The system correctly set the foreign key (zemploy01-department) with the check table (zdepartmentt02-department) - inside the red box in the diagram. If i do a system check, the system says: zemploy01-department is consistant.
It is apparent that the system takes all primary keys from the check table, and tries to match it to the keys of the foreign key table. The primary key columns of check table(zdepartmentt02) are: MANDT, CAREER, DEPARTMENT, LANGUAGE. These are matched to MANDT, CAREER, DEPARTMENT columns of the foreign key table(zemploy01). However, only one column should be matched i.e. zdepartmentt02-department and zemploy01-department. Why is the system trying to match all the primary keys of the check table.
The values filled in on this wizard are the fields you have set as primary keys in the check table
Related
I have a Winform that will display columns from 2 tables.
The form will have a bindingnavigator.
The primary table on the form is a "child" of the second table.
I need the opposite of the usual Customer/Orders linkage
where I want to scroll thru the "orders" and automatically
pull the "customer" info.
There is one common column (ID) between the 2 tables. The ID
column in the "orders" table is not unique but the ID column
in the "customer" table is.
The primary key in the "orders" table is a composite of the ID
column and an order# column which is unique.
I was also unable to create a foreign key relationship when
choosing the ID columns in both tables.
I get this error:
"The columns in table 'orders' do not match an existing
primary key or UNIQUE constraint"
What am I missing?
If you have a composite key (I mean your question is indicating it), then you have to add both columns (ID and order) in your foreign key reference.
When table have composite key, you can see a key symbol next to columns that are part of the primary key.
When you have composite key the order of creating FK is important.
Go to keys section of table and select script of primary key as create script to clipboard (right click on primary key and choose). Paste it in new command window and then create FK with order shown in that script of primary key. (So ID and order columns need to be defined in FK in right order)
I am talking about the normalization of a primary key. So let's say my primary key column is of type nvarchar, which violates the rules of normalization. After removing the primary key constraint and the identity specification from the desired column. I need to create a new column which will be the new primary key of that table.
My question is, what should happen with the previous primary key?
I've got an answer that sounds like: "the column should became a semantic key", but i can't understand this answer.
It's not unusual when designing a database schema to use a SURROGATE primary key. The idea is to give each record a unique and permanent identifier so it can be easily referenced by applications and foreign keys. This key has no meaning. Knowing the surrogate key gives you no information about the content of the record. The user of your application would never see this value.
On the other hand, your record may have a SEMANTIC primary key. This is a unique value that identifies this data to that makes sense to the user.
For example, let's say you have a table of Employees. The employer assigns each employee a unique Employee ID Number. Let's say you store this value as a string. To the user that value serves as the unique identifier that refers to that employee. Meanwhile, your table may have a numeric column that serves as the unique identifier for that record.
create table Employee ( EmployeeRecordID int identity(1,1) primary key,
EmployerAssignedID nvarchar(12),
EmployeeName nvarchar(60),
Salary money )
insert into Employee ( EmployerAssignedID, EmployeeName, Salary ) values
( '#ABC100', 'Fred', 25000.12 ),
( '#AZZ314', 'Mary', 37700.00 ),
( '#MAA719', 'Fran', 34444.04 ),
( '#MZA977', 'Mary', 36000.00 )
As each record is added, SQL Server generates a unique EmployeeRecordID for each record, starting with 1. This is the SURROGATE key. Within your database and within your application, you would use this value to reference the record.
But when your application is communicating with the users, you would use the EmployerAssignedID. This is the SEMANTIC primary key. It makes sense to your users to use this value to search for a particular employee.
A primary key is no more than a unique index which can't have NULL value as a key. Like any of indexes it can be clustered or nonclustered.
Deleting a clustered index makes table become a heap with changes in structure and behaviour. Deleting a nonclustered index is just deallocation its space and does not affect that table and other indexes on the table as well.
So after deleting you just have a column(s) with unique values and you are able to consider them as a semantic key until some duplicate values are inserted.
Say I have a table called Matchup that contains two sports teams. I also have a table called Pick that has a column that must match either Team_A OR Team_B. So it's a foreign key of one OR the other columns in Matchup. Is this possible?
Matchup
Team_A
Team_B
Pick
Pick_Team - FK Matchup (Must match Team_A or Team_B from Matchup).
I would split your Matchup table into two: Matchup proper and MatchupDetails.
The Matchup table would have a MatchupID column as its primary key.
The MatchupDetails one would consist of at least two columns: MatchupID to reference the Matchup table and TeamID to reference the Team table (you do have one, don't you?). The two columns would form the table's composite primary key.
Finally, there would be this Pick table. Since you've got multiple users (as per one of your comments), there would need to be a UserID reference. Two more columns, MatchupID & TeamID would serve as a composite foreign key referencing the corresponding column set in MatchupDetails. And to ensure that one user can pick no more than one team from a match-up, a composite primary key of (UserID, MatchupID) should do.
To summarise, here's a complete outline of the relevant part of the schema:
Matchup:
MatchupID
PRIMARY KEY (MatchupID)
MatchupDetails:
MatchupID
TeamID
FOREIGN KEY (MatchupID)
FOREIGN KEY (TeamID)
PRIMARY KEY (MatchupID, TeamID)
Pick:
UserID
MatchupID
TeamID
FOREIGN KEY (UserID)
FOREIGN KEY (MatchupID, TeamID)
PRIMARY KEY (UserID, MatchupID)
I dont think that this is the correct approach.
I would rather recomend that you add an additional field to table Matchup (lets say Pick) and add a CHECK CONSTRAINT to ensure that it is either Team_A or Team_B.
CHECK constraints enforce domain integrity by limiting the values that
are accepted by one or more columns. You can create a CHECK constraint
with any logical (Boolean) expression that returns TRUE or FALSE based
on the logical operators.
From FOREIGN KEY Constraints
In a foreign key reference, a link is created between two tables when
the column or columns that hold the primary key value for one table
are referenced by the column or columns in another table. This column
becomes a foreign key in the second table.
And it does not seem to be what you are looking for.
I'm having trouble figuring out how to create a foreign key constraint. My data model is fixed and out of my control, it looks like this:
CREATE TABLE Enquiry
(Enquiry_Ref INTEGER PRIMARY KEY CLUSTERED, Join_Ref INTEGER, EnquiryDate, EnquiryType...)
CREATE TABLE Contact
(Contact_Ref INTEGER PRIMARY KEY CLUSTERED, Surname, Forenames ....)
CREATE TABLE UniversalJoin
(Join_Ref INTEGER, Contact_Ref INTEGER, Rel_Type INTEGER)
Each Enquiry has exactly one Contact. The link between the two is the UniversalJoin table where
Enquiry.Join_Ref = UniversalJoin.Join_Ref AND
Rel_Type = 1 AND
UniversalJoin.Contact_Ref = Contact.Contact_Ref
The Rel_Type differs depending on what the source table is, so in the case of Enquiry, Rel_Type is 1 but for another table it would set to N.
My question is how do I create a foreign key constraint to enforce the integrity of this relationship? What I want to say, but can't, is:
CREATE TABLE Enquiry
...
CONSTRAINT FK_Foo
FOREIGN KEY (Join_Ref)
REFERENCES UniversalJoin (JoinRef WHERE Rel_Type=1)
You can't use conditional or filtered foreign keys in SQL Server
In these cases, you could have a multiple column FK between (JoinRef, Rel_Type) and set a check constraint on Rel_Type in UniversalJoin to make it 1.
However, I think you are trying to have a row with multiple parents which can't be done.
You might rather want to have a look at CHECK Constraints
CHECK constraints enforce domain
integrity by limiting the values that
are accepted by a column. They are
similar to FOREIGN KEY constraints in
that they control the values that are
put in a column. The difference is in
how they determine which values are
valid: FOREIGN KEY constraints obtain
the list of valid values from another
table, and CHECK constraints determine
the valid values from a logical
expression that is not based on data
in another column.
You could use a table trigger with INSERT and Update to layer the equivalent as a FK.
This way you are able to apply conditions i.e. if column value =1 check exists in table a if column value = 2 then check another table.
Would the following SQL statement automatically create an index on Table1.Table1Column, or must one be explicitly created?
Database engine is SQL Server 2000
CREATE TABLE [Table1] (
. . .
CONSTRAINT [FK_Table1_Table2] FOREIGN KEY
(
[Table1Column]
) REFERENCES [Table2] (
[Table2ID]
)
)
SQL Server will not automatically create an index on a foreign key. Also from MSDN:
A FOREIGN KEY constraint does not have
to be linked only to a PRIMARY KEY
constraint in another table; it can
also be defined to reference the
columns of a UNIQUE constraint in
another table. A FOREIGN KEY
constraint can contain null values;
however, if any column of a composite
FOREIGN KEY constraint contains null
values, verification of all values
that make up the FOREIGN KEY
constraint is skipped. To make sure
that all values of a composite FOREIGN
KEY constraint are verified, specify
NOT NULL on all the participating
columns.
As I read Mike's question, He is asking whether the FK Constraint will create an index on the FK column in the Table the FK is in (Table1). The answer is no, and generally. (for the purposes of the constraint), there is no need to do this The column(s) defined as the "TARGET" of the constraint, on the other hand, must be a unique index in the referenced table, either a Primary Key or an alternate key. (unique index) or the Create Constraint statment will fail.
(EDIT: Added to explicitly deal with comment below -)
Specifically, when providing the data consistency that a Foreign Key Constraint is there for. an index can affect performance of a DRI Constraint only for deletes of a Row or rows on the FK side. When using the constraint, during a insert or update the processor knows the FK value, and must check for the existence of a row in the referenced table on the PK Side. There is already an index there. When deleting a row on the PK side, it must verify that there are no rows on the FK side. An index can be marginally helpful in this case. But this is not a common scenario.
Other than that, in certain types of queries, however, where the query processor needs to find the records on the many side of a join which uses that foreign key column. join performance is increased when an index exists on that foreign key. But this condition is peculiar to the use of the FK column in a join query, not to existence of the foreign Key constraint... It doesn't matter whether the other side of the join is a PK or just some other arbitrary column. Also, if you need to filter, or order the results of a query based on that FK column, an index will help... Again, this has nothing to do with the Foreign Key constraint on that column.
No, creating a foreign key on a column does not automatically create an index on that column. Failing to index a foreign key column will cause a table scan in each of the following situations:
Each time a record is deleted from the referenced (parent) table.
Each time the two tables are joined on the foreign key.
Each time the FK column is updated.
In this example schema:
CREATE TABLE MasterOrder (
MasterOrderID INT PRIMARY KEY)
CREATE TABLE OrderDetail(
OrderDetailID INT,
MasterOrderID INT FOREIGN KEY REFERENCES MasterOrder(MasterOrderID)
)
OrderDetail will be scanned each time a record is deleted in the MasterOrder table. The entire OrderDetail table will also be scanned each time you join OrderMaster and OrderDetail.
SELECT ..
FROM
MasterOrder ord
LEFT JOIN OrderDetail det
ON det.MasterOrderID = ord.MasterOrderID
WHERE ord.OrderMasterID = #OrderMasterID
In general not indexing a foreign key is much more the exception than the rule.
A case for not indexing a foreign key is where it would never be utilized. This would make the server's overhead of maintaining it unnecessary. Type tables may fall into this category from time to time, an example might be:
CREATE TABLE CarType (
CarTypeID INT PRIMARY KEY,
CarTypeName VARCHAR(25)
)
INSERT CarType .. VALUES(1,'SEDAN')
INSERT CarType .. VALUES(2,'COUP')
INSERT CarType .. VALUES(3,'CONVERTABLE')
CREATE TABLE CarInventory (
CarInventoryID INT,
CarTypeID INT FOREIGN KEY REFERENCES CarType(CarTypeID)
)
Making the general assumption that the CarType.CarTypeID field is never going to be updated and deleting records would be almost never, the server overhead of maintaing an index on CarInventory.CarTypeID would be unnecessary if CarInventory was never searched by CarTypeID.
According to: https://learn.microsoft.com/en-us/sql/relational-databases/tables/primary-and-foreign-key-constraints?view=sql-server-ver16#indexes-on-foreign-key-constraints
Unlike primary key constraints, creating a foreign key constraint does not automatically create a corresponding index