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
Related
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.
Posting two questions:
1.
Let's say there is a query:
SELECT C1, C2, C3 from TABLE;
When this query is fired for the first time,it retrieves all the values in a certain order.
Next time, when the same query is fired, will the previous order be retained?
There are 2 tables, TABLE1 and TABLE2, both of them have identical data.
Will (SELECT * from TABLE1) and (SELECT * from TABLE1) retrieve the same order of rows?
SQL tables represent unordered sets. Period. There is no ordering in a result set unless you explicitly include an ORDER BY.
It is that simple. If you want data in a particular order, then you need to use ORDER BY. That is how relational databases work.
The same query can return results in different orders each time the query is executed. There are no guarantees about the order -- unless the query has an ORDER BY for the outermost SELECT.
No, unless you are fetching data from result cache!
No, unless they are very small tables and your query runs with low parallelism.
Sorry for extra answer, but I see Tim claims that the query will return same result as long as the underlying table(s) is not modified, and the query has same execution plan.
Snowflake executes the queries in parallel, therefore the order of data is not predictable unless ORDER BY is used.
Let's create a table (big enough to be processed in parallel), and run a simple test case:
-- running on medium warehouse
create or replace table my_test_table ( id number, name varchar ) as
select seq4(), 'gokhan' || seq4() from table(generator(rowcount=>1000000000));
alter session set USE_CACHED_RESULT = false;
select * from my_test_table limit 10;
You will see that it will return different rows every time you run.
To answer both questions short: No.
If your query has no ORDER BY-clause, the SELECT statement always returns an unordered set. This means: Even if you query the same table twice and the data didnt change, SELECT without ORDER BY can retrieve different row-orders.
https://docs.snowflake.com/en/sql-reference/sql/select.html
SELECT S.ColumnName,L.ColumnName ...
INTO #Tempdata
FROM [dbo].[M] M
LEFT JOIN [dbo].[S] S ON S.PrimaryKey=M.FKey
LEFT JOIN [dbo].[L] L ON S.FK=L.PK
ORDER BY S.ColumnName
SQL tables represent unordered sets. Period.
If you want to select the data in order, you need to use:
order by columnName
in the query used for the selecting the data.
SQL Server, though, does respect an ORDER BY in an INSERT in one important way. If you have an identity column, then the identity will follow the ORDER BY. This can be convenient. But to get ordered results (consistently) you need ORDER BY in the SELECT query.
Especially in tempdb, which is often stores data with multiple files, the rows inserted are splitted across the differents files. Retrieving the data using multiple threads (one thread by file to go faster) will give any combination of mingled subsets and give an impression of hasardous order of the rows.
As it has been said, there is no ways to have an ordered storage of rows in a table, and it is the reason why SQL have a ORDER BY clause !
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.