I have the following problem. On my server I have to maintain data in particular order (to by specific order of insertion). And it has to be stored in a file. I'm currently using SQLite to do this, but is it safe to assume that SQL db will keep the order of insertion, or should I use something else (in this case please give me a hint what to do).
Rows in a table are not stored in any particular order. You can only guarantee order with an order by clause.
To order rows by insertion, you'd typically use an autoincrement column. That assigns a higher values to rows that are inserted later. You can then order by autoinc_col to retrieve rows in the order in which they were inserted.
Related
I have a table for categories on my posts database, I wanna make a specific order for them, so when I'm trying to insert a new record, It's not necessary to be stored at the end of table.
What is the best way to do that?
The order of rows and columns is immaterial to the DBMS. Rows are not stored at the end and select without order by does not guarantee the order of rows. Even if data can be stored ordered in the files, due to parallel execution, result is not ordered.
If you want to get ordered dataset, use ORDER BY in your query.
If you want ot order the extracted data then you should use the order by clause
SELECT Name, Order
FROM MyTalbe
WHERE ...
ORDER BY Order
Col1 being your column with the number from 1 to N
When I insert a row in a Firebird database, it doesn't show at the end of the table and it inserted before other row. Why is this?
Firebird doesn't use clustered tables, nor does it order data by insert. When you insert a row, Firebird adds the row to the first datapage of the table it can find that has sufficient space available. As a result, it may end up anywhere relative to other rows of the table.
If you want to enforce a specific order to the data you view, you need to add an order by clause to your select statement, specifying by which columns you want to order the data. Without an order by, the order of the result of a select is not deterministic, and may be the result of physical order in data pages, order of data in an index used to find rows, and other factors.
I have a set of records in a table. I get records via select without any order statements just select * from table so there isn't any obvious order.
Will the order of records differs after updating some records?
There is no internal order in a Postgres database table (and this is also the case with most other RDBMS). The only order which will exist at selection time is the one you specify using an ORDER BY clause. So, the answer to your question is that you should always rely on ORDER BY if you want a consistent, reproducible order in your result set.
I'm using SQL server 2014,I'm fetching data from a view.The order of items is getting changed once i use Group by ,how can i get the order back after using this Group by,There is one date column,but its not saving any time,So i can't sort it based on date also..
How can I display the data in the same order as it displayed before using Group by?Anyone have any idea please help?
Thanks
Tables and views are essentially unordered sets. To get rows in a specific order, you should always add an ORDER BY clause on the columns you wish to order on.
I'm assuming you previously selected from the VIEW without an ORDER BY clause. The order in which rows are returned from a SELECT statement without an ORDER BY statement is undefined. The order you are getting them in, can change due to any number of reasons (eg some are listed here).
Your problem stems from the mistake you made on relying on the order from a SELECT from a VIEW without an ORDER BY. You should have had an ORDER BY clause in your SELECT statement to begin with.
How can I display the data in the same order as it displayed before using Group by?
The answer: You can't if your initial statement did not have an ORDER BY clause.
The resolution: Determine the order you want the resultset in and add an ORDER BY clause on those columns, both in your initial query and the version with the GROUP BY clause.
Maybe you can use the row_number() function without any OVER and ORDER BY keywords? This should be done in a sub-select and when you group the data in the outer SELECT, use the AVG() function on the numbered column and ORDER the result by this. The problem is, that when you group rows, the original rows disappear. That's kind if the purpose of GROUP BY. ;) Depending on what you GROUP BY, what you're asking might be logically impossible.
EDIT:
Found this solution Googling: http://blog.sqlauthority.com/2015/05/05/sql-server-generating-row-number-without-ordering-any-columns/
So you can number rows like this to maintain the order of rows from the table before you GROUP BY:
row_number() OVER (ORDER BY (SELECT 1))
The only way you can enforce a specific order is to explicitly use a ORDER BY clause. Otherwise the order of rows is not guaranteed (take a look at this article for more details) and the database engine will return the rows based on "as fast as it can" or "as fast as it can retrieve them from disk" rule. So, order can also vary between executions of the same query in the span of a few seconds.
When doing a DISTINCT, GROUP BY or ORDER BY, SQL Server automatically does a SORT of the data based on an index it uses for that query.
Looking at the execution plan of your query will show you what index (and implicitly columns in that index) is being used to sort the data.
i've got an issue due to database conception.
My data are grouped in a table which looks like :
IdGroup | IdValue
So for each group i've got the list of value.
Indeed, we should have had an order column or an id, but i can't.
Do you know anyway which can prove the order of the select value based on the insert order ?
I mean, if I inserted 1003,1001,1002 could i garantuee it to be retrieve in this order ?
IdGroup | IdValue
1 | 1003
1 | 1001
1 | 1002
Of course, using an order by doesn't seems to fit because i don't have any column usable.
Any idea ? Using a system proc or something like this.
Thanks a lot :)
Stop telling me to use an order by and altering the table, it doesn't fit and yes i know it's the good pratice to do... thanks :)
A couple of ideas:
DBCC PAGE (undocumented) can be used to look at the raw data pages of the table. It may be possible to determine insert order by looking at the low level information.
If you cannot alter the table, can you add a table to the database? If so, consider creating a table with an identity column and use a trigger on the original table to insert the records in the new table.
Also, you should include which version(s) of SQL Server are involved. Doing anything this unusual will very often be version specific.
You shouldn't rely on the data being returned in a particular order; use an ORDER BY clause to guarantee the order.
(Despite the fact that data appears to be returned in clustered index order, this might not always be the case).
Whilst some small scale tests will show that it returns it in what appears to be the right order, it just will not hold.
The golden rule remains - unless an order by clause is specified, there are no guarentees provided on the order of the returned data.
edit : If you place a non-clustered index on the idgroup column it is forced to add a hidden field, the uniqueifier since the values are the same - the problem it, you can't access it in an order by clause, but from a forensic perspective, you can determine the order it was inserted in.
As others have said, the only way to guarantee an ordering is with an ORDER BY clause. What isn't highlighted in their answers is that, the only place that this ORDER BY matters is in the SELECT statement. It doesn't* matter if you apply an ORDER BY clause during the INSERT statement; the system is free to return results from a select in whatever order it finds most efficient, unless an ORDER BY is specified at that time.
*There's a particular way to ensure what order IDENTITY values are assigned to a result set during an INSERT, using an ORDER BY, but I can't remember the exact details, and it still doesn't effect the order of SELECT.
Can you add the Created Date column? In this way you can get the records using Order by Clause Created Date. Moreover set it's default value Getdate()