance