Oracle newbie here. I need to build a database which fulfils the requirements below:
A department is allowed to register for only two programs in a year
The maximum participants in each program must not exceed the number of people in respective departments.
*There are 14 departments in total.
As per requirement, seems like I have to restrict the number of rows inserted.
For example, if the total number of people in Department A is 100, the 101st row has to be rejected.
Apologies if there are many errors as I'm writing this question because now is 1.30AM. I tried to keep the table simple with less columns so it's easier to test the code.
CREATE TABLE department(
DEPT_ID CHAR(5) not null primary key,
TOTAL_P NUMBER);
CREATE TABLE participant(
P_ID CHAR(5) not null primary key,
DEPT_ID CHAR(5) not null);
CREATE TABLE program(
PROG_ID CHAR(5) not null primary key,
PROG_NAME VARCHAR(30),
DEPT_ID CHAR(5),
START_DATE DATE,
END_DATE DATE,
FOREIGN KEY(DEPT_ID) references department(DEPT_ID) on delete cascade);
and I have tried using trigger, but I keep getting warning: trigger created with compilation errors.
(I tried to count the rows in table program and group them by dept_id, then proceed to check the condition)
CREATE OR REPLACE TRIGGER prog
BEFORE INSERT ON program
DECLARE
CountRows NUMBER;
BEGIN
SELECT COUNT(*)
INTO CountRows
GROUP BY DEPT_ID
FROM program;
IF CountRows > 2 THEN
RAISE_APPLICATION_ERROR(-20001, 'Only 2 programs are allowed');
END IF;
END;
/
I don't even know if my idea does make sense or not. I tried many other ways like putting the condition where(to specify dept_id) before begin, after begin, I still get the warning. I have been experimenting a whole day and still cannot figure it out.
MY QUESTIONS:
Is it better to create multiple conditions in one trigger as I will have 14 departments?
if so, how to do that without getting the warning?
any alternative way to restrict the number of rows?
Any help, hints, tips, anything, is deeply appreciated. thanks.
You could do it by maintaining in the trigger handling insert/update/delete operation on the "child" table (e.g. the program), an intersection table "parent_child" (e.g. department_program) containing the 2 foreign keys on the parent/child tables, and an index on which you will put the check constraint (e.g. < 3 for the number of program per department) + any other column defining the scope of the constraint (e.g. here the year of the start_date of the program). The 2 columns with the FK, the index and the other scope columns should be the PK of this intersection table.
e.g.
CREATE TABLE program_department
(
DEPT_ID CHAR(5),
PROG_ID CHAR(5),
PROG_YEAR NUMBER(4),
PROG_IDX NUMBER(10,0) DEFAULT 0,
-- to force always equal to a number the constraint must be defererrable
CONSTRAINT CK_PROG_IDX CHECK (PROG_IDX >= 0 AND PROG_IDX < 3) ENABLE,
PRIMARY KEY (PROG_ID, DEPT_ID, PROG_YEAR, PROG_CNT)
)
;
The idea is to maintain the PROG_IDX that will contain the index of the relation between the department and the program of the specific year.
In the trigger on the table program, you have to update the program_department according to each action, when updating/removing this may imply/implies decrementing the PROG_IDX of the ones having PROG_IDX greater than the one removed.
And of course you will have to apply about the "same" logic for the participant's relationship, however there you can't hardcode the constrain by a CHECK since the # of people in each department is not known at compile time. This case is more complex also because you have to think about the consequence of changes of the # of persons in a department. Probably you will have to keep in the intersection table, the # of people in the department at the start_date of the program.
Related
I should create a trigger in Oracle SQL that
Add department_amount column to locations table
Add comment 'Contains the amount of departments in the location'
Create a trigger ,which will update the amount of departments in location every time a row is inserted/deleted from departments.
Tables:
CREATE TABLE departments
(
department_id NUMBER(4) PRIMARY KEY,
department_name VARCHAR2(30) NOT NULL,
manager_id NUMBER(6),
location_id NUMBER(4) NOT NULL
);
CREATE TABLE locations
(
location_id NUMBER(4) PRIMARY KEY,
street_address VARCHAR2(40),
postal_code VARCHAR2(12),
city VARCHAR2(30),
state_province VARCHAR2(25),
country_id CHAR(2) NOT NULL
);
To answer your questions
Add department_amount column to locations table
alter table locations add department_amount number ;
Add comment 'Contains the amount of departments in the location'
comment on column locations.deparment_amount is 'Contains the amount of departments in the location';
Create a trigger ,which will update the amount of departments in location every time a row is inserted/deleted from departments.
create or replace trigger trg_loc
after insert or delete on departments
declare
begin
merge into locations t
using ( select count(department_id) as dpt_amount, location_id as loc_id
from departments b
group by location_id ) s
on (t.location_id = s.loc_id)
when matched then
update set t.department_amount = s.dpt_amount ;
end;
/
You have below a db<>fiddle with data example and the trigger demonstration that updates the department_amount in locations table when you insert or delete a department for each location.
db<>fiddle
I think that this task is so much of a don't-do-that that it serves no purpose. Not even for training.
What you'd have to do is
Check whether the column locations.department_amount exists. You can do this by looking into the system view dba_tables (or all_tables or user_tables if appropriate).
If that column does not exist, create it via ALTER TABLE. This, however is DDL, not DML, so you must run it dynamically via EXECUTE IMMEDIATE.
In case you've just created the column, also create the comment.
Now, in that same trigger add or subtract 1 from the department_amount. Thus you guarantee that the very first action (the one that leads to creating the column) already updates it.
Points 1 to 3 only have to happen on statement level, while point 4 should happen on row level. For this reason and in order to avoid problems with other triggers, this trigger should be a compound trigger.
But then, if you can create the trigger in the database, why can't you add the column and its comment, too? Why must you write the trigger such that it has to check whether the column exists? As mentioned, this just makes no sense, so the best thing here is not to do that.
How do I implement a sql command which outputs NULL for salary if it is a voluntary worker?
Here are the tables I created first:
create Table worker (
pid integer references Person,
salary float);
create Table person(
pid integer primary key,
name varchar(30),
adress varchar(30));
Since I'm not sure how to distinguish a normal worker from a voluntary one, I decided to make another table. Unfortunately, I don't know how to insert NULL values for salary for all voluntary workers. That is what I tried out:
create table voluntaryworker(
pid integer references Person,
salary = null);
insert into Person (pid, name, adress) values (1345, anna, 'festreet');
insert into voluntaryworker (pid, salary) values (1345, null);
pid = person ID
Most databases support generated columns. If you really want a salary column in voluntaryworker, then you can use such a column:
create table voluntaryworker (
pid integer references Person,
salary generated always as (cast(null as float))
);
The exact syntax may vary, depending on the database.
Note that having a separate table seems utterly superfluous. Why not just have a flag in the worker table.
Also, representing the salary as a float is quite troublesome. In general, you really should never use floating point representations for monetary amounts. decimal/numeric is much more appropriate for money.
Like others commented, you certainly don't need another table to implement this. All you need is some way to remember whether a worker is voluntary.
To make sure salary sticks to your rule, you can add a CHECK constraint:
CREATE TABLE worker (
pid integer PRIMARY KEY REFERENCES person
, voluntary boolean NOT NULL DEFAULT false
, salary numeric
, CONSTRAINT voluntary_has_no_salary CHECK (NOT voluntary OR salary IS NULL)
);
Meaning: voluntary workers cannot have a nonnull salary.
Alternatively, you might drop the table worker, too, and just add the columns worker_salary and worker_voluntary to table person. (You may need an additional flag worker, or integrate this information in the other two columns ...)
If you are still interested in generated columns (not needed here), see this example with correct syntax and instructions:
Computed / calculated / virtual / derived columns in PostgreSQL
Related:
PostgreSQL: Which Datatype should be used for Currency?
At least insert into voluntaryworker (pid) values (1354); leaves it NULL.
I am using PostgreSQL and am trying to restrict the number of concurrent loans that a student can have. To do this, I have created a CTE that selects all unreturned loans grouped by StudentID, and counts the number of unreturned loans for each StudentID. Then, I am attempting to create a check constraint that uses that CTE to restrict the number of concurrent loans that a student can have to 7 at most.
The below code does not work because it is syntactically invalid, but hopefully it can communicate what I am trying to achieve. Does anyone know how I could implement my desired restriction on loans?
CREATE TABLE loan (
id SERIAL PRIMARY KEY,
copy_id INTEGER REFERENCES media_copies (copy_id),
account_id INT REFERENCES account (id),
loan_date DATE NOT NULL,
expiry_date DATE NOT NULL,
return_date DATE,
WITH currentStudentLoans (student_id, current_loans) AS
(
SELECT account_id, COUNT(*)
FROM loan
WHERE account_id IN (SELECT id FROM student)
AND return_date IS NULL
GROUP BY account_id
)
CONSTRAINT max_student_concurrent_loans CHECK(
(SELECT current_loans FROM currentStudentLoans) BETWEEN 0 AND 7
)
);
For additional (and optional) context, I include an ER diagram of my database schema.
You cannot do this using an in-line CTE like this. You have several choices.
The first is a UDF and check constraint. Essentially, the logic in the CTE is put in a UDF and then a check constraint validates the data.
The second is a trigger to do the check on this table. However, that is tricky because the counts are on the same table.
The third is storing the total number in another table -- probably accounts -- and keeping it up-to-date for inserts, updates, and deletes on this table. Keeping that value up-to-date requires triggers on loans. You can then put the check constraint on accounts.
I'm not sure which solution fits best in your overall schema. The first is closest to what you are doing now. The third "publishes" the count, so it is a bit clearer what is going on.
I'm new to SQL and ask for your help.
There are 3 tables, these are "Employees", "Positions" and "EmployeesPositions".
For example, 2 positions can be attached to one employee.
How to link tables so that duplicates do not occur? I read about foreign keys and JOIN, but I have not yet figured out how to do it correctly.
Table structure:
Employees (id, Name);
Positions (id, Post, Rate); EmployeesPositions - I do not know how to make it right.
What I need: when adding an employee to the "Employees" table, associate an entry with posts from the "Positions" table, but as I wrote above, one employee can be associated with 2 posts (but not always). How correctly to implement the third table (EmployeesPositions), because in Positions only posts and rates are stored, and in EmployeesPositions there should be records, for example, Name1 => Post1 and Post2, and Name2 only Post 1?
If I thought something wrong, tell me please how best to implement it.
There are several ways to solve your problem, each with their own pros and cons.
First if we simplify your problem to "an employee has zero or more positions", then you can use the following table to associate an employee with a position:
create table employeespositions (
employee_id integer not null,
position_id integer not null,
constraint pk_employeespositions
primary key (employee_id, position_id),
constraint fk_employeespositions_employee
foreign key (employee_id) references employees (id),
constraint fk_employeespositions_position
foreign key (position_id) references positions (id)
)
The foreign keys enforce the existence of the employee and the position, while the primary key ensures a combination of employee and position only exists once.
This solution has two downsides:
It does not enforce that an employee has at least one position
It allows an employee to have more than two positions
The second problem is easily fixed by adding a trigger that checks if there is at most 1 position for an employee when attempting to insert (this allows a maximum of two):
create exception tooManyPositions 'Too many positions for employee';
set term #;
recreate trigger employeespositions_bi
active before insert on employeespositions
as
declare position_count integer;
begin
select count(*)
from employeespositions
where employee_id = new.employee_id
into position_count;
if (position_count > 1) then
exception tooManyPositions;
end#
set term ;#
However this solution does not enforce that an employee has at least one position. You could add a before delete trigger that ensures that the last position cannot be deleted, but that does not ensure that a newly created employee has at least one position. If you want to enforce that, you may want to consider using stored procedures for inserting and updating employees and their positions, and have the code of those stored procedures enforce that (eg by requiring a position when creating an employee).
Alternatively, you could also consider denormalizing your design, and making the positions part of the employees record, where the employee has a 'primary' and (optionally) a 'secondary' position.
create table employees (
-- using Firebird 3 identity column, change if necessary
id integer generated by default as identity primary key,
name varchar(100),
primary_position_id integer not null,
secondary_position_id integer,
constraint fk_employees_primary_position
foreign key (primary_position_id) references positions (id),
constraint fk_employees_secondary_position
foreign key (secondary_position_id) references positions (id),
constraint chk_no_duplicate_position
check (secondary_position_id <> primary_position_id)
)
The not null constraint on primary_position_id enforces the existence of this position, while the check constraint prevents assignment of the same position to both columns. Optionally you could consider adding a before insert or update trigger that when primary_position_id is set null, will set it to the value of secondary_position_id and sets secondary_position_id to null.
This solution has the advantage of allowing the enforcement of the existence of a primary position, but may lead to additional complexities when querying positions. This disadvantage can be overcome by creating a view:
create view employeespositions
as
select id as employee_id, primary_position_id as position_id
from employees
union all
select id as employee_id, secondary_position_id as position_id
from employees
where secondary_position_id is not null;
This view can then be used as if it is a table (although you can't insert into it).
I Have table three tables:
The first one is emps:
create table emps (id number primary key , name nvarchar2(20));
The second one is cars:
create table cars (id number primary key , car_name varchar2(20));
The third one is accounts:
create table accounts (acc_id number primary key, woner_table nvarchar2(20) ,
woner_id number references emps(id) references cars(id));
Now I Have these values for selected tables:
Emps:
ID Name
-------------------
1 Ali
2 Ahmed
Cars:
ID Name
------------------------
107 Camery 2016
108 Ford 2012
I Want to
Insert values in accounts table so its data should be like this:
Accounts:
Acc_no Woner_Table Woner_ID
------------------------------------------
11013 EMPS 1
12010 CARS 107
I tried to perform this SQL statement:
Insert into accounts (acc_id , woner_table , woner_id) values (11013,'EMPS',1);
BUT I get this error:
ERROR at line 1:
ORA-02291: integrity constraint (HR.SYS_C0016548) violated - parent key not found.
This error occurs because the value of woner_id column doesn't exist in cars table.
My work require link tables in this way.
How Can I Solve This Problem Please ?!..
Mean: How can I reference tables in previous way and Insert values without this problem ?..
One-of relationships are tricky in SQL. With your data structure here is one possibility:
create table accounts (
acc_id number primary key,
emp_id number references emps(id),
car_id number references car(id),
id as (coalesce(emp_id, car_id)),
woner_table as (case when emp_id is not null then 'Emps'
when car_id is not null then 'Cars'
end),
constraint chk_accounts_car_emp check (emp_id is null or car_id is null)
);
You can fetch the id in a select. However, for the insert, you need to be explicit:
Insert into accounts (acc_id , emp_id)
values (11013, 1);
Note: Earlier versions of Oracle do not support virtual columns, but you can do almost the same thing using a view.
Your approach should be changed such that your Account table contains two foreign key fields - one for each foreign table. Like this:
create table accounts (acc_id number primary key,
empsId number references emps(id),
carsId number references cars(id));
The easiest, most straightforward method to do this is as STLDeveloper says, add additional FK columns, one for each table. This also bring along with it the benefit of the database being able to enforce Referential Integrity.
BUT, if you choose not to do, then the next option is to use one FK column for the the FK values and a second column to indicate what table the value refers to. This keeps the number of columns small = 2 max, regardless of number of tables with FKs. But, this significantly increases the programming burden for the application logic and/or PL/SQL, SQL. And, of course, you completely lose Database enforcement of RI.