I have a database with two tables
One with games
and one with participants
A game is able to have more participants and these are in a different table.
Is there a way to combine these two into one query?
Thanks
You can combine them using the JOIN operator.
Something like
SELECT *
FROM games g
INNER JOIN participants p ON p.gameid = g.gameid
Explanation on JOIN operators
INNER JOIN - Match rows between the two tables specified in the INNER
JOIN statement based on one or more
columns having matching data.
Preferably the join is based on
referential integrity enforcing the
relationship between the tables to
ensure data integrity.
o Just to add a little commentary to the basic definitions
above, in general the INNER JOIN
option is considered to be the most
common join needed in applications
and/or queries. Although that is the
case in some environments, it is
really dependent on the database
design, referential integrity and data
needed for the application. As such,
please take the time to understand the
data being requested then select the
proper join option.
o Although most join logic is based on matching values between
the two columns specified, it is
possible to also include logic using
greater than, less than, not equals,
etc.
LEFT OUTER JOIN - Based on the two tables specified in the join
clause, all data is returned from the
left table. On the right table, the
matching data is returned in addition
to NULL values where a record exists
in the left table, but not in the
right table.
o Another item to keep in mind is that the LEFT and RIGHT OUTER
JOIN logic is opposite of one another.
So you can change either the order of
the tables in the specific join
statement or change the JOIN from left
to right or vice versa and get the
same results.
RIGHT OUTER JOIN - Based on the two tables specified in the join
clause, all data is returned from the
right table. On the left table, the
matching data is returned in addition
to NULL values where a record exists
in the right table but not in the left
table.
Self -Join - In this circumstance, the same table is
specified twice with two different
aliases in order to match the data
within the same table.
CROSS JOIN - Based on the two tables specified in the join clause, a
Cartesian product is created if a
WHERE clause does filter the rows.
The size of the Cartesian product is
based on multiplying the number of
rows from the left table by the number
of rows in the right table. Please
heed caution when using a CROSS JOIN.
FULL JOIN - Based on the two tables specified in the join clause,
all data is returned from both tables
regardless of matching data.
example
table Game has columns (gameName, gameID)
table Participant has columns (participantID, participantName, gameID)
the GameID column is the "link" between the 2 tables. you need a common column you can join between 2 tables.
SELECT gameName, participantName
FROM Game g
JOIN Participat p ON g.gameID = p.gameID
This will return a data set of all games and the participants for those games.
The list of games will be redundant unless you structure it some other way due to multiple participants to that game.
sample data
WOW Bob
WOW Jake
StarCraft2 Neal
Warcraft3 James
Warcraft3 Rich
Diablo Chris
Related
I have to two tables, both have a composite primary key:
OrderNr + CustNr
OrderNr + ItemNr
Can I join both tables with the OrderNr and OrderNr which is each a part of a composite primary key?
Yes, but you may find you get rows from each table that repeat as they combine to make a unique combination. This is called a Cartesian product
Table A
OrderNr, CustNr
1,C1
1,C2
2,C1
2,C2
TableB
OrderNr,ItemNr
1,i1
1,i2
SELECT * FROM a JOIN b ON a.OrderNr = b.OrderNr
1,C1,1,i1
1,C1,1,i2
1,C2,1,i1
1,C2,1,i2
This happens because composite primary keys can contain repeated elements so long as the combination of elements is unique. Joining on only one part of the PK, and that part being an element that is repeated (my custnr 1 repeats twice in each table, even though the itemnr and CustNr mean the rows are unique) results in a multiplied resultset - 2 rows from A that are custnr 1, multiplied by 2 rows from B that are custnr 1, gives 4 rows in total
Does it work with the normal/naturla join too?
Normal joins (INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER) will join the rows from two tables or subqueries when the ON condition is valid. The clause in the ON is like a WHERE clause, yes - in that it represents a statement that is true or false (a predicate). If the statement is true, the rows are joined. You don't even have to make it about data from the tables - you can even say a JOIN b ON 1=1 and every rows from A will get joined to every row from B. As commented, primary keys aren't involved in JOINS at all, though primary keys often rely on indexes and those indexes may be used to speed up a join, but they aren't vital to it.
Other joins (CROSS, NATURAL..) exist; a CROSS join is like the 1=1 example above, you don't specify an ON, every row from A is joined to every row from B, by design. NATURAL JOIN is one to avoid using, IMHO - the database will look for column names that are the same in both tables and join on them. The problem is that things can stop working in future if someone adds a column with the same name but different content/meaning to the two tables. No serious production system I've ever come across has used NATURAL join. You can get away with some typing if your columns to join on are named the same, with USING - SELECT * FROM a JOIN b USING (col) - here both A and B have a column called col. USING has some advantages, especially over NATURAL join, in that it doesn't fall apart if another column of the same name as an existing one but it has some detractors too - you can't say USING(col) AND .... Most people just stick to writing ON, and forget USING
NATURAL join also does NOT use primary keys. There is no join style (that I know of) that will look at a foreign key relationship between two tables and use that as the join condition
And then is it true that if I try to join Primary key and foreign key of two tables, that it works like a "where" command?
Hard to understand what you mean by this, but if you mean that A JOIN B ON A.primarykey = B.primary_key_in_a then it'll work out, sure. If you mean A CROSS JOIN B WHERE A.primarykey = B.primary_key_in_a then that will also work, but it's something I'd definitely avoid - no one writes SQLs this way, and the general favoring is to drop use of WHERE to create joining conditions (you do still see people writing the old school way of FROM a,b WHERE a.col=b.col but it's also heavily discouraged), and put them in the ON instead
So, in summary:
SELECT * FROM a JOIN b ON a.col1 = b.col2
Joins all rows from a with all rows from b, where the values in col1 equal the values in col2. No primary keys are needed for any of this to work out
You can join any table if there is/are logical relationship between them
select *
from t1
JOIN t2
on t1.ORderNr = t2.OrderNr
Although if OrderNr cannot provide unicity between tables by itself, your data will be multiplied.
Lets say that you have 2 OrderNr with value 1 on t1 and 5 OrderNr with value 1 on t2, when you join them, you will get 2 x 5 = 10 records.
Your data model is similar to a problem commonly referred to as a "fan trap". (If you had an "order" table keyed solely by OrderNr if would exactly be a fan trap).
Either way, it's the same problem -- the relationship between Order/Customers and Order/Items is ambiguous. You cannot tell which customers ordered which items.
It is technically possible to join these tables -- you can join on any columns regardless of whether they are key columns or not. The problem is that your results will probably not make sense, unless you have more conditions and other tables that you are not telling us about.
For example, a simple join just on t1.OrderNr = t2.OrderNr will return rows indicating every customer related to the order has ordered every item related to the order. If that is what you want, you have no problem here.
I am trying to get two tables that aren't related to pull a column of data each.
I have one table called AlphaData and one called TLAuth. Each includes a column that is labelled invoice and I need to pull both columns so I can at least start a comparison. TLAuth will include some of the invoice numbers from AlphaData, but not all of them.
Right now I am using the following code:
SELECT Alphadata.Invoice, TLAuth.Invoice
FROM Alphadata
INNER JOIN TlAuth
ON TLauth.TLAUthID = Alphadata.TLAUthID;
But every time I run this it comes up totally blank. There is definitely data in there, I can pull one column of data from each, but not both at the same time. I have even setup a relationship (1 to Many from TL Auth to Alphadata) and it doesn't seem to work so any help would be grand.
If the tables could not match you should use left join
SELECT Alphadata.Invoice, TLAuth.Invoice
From Alphadata
LEFT JOIN TlAuth ON TLauth.TLAUthID=Alphadata.TLAUthID;
I have two tables, Invoices and members, connected by PK/FK relationship through the field InvoiceNum. I have created the following sql and it works fine, and pulls 44 records as expected.
SELECT
INVOICES.InvoiceNum,
INVOICES.GroupNum,
INVOICES.DivisionNum,
INVOICES.DateBillFrom,
INVOICES.DateBillTo
FROM INVOICES
INNER JOIN MEMBERS ON INVOICES.InvoiceNum = MEMBERS.InvoiceNum
WHERE MEMBERS.MemberNum = '20032526000'
Now, I want to replace INVOICES.GroupNum and INVOICES.DivisionNum in the above query with GroupName and DivisionName. These values are present in the Groups and Divisions tables which also have the corresponding Group_num and Division_num fields. I have created the following sql. The problem is that it now pulls 528 records instead of 44!
SELECT
INVOICES.InvoiceNum,
INVOICES.DateBillFrom,
INVOICES.DateBillTo,
DIVISIONS.DIVISION_NAME,
GROUPS.GROUP_NAME
FROM INVOICES
INNER JOIN MEMBERS ON INVOICES.InvoiceNum = MEMBERS.InvoiceNum
INNER JOIN GROUPS ON INVOICES.GroupNum = GROUPS.Group_Num
INNER JOIN DIVISIONS ON INVOICES.DivisionNum = DIVISIONS.Division_Num
WHERE MEMBERS.MemberNum = '20032526000'
Any help is greatly appreciated.
You have at least one relation between your tables which is missing in your query. It gives you extra records. Find all common fields. Say, are divisions related to groups?
The statement is fine, as far as the SQL syntax goes.
But the question you have to ask yourself (and answer it):
How many rows in Groups do you get for that given GroupNum?
Ditto for Divisions - how many rows exist for that DivisionNum?
It would appear that those numbers aren't unique - multiple rows exist for each number - therefore you get multiple rows returned
It seems to me that there are two scenarios in which to use JOINs:
When data would otherwise be duplicated
When data from one query would otherwise be used in another query
Are these scenarios right? Are there any other scenarios in which to use JOIN?
EDIT: I think I've miscommunicated. I understand how a JOIN works, what I'm not so sure about is when to use one.
JOINS are used to JOIN tables together with related information.
Tipical situations are where you have lets say
A user table where the user has specific security settings. The join would be used such that you can determine which settings the user has.
Users
-UserID
-UserName
UserSecurityRoles
-UserID
-SecurityRoleID
SecurityRoles
-SecurityRoleID
-SecurityRole
SELECT *
FROM Users u INNER JOIN
UserSecurityRoles usr ON u.UserID = usr.UserID INNER JOIN
SecurityRoles sr ON usr.SecurityRoleID = sr.SecurityRoleID
WHERE sr.SecurityRole = 'Admin'
LEFT joins will be used in the cases where you wish to retrieve all the data from the table in the left hand side, and only data from the right that match.
JOINS are used when you start normalizing your table structure. You can crete a table with 100s on columns, where a lot of the data could possibly be NULL, or you can normalize the tables, such that you avoid having too many columns with null values, where you group the appropriate data into table structures.
The answer to this Question has a VERY good link that has graphical display of using JOINs
JOINS are used to return data that is related in a relational database. Data can be related in 3 ways
One to Many relationship (A Person can have many Transactions)
Many to Many relationship (A Doctor can have many Patients, but a Patient can have more than one Doctor)
One to One relationship (One Person can have exactly one Passport number)
JOINS come in various flavours:
AN INNER JOIN will return data from both tables where the keys in each table match
A LEFT JOIN or RIGHT JOIN will return all the rows from one table and matching data from the other table
A CROSS JOIN will return the product of each table
You use joins when you need information from more than one table :)
I am trying to do an OUTER JOIN, with multiple join conditions. Here is my query (I will explain issue below):
SELECT ad.*, cp.P_A, cp.P_B, cp.P_C
INTO #AggData3
FROM #AggData2 ad
LEFT OUTER JOIN #CompPriceTemp cp
ON ad.PART=cp.Part_No
and ad.[Month]=cp.[Month]
and ad.[Year]=cp.[Year]
GO
For each record in #AggData2, which is average price and volume by month for each part, I want to join the prices of the three competitors (A, B & C). Thus, I want to join based on Part, Month, and Year. Because some competitors don't offer all parts, I am using a LEFT OUTER JOIN. So, the resulting table (#AggData3), should have the exact same number of rows as the initial table (#AggData2), just with the three additional columns with competitor prices.
However, the new table (#AggData3), has ~35,000 more rows than #AggData2.
Any ideas why that is happening, and how to fix my query.
Because there are multiple rows in Table #CompPriceTemp that match to one row in #AggData2.
Is there one for each of three competitors perhaps? If that is so, then you need three joins, each to the same table, one for each of the 3 competitors?
But if there is supposed to be one row in #CompPriceTemp for each Month, Year, and product, with three separate columns one column for each competitor, then you have some bad data in there.
Wild guess:
ON ad.PART=cp.Part_No
and ad.[Month]=cp.[Month]
and ad.[Year]=cp.[Year]
This query does not uniquely identify rows in CP. Or CP has ~35000 duplicate rows.
Are you sure that you have only one matching row in CompPriceTemp for every single row in AggData2 ?