I have created two separate tables.
Table 1 (username,email,gender,age)
Table 2(taskname,description,createdon,duedate,workstatus).
Now I want to create a
table 3 (taskname,description,createdon,duedate,staffs,workstatus)
where staffs will show all the registered members' name and admin can select multiple users to allocate in a task.
I am a premature php learner.
Will anyone help me out to solve the problem,please ?
First of all, create two primary keys as user_id and task_id in table 1 and table 2 respectively. Once you are done with it, create table 3 with fields user_id and task_id as two foreign keys referencing table 1 and table 2 respectively and your requirement should get fulfilled.
Related
in a school management system, we need to incorporate 3 semester grades for each subject by each student.
after a discussion, i came up with two solutions.
solution 1
create 3 tables for each semester. (gradeSemester1, gradeSemester2, gradeSemester3)
solution 2
create 1 table called, semesterGrades and with a type handle all 3 semesters.
the reason for solution 1 is to stop data duplication. for example, if there is 8 subjects for a student. this can only contain 8 records in a table where as in solution 2 it can contain up to 24 records of a single student.
what will be the best solution when performance is a major concern ? why solution 2 is better ?
You don't want to change the Database structure depending on the data, so creating a new table whenever you need a new semester is bad design.
All you need is one extra table to store the grades. However I would personally not store them in columns, but in rows to be more flexible (maybe some day you want more than the grades of only 3 semesters).
The table would look like this:
ID | StudentID | SemesterID | SubjectID | Grade
Another advantage of this approach is that you know which semester a grade belongs to. If you have 3 columns for the grades, you only know the grades but you have no information about the semester (I'm guessing he could take more than 3 semesters if needed).
Also I would not worry about performance with this approach. You will have to join tables together but with the proper indexes set up that should not be an issue.
Joining tables is way more costly than a single select because in a join you're selecting and THEN pairing to create a single huge table.
Why not Solution 3:
Create 1 table with 3 columns (one for each semester). That's effectively what your join will be doing each time anyway. Is there any reason to keep them separate?
EDIT (explanation):
Unless I'm misunderstanding something...
A single row in this table would relate a student to a subject and could contain three columns (one for each semester grade). Assuming you have a table for students AND another table for subjects. Containing three semester grades in this table would still be normalized.
TABLE
----------------------
student_id
subject_id
semester1grade
semester2grade
semester3grade
table
ID|owner_id|work_id|lorem|etc|
1 |00123 | 00213 |XXXXX|XXX|
2 |00124 | 00213 |XXXXX|XXX|
owner_id (fk) owners.id (owners[id,name,etc])
work_id (fk) work.id (work[id,name,etc])
question is can I set codeigniter that when I
select(table.*,work.name as work,owners.name as owner) from table
it automatically handle joins since that table already contain the fk-ref ? or I must include join('owner','owner.id=table.owner_id) ?
actually all what I want is that when I select a table that contains a fk this fk column is replaced with one column from ref row by just passing the column name on ref table without having to worry about creating a specific function in my module for that each query.
My current solution:
was to create a view for each table that contain a relation and replace all fk columns with desired ref value, but since i have 6 tables 5 of them with fk,i now have 6 tables and 5 view (11 tables)in db which is really kind confusing for me, so any smarter way to do this ?
I think you are making some confusion on what FK is and what it does within a table.
FK constraint grants data integrity when it's present and relates data within tables. It doesn't join anything.
If you want to select records across related tables, you either use a
SELECT * FROM table1,table2 WHERE table1.K1 = table2.FK1
or
SELECT * FROM table1 JOIN table2 ON table1.K1 = table2.FK1
AND YES, you need to tell CodeIgniter to do those queries
I made three tables.
Table1=users.And the column names are userid (auto_increment) and username.
Table2=hobbies. column names are hobbyid (auto_increment) and hobbyname.
Table3=users_hobbies. column names are FK_userid and FK_hobbyid.
Now whenever I register new user and his/her hobbies from a html form, I select the
corresponding userid and hoobyid that is generated from table 1 and table 2
and insert them to table 3 using query
So what is the use of relationship, if I create it between table 1 and 3 and table 2 and 3?
Will the corresponding userid and hobbyid automatically go to table 3 without using query?
No, the userid and hobbyid won't go automatically anywhere.
The major point of relationships or rather constraints is to enforce data integrity. That means you shouldn't be able to add an entry containing id 2, 2 into the users_hobbies table without a user with id 2 and a hobby with id 2.
In order to keep this integrity you can also specify cascadings. (Depending on the Database system, I hardly work with mysql, so I am not sure about that).
That means, you can specify that all users_hobbies for user with id 1 are deleted if the user himself is deleted.
Apologies if this is really stupid but I don't have any experience in php and mysql to know how things should be done. I have a customer table in a mysql db and a group table:
customers - ID name email phone group
groups - ID name description
So I need to assign groups to customers if necessary, this can be more than one group to each customer. So e.g. customer 1 is in group 4,5,6
What way should I assign groups in the group column of the customer table. Should I just add the group ID's separated by commas, then just use explode when I need to get the individual ID's out?
Maybe this isn't the right approach at all, could someone enlighten me please. I would appreciate knowing the right way to do this, thanks.
Do not store multiple IDs in one column. This is a denormalization that will make it much harder to query and change your data, as well as hurting performance.
Instead, create a separate CustomerGroup table (with CustomerID and GroupID columns), and have one row per Customer/Group relationship.
Here is an example of tables to show how you should implement this :
Table 1 CONSUMERS:
id name email
1 john john#something.com
2 ray ray#something.com
Table 2 GROUPS :
id group_name description
1 music good music group
2 programming programmers
Table 3 CONSUMERS_GROUPS
consumer_id group_id
1 1
1 2
2 1
Now the table 3 is listing consumers ids which belong to which group id.
This type of relationship is called one to many relation where, one consumer can have many groups. Reverse might also be true where one group can have many consumers. In that case relationship is called many to many
Should I just add the group ID's separated by commas, then just use explode when I need to get the individual ID's out?
No! If you do that then you won't quickly be able to (for example) query for which users there are in a specific group.
Instead use a join table with two columns, each of which has a foriegn key constraint to the corresponding table.
group_id customer_id
4 1
5 1
6 1
I have 3 employers IDs: 1, 2 and 3. I am generating tasks for each one by adding a line in database and in column "for_emp" I insert IDs I want to assign this task for and could be all 3 of them separated by comma. So let's say I got a task and "for_emp" is "1,2,3", the employers IDs. If I would like to select all tasks for the ID 2, will I be able to select from the row that has "1,2,3" as IDs and just match "2" there ? If not, how do you suggest I insert my emp IDs into one row in database ? The db is MySQL.
Any ideas ? Thanks.
Don't do it like that, you should normalize your database.
What you want to do is have a table such as task, and then task_assignee. task_assignee would have fields task_id and user_id. If a task has eg. three assignees (IDs 1, 2 and 3), then you'll create three rows in the task_assignee table for that one task, like this:
+--------+---------+
|task_id | user_id |
+--------+---------+
| 1 | 1 [
| 1 | 2 [
| 1 | 3 [
+--------+---------+
Then it's just a simple matter of querying the task_assignee table to find all tasks that are assigned to a given user.
Here's an example of how to get all the tasks for user_id 2:
SELECT t.* FROM task AS t INNER JOIN task_assignee AS ta WHERE ta.user_id = 2
EDIT.
Just as a related note, even if you didn't do it the right way (which I described in my answer previously), doing it with hacks such as LIKE would still be far from the optimal solution. If you did store a list of comma-separated values, and needed to check if eg. the value 2 is in the list, you could use the MySQL's FIND_IN_SET function:
SELECT * FROM task WHERE FIND_IN_SET(2, for_emp)
But you shouldn't do this unless you have no choice (eg. you're working with someone's shitty DB design), because it's way more inefficient and won't let you index the the employee ID.
The following query should do what you want:
SELECT * FROM tasks WHERE for_emp LIKE '%2%';
However, be aware that that would also match employers 12, 20, 21 etc; so take care if you expect you might end up in double-digits.
However, the other answers about renormalising your database are definitely preferable.
You're doing it wrong. Create a relation table with two fields: employee id and task id. If one task should be assigned to three employees, insert three rows in the relation table.
You then use JOIN to join the task, employee and relation tables.
then its no proper relation...
I would suggest a "mapping table" for the n:m relation
employee
id
task
id
employeetask
task_id
employee_id
Make a table for your employers. Insert your three rows in it.
Then make a table for mapping tasks to employers. If a task is assigned to three employers, insert three rows into this table. This is basic entity-relation work.
I would make 2 different tables.
1 with employees, and 1 with tasks.
then make another table which combines the two tables, I will call it Assigned Tasks.
Then in assigned tasks I make a assigned id, a employeenumber which is a FK to the employee table and a taskid which is a FK to the Tasks table.
If an employee has more than 1 task. Just insert another row in the assigned table. ;)
When its about Databases, try to think in solo entities! Combining those entities is able in antoher table.
sql example:
Select * from Assignedtasks where employeeID = 1 will give you all his/her tasks. :)
You could use a LIKE '%,2,%' clause in your SELECT statement.
eg:
SELECT * FROM table where for_emp LIKE '%,2,%'
However performance of such non-sargable queries is usually quite bad.
I would suggest that you insert a row each for each employee who is assigned to the task using a separate TASK_EMPLOYEE_MAPPING table with taskId, employeeId as a composite primary key.
With such a design, your query will be
SELECT * FROM TASK_EMPLOYEE_MAPPING WHERE employeeId = '2'