I'm using MS Access 2016.
Suppose I have a Student table, a Subject table (ie Geography, History etc) and a StudentSubject table that records which subjects each student has chosen.
StudentSubject contains StudentId and SubjectId as foreign keys.
No student is to choose more that 5 subjects.
Is there any way to define a validation rule on the table such that a given StudentId may not appear in the StudentSubject table more than 5 times?
I could enforce the contraint by introducing an additional table, but if possible, I like to avoid that.
I also want to define a constraint at table level, rather than use vba code that gets invoked when a record is inserted via a form. Access, as far as I know, has no such thing as triggers, as one would have in an sql system.
You can use a CHECK constraint to limit the possibilities:
ALTER TABLE StudentSubject
ADD CONSTRAINT Max5Subjects
CHECK(
NOT EXISTS(
SELECT 1
FROM StudentSubject
GROUP BY StudentID
HAVING Count(StudentID) > 5
)
)
Note that this might slow down data entry a bit, especially if StudentID is not indexed.
Check constraints need to be executed either in ANSI 92 compatible mode, or using ADO (e.g. using CurrentProject.Connection.Execute). More details here
To execute it using ADO, you can just use this single line in the immediate window:
CurrentProject.Connection.Execute "ALTER TABLE StudentSubject ADD CONSTRAINT Max5Subjects CHECK(NOT EXISTS( SELECT 1 FROM StudentSubject GROUP BY StudentID HAVING Count(StudentID) > 5))"
Also, keep in mind that if somehow there are records that violate the constraint (e.g. because they were present before the constraint got added), your table will get locked down fully.
Related
I am working on this assignment question and it is asking me:
To create a table called (TEMP_CUST) from an existing table Customers
View the content and constraints of TEMP_CUST table
What I have done so far is I have created my table, didn't add any constraints to the table TEMP_CUST and viewed the table using the DESC command.
Here is the code for table creation
CREATE TABLE TEMP_CUST
AS
(SELECT
CUSTOMER#, LASTNAME,
FIRSTNAME, ADDRESS, CITY,
STATE, ZIP, REFERRED,
REGION, EMAIL
FROM
CUSTOMERS);
DESC TEMP_CUST;
Now that I have done that I want to view the constraints of the table. I have used this command but am not sure if it is correct.
SELECT *
FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'TEMP_CUST';
i have used this command, but not sure if it is correct.
You haven't said why you don't think it's correct so we have to guess the reason for your doubt. Perhaps it's because the set of constraints you get is smaller than the set of constraints for the original CUSTOMERS table?
That is correct. When we use CREATE TABLE ... AS SELECT the statement creates a new table with the projection, column names and datatypes of the original tables (assuming a vanilla SELECT clause) and the data (determined by the WHERE clause, if any). However, the only constraints which are created are NOT NULL constraints on the primary key column(s) and any other mandatory columns. The new table does not have primary key, foreign key or check constraints. We have to create these explicitly.
Hence, this query ...
SELECT * FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'TEMP_CUST';
... might return fewer constraints than you were expecting.
Below are my table structures :
Table -Customer
CustomerID Blacklisted Customer Name
101 Y ABC
102 Y DEF
Table -Blacklist
CustomerID BlacklistID Customer Name
101 1011 ABC
102 1012 DEF
Table -Reason
BlacklistID ReasonID Reason Code
1012 02 Rcode2
Main table "Customer" is to store customer information.There is a trigger after update on table "Customer" to insert record in table "Blacklist" if somebody updates the blacklisted as Y in customer table.
We consider the customer as blacklisted if ,
Blacklisted column in Customer table as value 'Y' and.
There are records present for customer in Blacklist and Reason table
Now my requirement is to blacklist the customer from backend.For this i am writing stored procedure with below queries:
Update customer set blacklisted ='Y' where customerid='102';
select BlacklistID into var_id from blacklist where customerid='102';
Insert into reason(BlacklistID,ReasonID,ReasonCode)values(var_ id,111,'RCODE1');
Now to insert entry in Reason table(step-3),i need BlacklistID which is a foreign key and i will get the value of BlacklistID once the trigger on customer table gets exceuted.So my confusion is, can i assume the trigger on update of 'Customer' table will always get excuted before the cntrl reaches my INSERT INTO reason(step-3) statement. Please suggest.
If you need to be certain about the order of trigger execution, you can specify this order when creating the trigger.
This is done with the FOLLOWS ... and PRECEEDS ... options of the create trigger statement:
More details in the manual: http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/create_trigger.htm#CJAEJAFB
FOLLOWS | PRECEDES
Specifies the relative firing of triggers that have the same timing point. It is especially useful when creating crossedition triggers, which must fire in a specific order to achieve their purpose.
Use FOLLOWS to indicate that the trigger being created must fire after the specified triggers. You can specify FOLLOWS for a conventional trigger or for a forward crossedition trigger.
Use PRECEDES to indicate that the trigger being created must fire before the specified triggers. You can specify PRECEDES only for a reverse crossedition trigger.
Yes. Triggers are part of the statement. Although you cannot be fully certain *) of the order in which multiple triggers in the same statement are executed, you can be certain that they al are done when the statement itself is done. So by the time of step 2, all update triggers of step one have fired.
*) Actually, the default order is:
Statement level before triggers
Row level before triggers
Row level after triggers
Statement level after triggers
But if you have, say, two row level before trigger, by default you cannot be certain in which order those two are executed. But I learned from the comments that in Oracle 11, you can actually specify the order to cover even those cases.
I don't see a need for all these tables, and therefore no need for a trigger.
Why do you not just use a blacklist reason code in the customer table? The purpose of the blacklist table is unclear as it seems to add no data and just repeats data from the customer table with an additional id column?
Or if you need multiple reason codes then just use a blacklist_reason table that references the customer id and a reason code -- I don't think you even need the blacklisted column in the customer table.
Let assume that I have an empty database and I want to create two tables. There is a relationship between them. For example, one of attributes (call them b) in R is a foreign key. The definition SHOULD (database is empty, I have not executed any statement, yet) look:
create table R(
a type primary key,
b type references S(b)
);
create table S(
b type primary key
);
If I try run script with statements as above, I get error (according to line: b type references S(b)) , because S doesn't exist - that is normal and natural. It is possible to disable checking existing of tables, coresponding to foreign keys?
I know I can change order of statements. Or create tables without constraints, and add them later.
Why I'm asking. Let assume that we have many tables, with many relationships between them. Ordering tables manually will consume a lot of time, when we will want prepare kind of 'backup script'.
I know that it is possible to disable constraint as below:
alter table table_name disable constraint_name;
But that works for example with insert statement. There is a solution proper to create statement?
I have little problem with programming trigger for my dtb. I need to control 2 values in one 1 table. I have table called Concert and it has 2 foreign keys: 1 is the id of table Place. Second is not important for this I think.
Concert: id_concert, id_place<fk>, id_organizer<fk>, date, name, sponsor
Place: id_place, name, capacity, adress, town
What I want to eliminate is, that 2 concerts organized at same day cannot be on one place. So, I need to somehow control that user cannot insert the same date and same place for concert if there already concert with this values exists.
Thank you very much for your suggestions and sorry for bad english.
You need to add a unique constraint on your Concert table that consists of the (id_place, date) pair. This would instruct the database engine to not allow more than one Concert in the same place at the same time.
For Oracle, information can be found here: http://www.techonthenet.com/oracle/unique.php
CREATE TABLE Concert
(
... (filled in with your existing table definition)
CONSTRAINT concert_place_unique UNIQUE (id_place, date)
);
or to alter an existing table:
ALTER TABLE Concert
add CONSTRAINT concert_place_unique UNIQUE (id_place, date);
Constraints are the proper way to handle this condition, not triggers. Constraints are database intrinsic and have no race conditions and prevent the data from being added in the first place.
I have two tables, let's call them TableA and TableB. One record in TableA is related to one or more in TableB. But there's also one special record within them in TableB for each record from TableA (for example with lowest ID), and I want to have quick access to that special one. Data from both tables aren't deleted - it's a kind of history rarely cleared. How do that the best in terms of performance?
I thought of:
1) two-way relationship, but it will affect insert performance
2) design next table, with primary key as FK_TableA (for TableA record exactly one is "special") and second column FK_TableB and then create view
3) design next table, with primary key as FK_TableA, FK_TableB, make FK_TableA unique and then create view
I'm open for all other ideas :)
4) I'd consider an indexed view to hide the JOIN and row restriction
This is similar to your options 2 and 3 but the DB engine will maintain it for you. With a new table you'll either compromise data integrity or have to manage the data via triggers