I feel like this may be a bit of a unique problem, but hopefully someone out there has come across a similar situation.
My application uses this database table:
DT table
The issue is with Field1 - 9.
Depending on how the user decides to set up their instance of the app there can be any number of fields used (from 0 - 9). The information for these are held in this Table:
Field Table
So for this example there are only to be two fields. And when a record is created for the DT table, field 1 and 2 will have data entered and all other field columns will be NULL. Obviously this isn't good practice, as for one, if a field name was changed in the future, all previous data wouldn't make sense.
I've been trying to think of a way to structure it differently. all I can think of is somehow when a DT record is created it will hold foreign keys to the fields that were used, but it seems that it's not possible to have multiple foreign keys in one column.
Any help or suggestions would be greatly appreciated.
One way to normalize this would be to factor out the repeating fields to a separate table, where you would have one entry per field with DT_id as a foreign key to the DT table.
DT Table:
ID
Start
End
...
DT_field table:
ID
DT_id (foreign key)
Value
Data fields
I am designing a database table structure. Say that we need to record employee profiles from different companies. We have the following fields:
+---------+--------------+-----+--------+-----+
| Company | EmployeeName | Age | Gender | Tel |
+---------+--------------+-----+--------+-----+
It's possible that two employees from different company may have the same name (and assume that no 2 employee has the same name in the same company). In this case a composite primary key (Company, EmployeeName) would be necessary in my opinion.
Search
Now I need to get all information by using only one of the 2 attributes in the primary key. For example,
I want to search all employees' profile of Company A:
SELECT EmployeeName, Age, Gender, Tel FROM table WHERE Company = 'Company A'
And I can also search all employees from different company named Donald:
SELECT Company, Age, Gender, Tel FROM table WHERE EmployeeName = 'Donald'
Strategy
In order to implement this requirement, my strategy would be storing all data in a single table, which is easy to read and understandable. However I noticed that it may take a long time to search as the query may need to iterate through all rows. I would like to retrieve these information as quick as possible. Would there be a better strategy for this?
First, your rows should have a unique identifier for each row -- identity/auto-increment/serial, depending on the database. Second, you might reconsider names being unique. Why can't two people at the same company have the same name?
In any case, you have a primary key on, say, (company, name). For the opposite search you simply want another index on (name, company):
create index idx_profiles_name_company on profiles(name, company);
A note explaining Gordon's suggestion for an identity on each row. This is supplemental to his answer above.
In theory there is nothing wrong with a primary key that crosses columns and in a db like PostgreSQL I like to have identity values as secondary keys (i.e. not null unique) and specify natural primary keys. Of course on MS SQL Server or MySQL/InnoDB that would be a recipe for problems. I would also not say "all" but rather "almost all" since there are times when breaking this rule is good.
Regardless, having an identity row simplifies a couple of things and it provides an abstraction around keys in case you get things wrong. Composite keys provide a couple issues that end up eating time (and possibly resulting in downtime) later. These include:
Joins on composite keys are often more expensive than those on simple values, and
Adding or changing a natural primary key which crosses columns is far harder when joins are involved
So depending on your db you should either specify a unique secondary key or make your natural primary key separate (which you should do depends on storage and implementation specifics).
I have an order table in sql server and I need for the order number primary key to be like this
OR\20160202\01
OR is just a string
20160202 is the Date
01 is sequence number for that day
for second Order record the same day it would be
OR\20160202\02 and so on..
backlashes should also be included...
Whats the way to go about creating such a field in sql server (using version 2016)
EDIT: to add more context to what sequence number is, its just a way for this field composite or not to be unique. without a sequence number i would get duplicate records in DB because i could have many records the same day so date would remain the same thus it would be something like
OR\20160202 for all rows for that particular day so it would be duplicate. Adding a "sequence" number helps solve this.
The best way is to not create such a column in SQL. You're effectively combining multiple pieces of data into the same column, which shouldn't happen in a relational database for many reasons. A column should hold one piece of data.
Instead, create a composite primary key across all of the necessary columns.
composite pk
order varchar(20)
orDate DateTime
select *
, row_number() over (partition by cast(orDate as Date) order by orDate) as seq
from table
Will leave it to you on how to concatenate the data
That is presentation thing - don't make it a problem for the PK
About "sequence number for that day" (department, year, country, ...).
Almost every time I discussed such a requirement with end users it turned out to be just misunderstanding of how shared database works, a vague attempt to repeat old (separate databases, EXCEL files or even paper work) tricks on shared database.
So i second Tom H and others, first try not to do it.
If nevertheless you must do it, for legal or other unnegotiatable reasons then i hope you are on 2012+. Create SEQUENCE for every day.
Formatted PK is not a good idea.Composite key is a better approach.The combination of day as a date column and order number as a bigint column should be used.This helps in improving the query performance too.
You might want to explore 'Date Dimension' table. Date Dimension is commonly used table in data warehousing. It stores all the days of the calendar(based on your choice of years) and numeric generated keys for these days. Check this post on date dimension. It talks about creating one in SQL SERVER.
https://www.mssqltips.com/sqlservertip/4054/creating-a-date-dimension-or-calendar-table-in-sql-server/
Presently I'm learning (MS) SQL, and was trying out various aggregate function samples. The question I have is: Is there any scenario (sample query that uses aggregate function) where having a unique constraint column (on a table) helps when using an aggregate function.
Please note: I'm not trying to find a solution to a problem, but trying to see if such a scenario exist in real world SQL programming.
One immediate theoretical scenario comes to mind, the unique constraint is going to be backed by a unique index, so if you were just aggregating that field, the index would be narrower than scanning the table, but that would be on the basis that the query didn't use any other fields and was thus covering, otherwise it would tip out of the NC index.
I think the addition of the index to enforce the unique constraint is automatically going to have the ability to potentially help a query, but it might be a bit contrived.
Put the unique constraint on the field if you need the field to be unique, if you need some indexes to help query performance, consider them seperately, or add a unique index on that field + include other fields to make it covering (less useful, but more useful than the unique index on a single field)
Let's take following two tables, one has records for subject name and subject Id and another table contains record for student having marks in particular subjects.
Table1(SubjectId int unique, subject_name varchar, MaxMarks int)
Table2(Id int, StudentId int, SubjectId, Marks int)
so If I need to find AVG of marks obtained in Science subject by all student who have attempted for
Science(SubjectId =2) then I would fire following query.
SELECT AVG(Marks), MaxMarks
FROM Table1, Table2
WHERE Table1.SubjectId = 2;
What's the best way to store a linked list in a MySQL database so that inserts are simple (i.e. you don't have to re-index a bunch of stuff every time) and such that the list can easily be pulled out in order?
Using Adrian's solution, but instead of incrementing by 1, increment by 10 or even 100. Then insertions can be calculated at half of the difference of what you're inserting between without having to update everything below the insertion. Pick a number large enough to handle your average number of insertions - if its too small then you'll have to fall back to updating all rows with a higher position during an insertion.
create a table with two self referencing columns PreviousID and NextID. If the item is the first thing in the list PreviousID will be null, if it is the last, NextID will be null. The SQL will look something like this:
create table tblDummy
{
PKColumn int not null,
PreviousID int null,
DataColumn1 varchar(50) not null,
DataColumn2 varchar(50) not null,
DataColumn3 varchar(50) not null,
DataColumn4 varchar(50) not null,
DataColumn5 varchar(50) not null,
DataColumn6 varchar(50) not null,
DataColumn7 varchar(50) not null,
NextID int null
}
Store an integer column in your table called 'position'. Record a 0 for the first item in your list, a 1 for the second item, etc. Index that column in your database, and when you want to pull your values out, sort by that column.
alter table linked_list add column position integer not null default 0;
alter table linked_list add index position_index (position);
select * from linked_list order by position;
To insert a value at index 3, modify the positions of rows 3 and above, and then insert:
update linked_list set position = position + 1 where position >= 3;
insert into linked_list (my_value, position) values ("new value", 3);
A linked list can be stored using recursive pointers in the table. This is very much the same hierarchies are stored in Sql and this is using the recursive association pattern.
You can learn more about it here (Wayback Machine link).
I hope this helps.
The simplest option would be creating a table with a row per list item, a column for the item position, and columns for other data in the item. Then you can use ORDER BY on the position column to retrieve in the desired order.
create table linked_list
( list_id integer not null
, position integer not null
, data varchar(100) not null
);
alter table linked_list add primary key ( list_id, position );
To manipulate the list just update the position and then insert/delete records as needed. So to insert an item into list 1 at index 3:
begin transaction;
update linked_list set position = position + 1 where position >= 3 and list_id = 1;
insert into linked_list (list_id, position, data)
values (1, 3, "some data");
commit;
Since operations on the list can require multiple commands (eg an insert will require an INSERT and an UPDATE), ensure you always perform the commands within a transaction.
A variation of this simple option is to have position incrementing by some factor for each item, say 100, so that when you perform an INSERT you don't always need to renumber the position of the following elements. However, this requires a little more effort to work out when to increment the following elements, so you lose simplicity but gain performance if you will have many inserts.
Depending on your requirements other options might appeal, such as:
If you want to perform lots of manipulations on the list and not many retrievals you may prefer to have an ID column pointing to the next item in the list, instead of using a position column. Then you need to iterative logic in the retrieval of the list in order to get the items in order. This can be relatively easily implemented in a stored proc.
If you have many lists, a quick way to serialise and deserialise your list to text/binary, and you only ever want to store and retrieve the entire list, then store the entire list as a single value in a single column. Probably not what you're asking for here though.
This is something I've been trying to figure out for a while myself. The best way I've found so far is to create a single table for the linked list using the following format (this is pseudo code):
LinkedList(
key1,
information,
key2
)
key1 is the starting point. Key2 is a foreign key linking to itself in the next column. So your columns will link something link something like this
col1
key1 = 0,
information= 'hello'
key2 = 1
Key1 is primary key of col1. key2 is a foreign key leading to the key1 of col2
col2
key1 = 1,
information= 'wassup'
key2 = null
key2 from col2 is set to null because it doesn't point to anything
When you first enter a column in for the table, you'll need to make sure key2 is set to null or you'll get an error. After you enter the second column, you can go back and set key2 of the first column to the primary key of the second column.
This makes the best method to enter many entries at a time, then go back and set the foreign keys accordingly (or build a GUI that just does that for you)
Here's some actual code I've prepared (all actual code worked on MSSQL. You may want to do some research for the version of SQL you are using!):
createtable.sql
create table linkedlist00 (
key1 int primary key not null identity(1,1),
info varchar(10),
key2 int
)
register_foreign_key.sql
alter table dbo.linkedlist00
add foreign key (key2) references dbo.linkedlist00(key1)
*I put them into two seperate files, because it has to be done in two steps. MSSQL won't let you do it in one step, because the table doesn't exist yet for the foreign key to reference.
Linked List is especially powerful in one-to-many relationships. So if you've ever wanted to make an array of foreign keys? Well this is one way to do it! You can make a primary table that points to the first column in the linked-list table, and then instead of the "information" field, you can use a foreign key to the desired information table.
Example:
Let's say you have a Bureaucracy that keeps forms.
Let's say they have a table called file cabinet
FileCabinet(
Cabinet ID (pk)
Files ID (fk)
)
each column contains a primary key for the cabinet and a foreign key for the files. These files could be tax forms, health insurance papers, field trip permissions slips etc
Files(
Files ID (pk)
File ID (fk)
Next File ID (fk)
)
this serves as a container for the Files
File(
File ID (pk)
Information on the file
)
this is the specific file
There may be better ways to do this and there are, depending on your specific needs. The example just illustrates possible usage.
There are a few approaches I can think of right off, each with differing levels of complexity and flexibility. I'm assuming your goal is to preserve an order in retrieval, rather than requiring storage as an actual linked list.
The simplest method would be to assign an ordinal value to each record in the table (e.g. 1, 2, 3, ...). Then, when you retrieve the records, specify an order-by on the ordinal column to get them back in order.
This approach also allows you to retrieve the records without regard to membership in a list, but allows for membership in only one list, and may require an additional "list id" column to indicate to which list the record belongs.
An slightly more elaborate, but also more flexible approach would be to store information about membership in a list or lists in a separate table. The table would need 3 columns: The list id, the ordinal value, and a foreign key pointer to the data record. Under this approach, the underlying data knows nothing about its membership in lists, and can easily be included in multiple lists.
This post is old but still going to give my .02$. Updating every record in a table or record set sounds crazy to solve ordering. the amount of indexing also crazy, but it sounds like most have accepted it.
Crazy solution i came up with to reduce updates and indexing is to create two tables (and in most use cases you don's sort all records in just one table anyway). Table A to hold the records of the list being sorted and table B to group and hold a record of the order as a string. the order string represents an array that can be used to order the selected records either on the web server or browser layer of a webpage application.
Create Table A{
Id int primary key identity(1,1),
Data varchar(10) not null
B_Id int
}
Create Table B{
Id int primary key Identity(1,1),
GroupName varchat(10) not null,
Order varchar(max) null
}
The format of the order sting should be id, position and some separator to split() your string by. in the case of jQuery UI the .sortable('serialize') function outputs an order string for you that is POST friendly that includes the id and position of each record in the list.
The real magic is the way you choose to reorder the selected list using the saved ordering string. this will depend on the application you are building. here is an example again from jQuery to reorder the list of items: http://ovisdevelopment.com/oramincite/?p=155
https://dba.stackexchange.com/questions/46238/linked-list-in-sql-and-trees suggests a trick of using floating-point position column for fast inserts and ordering.
It also mentions specialized SQL Server 2014 hierarchyid feature.
I think its much simpler adding a created column of Datetime type and a position column of int, so now you can have duplicate positions, at the select statement use the order by position, created desc option and your list will be fetched in order.
Increment the SERIAL 'index' by 100, but manually add intermediate values with an 'index' equal to Prev+Next / 2. If you ever saturate the 100 rows, reorder the index back to 100s.
This should maintain sequence with primary index.
A list can be stored by having a column contain the offset (list index position) -- an insert in the middle is then incrementing all above the new parent and then doing an insert.
You could implement it like a double ended queue (deque) to support fast push/pop/delete(if oridnal is known) and retrieval you would have two data structures. One with the actual data and another with the number of elements added over the history of the key. Tradeoff: This method would be slower for any insert into the middle of the linked list O(n).
create table queue (
primary_key,
queue_key
ordinal,
data
)
You would have an index on queue_key+ordinal
You would also have another table which stores the number of rows EVER added to the queue...
create table queue_addcount (
primary_key,
add_count
)
When pushing a new item to either end of the queue (left or right) you would always increment the add_count.
If you push to the back you could set the ordinal...
ordinal = add_count + 1
If you push to the front you could set the ordinal...
ordinal = -(add_count + 1)
update
add_count = add_count + 1
This way you can delete anywhere in the queue/list and it would still return in order and you could also continue to push new items maintaining the order.
You could optionally rewrite the ordinal to avoid overflow if a lot of deletes have occurred.
You could also have an index on the ordinal to support fast ordered retrieval of the list.
If you want to support inserts into the middle you would need to find the ordinal which it needs to be insert at then insert with that ordinal. Then increment every ordinal by one following that insertion point. Also, increment the add_count as usual. If the ordinal is negative you could decrement all of the earlier ordinals to do fewer updates. This would be O(n)