So I have an unordered table movement that has columns timestamp,x,and y. I want to order by timestamp, and change and save the table to have the all rows ordered by timestamp.
I wanted to use UPDATE TABLE but I'm unsure on how the query would look... for example, I have this:
UPDATE movement
SET ?????
ORDER BY timestamp;
I don't want to set anything, I just want to change the order of the table. I need to do this for another query that I'm going to write, but the fact is that this table is very large and I need to break up things in steps so that the performance isn't terrible later. So how would I do this? I found this SO post that addressed a similar question, but I was wondering if there was another way to do it rather than use another table, because as I said, this table is very large(millions of rows) and recopying the table would take a long time.
Tables don't inherently have an order; you don't have to update them into any particular order.
What you do is choose the order of what you SELECT from the table. Then it can be in any order you choose!
Example:
SELECT * FROM movement
ORDER BY timestamp;
But then somewhere else maybe you want to:
SELECT * FROM movement
ORDER BY timestamp DESCENDING;
You can't use ORDER BY in UPDATE statement. ORDER BY should be used with SELECT statement only. Again, there is no need of having the records stored in particular order cause you can just display the records in particular order with a SELECT statement like
select * from movement
order by timestamp
Relational database tables represent unordered sets. There is no syntax for sorting a table, simply because there is no such concept as the order of rows in a table. When you issue a query without an explicit order by clause, the database may return the rows to you in any order it may see fit, which might be influenced by the order they were inserted and written to disk, their presence in some memory cache, indexes, or a host of other implementation details.
If you want to query the table's rows sorted by their timestamp, just explicitly state it in the order by clause:
SELECT *
FROM `movement`
ORDER BY `timestamp`
It is actually possible. This is in MySQL format... Update is for editing already existing information. If you want to make more direct changes, use ALTER or MODIFY according to syntax.
ALTER TABLE movement
ORDER BY timestamp;
Related
I have the below syntax where I sorted the the date order, my default is autocommit enabled, however when I reopen the table it is not sorted again, is there some other syntax I should write to the below to make it permanent?
Thanks
SELECT *
FROM stock_price.spy1996
ORDER BY Date ASC;
You are misunderstanding several very important concepts in SQL.
First, changes to the database occur (generally) through UPDATE, INSERT, and DELETE. These are the changes that are committed.
Second, SELECT does not change the database.
Third, SQL tables represent unordered sets. If you want the results from a query to be in a particular order, then you have to use an ORDER BY clause.
If you like, you can implement a view that does this automatically:
CREATE VIEW v_spy_1996 as
SELECT *
FROM stock_price.spy1996
ORDER BY Date ASC;
Then when you query from the view, you will not have to repeat the ORDER BY.
There are many solutions in stackoverflow itself where the objective was to read the last n rows of the table using either an auto-increment field or timestamp: for example, the following query fetches the last ten records from a table named tab in the descending order of the field values named id which is an auto increment field in the table:
Select * from tab order by id desc limit 10
My question is: Is there any alternative way without having to get an auto increment field or timestamp to accomplish the task to get the same output?
Tips: The motivation to ask this question comes from the fact that: as we store records into tables and when query the database with a simple query without specifying any criteria like :
Select * from tab
Then the order of the output is same as the order of the records as inserted into the table. So is there any way to get the records in the reverse order of what they were entered into the database?
Data in mysql is not ordered- you don't have any guarantee on the order of the records you'll get unless you'll specify order by in your query.
So no, unless you'll order by timestamp, id, or any other field, you can't get the last rows, simply because there's no 'last' without the order
In the SQL world, order is not an inherent property of a set of data.
Thus, you get no guarantees from your RDBMS that your data will come
back in a certain order -- or even in a consistent order -- unless you
query your data with an ORDER BY clause.
So if you don't have the data sorted by some id or some column then you cannot track the data based on its sorting. So it is not guaranteed how MYSQL will store the data and hence you cannot get the last n records.
You can also check this article:
Caveats
Ordering of Rows
In the absence of ORDER BY, records may be returned in a different
order than the previous MEMORY implementation.
This is not a bug. Any application relying on a specific order without
an ORDER BY clause may deliver unexpected results. A specific order
without ORDER BY is a side effect of a storage engine and query
optimizer implementation which may and will change between minor MySQL
releases.
I've searched and searched and can't find an answer to this question, I'm probably asking it in the wrong way.
I am querying an employee database.
I need to get details based on a position id, however there could be multiple records for that position id as this organisation has permanent employees and temporary employees that act against the same position.
So, in order to get the CURRENT occupant of the position id, I need my query to SELECT the FIRST record that matches the position string, from the TOP DOWN.
will this select the first matched record from the top?
SELECT * WHERE `position_id`="00000000" LIMIT 1;
Thanks in advance.
You need an ORDER BY clause to define the ordering between the individual records your table. If you do not use ORDER BY you can assume no fixed order between the records, and you could get a new order each time you executed the query.
From the manual:
With one argument, the value specifies the number of rows to return from the beginning of the result set
So with LIMIT 1 you get the first row from the result set. What the result set is depends on engine used and which indexes you have. If you want the first row added, you need to create another column to define that.
It just gets one at random*. There's no way to tell which one it will be, unless you add an ORDER BY clause.
* Not really at random, of course. It depends on the way the records are stored and repeated queries will probably return the same result every time, at least as long as you don't modify the table or its contents. I actually mean, you cannot be sure.
I have a table (rather ugly designed, but anyway), which consists only of strings. The worst is that there is a script which adds records time at time. Records will never be deleted.
I believe, that MySQL store records in a random access file, and I can get last or any other record using C language or something, since I know the max length of the record and I can find EOF.
When I do something like "SELECT * FROM table" in MySQL I get all the records in the right order - cause MySQL reads this file from the beginning to the end. I need only the last one(s).
Is there a way to get the LAST record (or records) using MySQL query only, without ORDER BY?
Well, I suppose I've found a solution here, so my current query is
SELECT
#i:=#i+1 AS iterator,
t.*
FROM
table t,
(SELECT #i:=0) i
ORDER BY
iterator DESC
LIMIT 5
If there's a better solution, please let me know!
The order is not guaranteed unless you use an ORDER BY. It just happens that the records you're getting back are sorted the way need them.
Here is the importance of keys (primary key for example).
You can make some modification in your table by adding a primary key column with auto_increment default value.
Then you can query
select * from your_table where id =(select max(id) from your_table);
and get the last inserted row.
I'm dealing with a legacy database table that has no insertion date column or unique id column, however the natural order of insertion is still valid when examined with a simple SELECT * showing oldest to newest.
I'd like like to fetch that data with pagination but reverse the order as if it was ORDER BY date DESC
I've thought about wrapping the query, assigning a numeric id to the resulting rows and then do an ORDER BY on the result but wow that seems crazy.
Is there a more simple solution I am overlooking?
I cannot add columns to the existing table, I have to work with it as is.
Thanks for any ideas!
Use #rownum in your query to number each row and then order by the #rownum desc. Here's an example.
select #rownum:=#rownum+1 ‘rank’, p.* from player p, (SELECT #rownum:=0) r order by score desc limit 10;
Finally, beware that relying on the current order being returned long-term isn't recommended.
If you're writing an application to process the data, another approach might be to run your current query, then iterate over the returned records from last to first.
If you have too many records, then you may wish to instead use a view. This is a Database object which can be used to combine data from different tabls, or present a modified view of a single table, amongst other things. In this case, you could try creating a view of your table and add a generated ID column. You could then run SELECT statements against this view ordering by the new column you have added.
However be aware of the advice from another poster above: the order in which rows are returned without an ORDER BY clause is arbitrary and may change without notification. It would be best to amend your table if at all possible.
mySQL CREATE VIEW syntax