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.
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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
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 have a table called points. I executed the following query and expected a list of lexicographicaly sorted list of ROWIDs but that did not happen. How does Order by rowid sorts the row?
select rowid from points order by rowid
I had rows like following
AAAE6MAAFAAABiSAAA
AAAE6MAAFAAABi+AAA
2nd row is lexicographicaly smaller than first row. So what is sorting criteria if it is not lecxicographical sorting?
Why you see is only a representation used for display purposes.
The actual rowid contains binary information about the data block, the row in the block, the file where the block is located and the internal object id of the table (See the manual for details)
When you use order by rowid Oracle sorts the rows based on that (internal) information, not based on the "string representation".
If you change your query to:
select rowid,
dbms_rowid.rowid_relative_fno(rowid) as rel_fno,
dbms_rowid.rowid_row_number(rowid) as row_num,
dbms_rowid.rowid_block_number(rowid) as block_num,
dbms_rowid.rowid_object(rowid)
from points
order by rowid
You will most probably see the logic behind the ordering of the rownumber.
Note that the value for dbms_rowid.rowid_object() will always be the same. And if you only have two rows in your table, both will most probably also have the same value for rowid_block_number()
The sequence of rowid is not guranteed. It depends on how you have set the NLS settings. Also rowid represents the physical allocation of the row in the database. A rowid is considered immutable(does not change) but if you delete a row and insert it again then it changes.
If you delete a row, then Oracle may reassign its rowid to a new row
inserted later.
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.
Insert 200 data througn for-loop into sqlserver 2000 database, the order change, why ?
When I use mysql, it doesn't have the matter.
i mean:
when you insert 2, then insert 3, then insert 1, in mysql you will see 2,3,1 like the order you insert. But in sqlsever2000 that may not.
The order in which rows are stored doesn't really matter. Actually SQL tables don't have any ordering. If order matters to you, you need to query using an ORDER BY clause.
There are no guarantees on the order of the results of a select statement, unless you add an ORDER BY clause. You might get the results back in the order of insertion, but it's not guaranteed.
If you have an index on the table, it might appear that your data is being ordered in a way that you're not expecting. Normally, you'll just have an identity column as your surrogate primary key, which means your inserted data will show up in "order" if you just do a select *, but if you have indexes on other columns the data might be ordered differently when you do select *.
You can control physical ordering by creating a clustered index on that columns. Only one clustered index is allowed per table. Cluster index make sure when you perform SELECT you will always get data in order which was defined when creating cluster index.