I have three tables with foreign key relationship with each other.
The table school will be uploaded manually. A student will login to the website and check their marks
The entire data is to be uploaded to another new database of different instance
The Login Id(stud_id) of the student in DB1 is 10 and Login Id(stud_id) of the student in DB2 is 1 in another instance.
For retaining the data of student_marks table, I intend to do the following steps,
1. Dump student_marks table from DB1
2. Copy it to DB2
NOTE: stud_id would be different for both the databases
Is there any way to do the above.
Refer the table below,
school:
id | name| place
-----+-------------
1 | sch1 | test
student:
id | school_id| stud_name
-----+-------------
1 | 1 | stud1
student_marks:
id | stud_id| subj1 | subj2
-----+-----------------------
1 | 1 | 30 | 30
Thanks in advance!
First Disable the foreign key constraint, then dump the data and after that again enable the foreign key constraint or you can put foreign key constraint after migrating the data.
Related
I have a OneToMany Relation Ship in my entities. There is a entity article an a entity draft.
Every draft has a unique version in connected with an article.
aritcle_id | draft_id | version
-------------------------------
1 | 1 | 1
1 | 2 | 2
1 | 3 | 3
2 | 4 | 1
If I make the version 'unique' it is version-row-far unique is it possible to declare it article-row-far unique? So you can have duplicated versions but only if the version is attached to another article?
Every draft has a unique version
You can implement this with a unique constraint. I think it is:
alter table t add constraint unq_t_draft_version unique (draft_id, version);
If draft_id can be repeated on different articles, you can include all three columns:
alter table t add constraint unq_t_article_draft_version unique (article_id, draft_id, version);
How can we auto generate column/fields in microsoft access table ?
Scenario......
I have a table with personal details of my employee (EmployDetails)
I wants to put their everyday attendance in an another table.
Rather using separate records for everyday, I want to use a single record for an employ..
Eg : I wants to create a table with fields like below
EmployID, 01Jan2020, 02Jan2020, 03Jan2020,.........25May2020 and so on.......
It means everyday I have to generate a column automatically...
Can anybody help me ?
Generally you would define columns manually (whether that is through a UI or SQL).
With the information given I think the proper solution is to have two tables.
You have your "EmployDetails" which you would put their general info (name, contact information etc), and the key which would be the employee ID (unique, can be autogenerated or manual, just needs to be unique)
You would have a second table with a foreign key to the empployee ID in "EmployDetails" with a column called Date, and another called details (or whatever you are trying to capture in your date column idea).
Then you simply add rows for each day. Then you do a join query between the tables to look up all the "days" for an employee. This is called normalisation and how relational databases (such as Access) are designed to be used.
Employee Table:
EmpID | NAME | CONTACT
----------------------
1 | Jim | 222-2222
2 | Jan | 555-5555
Detail table:
DetailID | EmpID (foreign key) | Date | Hours_worked | Notes
-------------------------------------------------------------
10231 | 1 | 01Jan2020| 5 | Lazy Jim took off early
10233 | 2 | 02Jan2020| 8 | Jan is a hard worker
10240 | 1 | 02Jan2020| 7.5 | Finally he stays a full day
To find what Jim worked you do a join:
SELECT Employee.EmpID, Employee.Name, Details.Date, Details.Hours_worked, Details.Notes
FROM Employee
JOIN Details ON Employee.EmpID=Details.EmpID;
Of course this will give you a normalised result (which is generally what's wanted so you can iterate over it):
EmpID | NAME | Date | Hours_worked | Notes
-----------------------------------------------
1 | Jim | 01Jan2020 | 5 | ......
1 | Jim | 02Jan2020 | 7 | .......
If you want the results denormalised you'll have to look into pivot tables.
See more on creating foreign keys
Consider the following postgres (version 9.4) database:
testbase=# select * from employee;
id | name
----+----------------------------------
1 | johnson, jack
2 | jackson, john
(2 rows)
testbase=# select * from worklog;
id | activity | employee | time
----+----------------------------------+----------+----------------------------
1 | department alpha | 1 | 2018-01-27 20:32:16.512677
2 | department beta | 1 | 2018-01-27 20:32:18.112356
5 | break | 1 | 2018-01-27 20:32:22.255563
3 | department gamma | 2 | 2018-01-27 20:32:20.073173
4 | department gamma | 2 | 2018-01-27 20:32:21.05962
(5 rows)
The column 'name' in table 'employee' is of type character(32) and unique, the column 'employee' in 'worklog' references 'id' from the table 'employee'. The column 'id' is the primary key in either table.
I can see all activities from a certain employee by issuing:
testbase=# select * from worklog where employee=(select id from employee where name='johnson, jack');
id | activity | employee | time
----+----------------------------------+----------+----------------------------
1 | department alpha | 1 | 2018-01-27 20:32:16.512677
2 | department beta | 1 | 2018-01-27 20:32:18.112356
5 | break | 1 | 2018-01-27 20:32:22.255563
(3 rows)
I would rather like to simplify the query to
testbase=# select * from worklog where employee='johnson, jack';
For this I would change 'employee' to type character(32) in 'worklog' and declare 'name' as primary key in table 'employee'. Column 'employee' in 'worklog' would, of course, reference 'name' from table 'employee'.
My question:
Will every new row in 'worklog' require additional 32 bytes for name of the 'employee' or will postgres internally just keep a pointer to the foreign field without duplicating the name for every new row?
I suppose that the answer for my question is somewhere in the documentation but I could not find it. It would be very helpful if someone could provide an according link.
PS: I did find this thread, however, there was no link to some official documentation. The behaviour might also have changed, since the thread is now over seven years old.
Postgres will store the data that you tell it to store. There are some new databases that will do compression under the hood -- and Postgres might have features to enable that (I do not know all Postgres features).
But, you shouldn't do this. Integer primary keys are more efficient than strings for three reasons:
They are fixed length in bytes.
They are shorter.
Collations are not an issue.
Stick with your original query, but write it using a join:
select wl.*
from worklog wl join
employee e
on wl.employee = e.id
where e.name = 'johnson, jack';
I suggest this because this is more consistent with how SQL works and makes it easier to choose multiple employees.
If you want to see the name and not the id, create a view (say v_worklog) and add in the employee name.
In our entitlement framework each "resource" (resource is nothing but any entity that you want to protect by assigning privileges to roles which can access or not access based on privileges) is stored in a resource table like below.
DESIGN1
RESOURCE TABLE
id (int) | namespace (varchar) | entity_id | black_listed (boolean)
1 | com.mycompany.core.entity1 |24 | false
2 | com.mycompany.core.entity2 |24 | false --note entity2
3 | com.mycompany.core.entity10 |3 | false -- note entity10
each resource in the table represent different entity e.g. entity1,entity2,..,entity10. basically that's nothing but entity1.id, entity2.id, entity3.id, ... and so on. because RESOURCE table keeps resources for all kinds of entity - entity_id column in RESOURCE table can't have proper foreign key relationship constraint. we are thinking to refactor this schema such as follow
DESIGN 2
RESOURCE TABLE
id | description | entity1_id | entity2_id | entity3_id | entity4_id | entity5_id | entity6_id | black_listed(boolean)
1 | com.mycompany.core.entity1|24 | null | null | null |null | null
2 | com.mycompany.core.entity2|null | 24 | null | null |null | null
now entity1_id will have a proper FK to entity1 table , entity2_id will have proper FK to entity2 and so on. downside of this approach is that this table will always have null values for all the columns BUT one. e.g. you can only have one entity resource per row. also having null seems to be anti pattern especially for FK relationship. One other way would be normalize the schema and create a resource table for each enitty type. but that will be pretty insane to maintain and quickly become a headache. not saying it's good or bad but doesn't look like a practical design.
is there a better way to design such a table where proper FK relatinoships are also maintained? or you'll endorse Design 2?
You need to create one table for all entities with id (surrogate primary key) or entity_type, entity_id as unique key.
id entity_type entity_id
1 entity1 24
2 entity2 24
Then you need to have only one column in RESOURCE referring to this table (say entities). Your RESOURCE table will look like as in the first example, but the difference is there will be only one entities table, not 10.
I am building a Hospital Management System and I wanted to make a table in the database that contains all the hospitals. Each hospital has another table that contains info about its employees.
If I wanted to add a new record (a new Hospital) in the hospital table after the system is released, can the system generate a new table for that new hospital's employees?
It would have a standard form and the system would ask the user to fill it (through GUI, or any other way).
Is it technically possible? And if not is there other ways to do it?
Why would you need to create a seperate table for each hospital? Add Hospital_ID as a column to the Employee table and then you can tell, from that one table, what hospital an mployee works for.
SQLFiddle
First you should read about Table relationships. It's really important that you understand how it works before desgnining your database.
Now as for your question, tables don't contain tables. This is why you should review you design.
You should have a those tables :
Hospital
---------------------------------------------------
| ID | Name | Address | Phone |
---------------------------------------------------
| 1 | SomeName | 13 Mercy Street | 555-555-5555 |
---------------------------------------------------
Employee
-------------------------
| ID | Hospital_ID | Name |
-------------------------
| 01 | 1 | John |
-------------------------
This way every employee is associated with an hospital. We now know John works in the SomeName hospital.
To help in your research,
Hospital.ID is a PRIMARY KEY
Employee.ID is a PRIMARY KEY
Employee.Hospital_ID is a FOREIGN KEY