i am trying to teach myself SQL and of course I would like to follow best practices.
I have created two querys to find the latest record :
select * from AppSurvey where
DateLastUsed >= ( SELECT MAX(DateLastUsed) FROM AppSurvey)
and
select top 1 * from AppSurvey order by DateLastUsed desc
is one of these methods more efficent than the other or does it really matter
There is a similiar post on this site to what you are trying to get at.
For autoincrement fields: MAX(ID) vs TOP 1 ID ORDER BY ID DESC
The preferred answer seems to be: "In theory, they will use same plans and run almost same time"
The first one could get more than one row, if your DateLastUsed column isn't unique.
Related
I need to fetch 4 random values from each category. What should be the correct sql syntax for maria db. I have attached one image of table structure.
Please click here to check the structure
Should i write some procedure or i can do it with basic sql syntax?
You can do that with a SQL statement if you only have a few rows:
SELECT id, question, ... FROM x1 ORDER BY rand() LIMIT 1
This works fine if you have only a few rows - as soon as you have thousands of rows the overhead for sorting the rows becomes important, you have to sort all rows for getting only one row.
A trickier but better solution would be:
SELECT id, question from x1 JOIN (SELECT CEIL(RAND() * (SELECT(MAX(id)) FROM x1)) AS id) as id using(id);
Running EXPLAIN on both SELECTS will show you the difference...
If you need random value for different categories combine the selects via union and add a where clause
http://mysql.rjweb.org/doc.php/groupwise_max#top_n_in_each_group
But then ORDER BY category, RAND(). (Your category is the blog's province.)
Notice how it uses #variables to do the counting.
If you have MariaDB 10.2, then use one of its Windowing functions.
SELECT column FROM table WHERE category_id = XXX
ORDER BY RAND()
LIMIT 4
do it for all categories
I want to get the biggest ZIP code in DB. Normally I do this
SELECT *
FROM (
Select * From tbuser ORDER BY zip DESC
)
WHERE rownum = 1
with this code I can get the biggest zip code value without a duplicate row (since zip code is not a primary key).
But the main company at Japan said that I cant use it since when the connection is slow or the DB have very large data, you cant get the right row of it. It will be a great help for me if someone can helps.
I want to get the biggest ZIP code in DB.
If you really only want the zip code, try that:
SELECT MAX(zip) FROM TBUSER;
This will use the index on the zip column (if it exists).
That being said, Oracle is usually smart enough to properly optimize sub-query selection using ROWNUM. Maybe your main company is more concerned about the possible "full table" ̀€ORDER BY` in the subquery ? OTH, if the issue is really with "slow network", maybe worth taking some time with your DBA to look on the wire using a network analyzer or some other tool if your approach really leads to "excessive bandwidth consumption". I sincerely doubt about that...
If you want to retrieve the whole row having the maximum zip code here is a slight variation on an other answer (in my opinion, this is one of the rare case for using a NATURAL JOIN):
select * from t
natural join (select max(zip) zip from t);
Of course, in case of duplicates, this will return multiple rows. You will have to combine that with one of the several options posted in the various other answers to return only 1 row.
As an extra solution, and since you are not allowed to use ROWNUM (and assuming row_number is arbitrary forbidden too), you can achieve the desired result using something as contrived as:
select * from t
where rowid = (
select min(t.rowid) rid from t
natural join (select max(zip) zip from t)
);
See http://sqlfiddle.com/#!4/3bd63/5
But honestly, there isn't any serious reason to hope that such query will perform better than the simple ... ORDER BY something DESC) WHERE rownum <= 1 query.
This sounds to me like bad advice (masquerading as a rule) from a newbie data base administrator who doesn't understand what he's looking at. That insight isn't going to help you, though. Rarely does a conversation starting with "you're an obstructionist incompetent" achieve anything.
So, here's the thing. First of all, you need to make sure there's an index on your zip column. It doesn't have to be a primary key.
Second, you can try explaining that Oracle's table servers do, in fact, optimize the ... ORDER BY something DESC) WHERE rownum <= 1 style of query. Their servers do a good job of that. Your use case is very common.
But if that doesn't work on your DBA, try saying "I heard you" and do this.
SELECT * FROM (
SELECT a.*
FROM ( SELECT MAX(zip) zip FROM zip ) b
JOIN ZIP a ON (a.zip = b.zip)
) WHERE rownum <= 1
This will get one row with the highest numbered zip value without the ORDER BY that your DBA mistakenly believes is messing up his server's RAM pool. And, it's reasonably efficient. As long as zip has an index.
As you are looking for a way to get the desired record without rownum now, ...
... here is how to do it from Oracle 12c onward:
select *
from tbuser
order by zip desc fetch first 1 row only;
... and here is how to do it before Oracle 12c:
select *
from (select tbuser.*, row_number() over(order by zip desc) as rn from tbuser)
where rn = 1;
EDIT: As Sylvain Leroux pointed out, it is more work for the dbms to sort all records rather than just find the maximum. Here is a max query without rownum:
select *
from tbuser where rowid =
(select max(rowid) keep (dense_rank last order by zip) from tbuser);
But as Sylvain Leroux also mentioned, it makes also a difference whether there is an index on the column. Some tests I did show that with an index on the column, the analytic functions are slower than the traditional functions. Your original query would just get into the index, go to the highest value, pick the record and then stop. You won't get this any faster. My last mentioned query being quite fast on a none-indexed column is slower than yours on an indexed column.
Your requirements seem arbitrary, but this should give you the result you've requested.
SELECT *
FROM (SELECT * FROM tbuser
WHERE zip = (SELECT MAX(zip) FROM tbuser))
WHERE rownum = 1
OK - try something like this:
SELECT *
FROM TBUSER
WHERE ZIP = (SELECT MAX(ZIP) FROM TBUSER);
Fetch a single row from a cursor based on the above statement, then close the cursor. If you're using PL/SQL you could do it like this:
FOR aRow IN (SELECT *
FROM TBUSER
WHERE ZIP = (SELECT MAX(ZIP) FROM TBUSER))
LOOP
-- Do something with aRow
-- then force an exit from the loop
EXIT;
END LOOP;
Share and enjoy.
I was wondering that nobody posted this answer yet. I think that is the way, you should do something like that.
SELECT *
FROM (
Select a.*, max(zip) over () max_zip
From tbuser a
)
WHERE zip=max_zip
and rownum = 1
Your query gets exactly one random row of all records having the max zip code. So it cannot be the problem that you retrieve a record with another zip code or more than one record or zero records (as long as there is at least one record in the table).
Maybe Japan simply expects one of the other rows with that zip code? Then you may just have to add another order criteria to get that particular desired row.
Another thought: As they are talking about slow connection speed, it may also be that they enter a new max zip code on one session, query with another and get the old max zip, because the insert statement of the other session hasn't gone through yet. But well, that's just the way this works of course.
BTW: A strange thing to select a maximum zip code. I guess that's just an example to illustrate the problem?
IF you are getting multiple records using MAX function (which is not possible, but in your case you are getting, I don't know how until you post screenshot) then You can use DISTINCT in your sql query to get single record
SELECT DISTINCT MAX(zipcode) FROM TableUSER
SQL FIDDLE
I'm new to SQL and I don't think this question should be hard to answer. I have a high-score table for a game that contains the columns name, score, and rank. I want to know how I can order the table by descending order each time a new score is added so the table can always stay ordered by score.
I know this is the wrong way of doing this, but I hope this makes my point kind of clearer.
UPDATE `HSTable`.`Highscores` ORDER BY `Highscores`.`score` DESC;
What is the correct way of approaching this?
One more thing, is there a way I can set it so that the ranking value always stays where it's suppose to be from the SQL, for example, 1st place is always at the top regardless the score?
generally we use order by while fetching data from table
so we use
SELECT * FROM `HSTable` ORDER BY `score` DESC
Use a ranking function like RANK or DENSE_RANK to rank the results. For better performance you can add a clustered index that starts with score descending.
You can get ordered results using SELECT clause:
SELECT * FROM `HSTable` ORDER BY `score` DESC
I don't believe the order of the data in your database table should really matter because when you query the data you can sort it any which way you would like. What is the reason why you want to have the data stored in the database in the correct order?
That is not the correct way to select records in SQL. Use this instead:
SELECT * FROM `tablename` ORDER BY `id` DESC
I'm trying to select a record with the most effective votes. Each record has an id, the number of upvotes (int) and the number of downvotes (int) in a MySQL database.
I know basic update, select, insert queries but I'm unsure of how to form a query that looks something like:
SELECT *
FROM topics
WHERE MAX(topic.upvotes - topic.downvotes)
You are not using MAX() right.
Here is pretty fast query:
SELECT id, (upvotes - downvotes) AS popular
FROM topics
ORDER BY popular DESC LIMIT 1
To run an update:
UPDATE topics,
(here you put your select statement from above in parenthesis ) AS popular_query
SET topic.description = "I'm popular"
WHERE topics.id = popular_query.id
I just ran that on a table with 250,000 records (it's very similar - inventory usage - looking for a most popular part) and it took .203 of a second - on my dev machine - this is not even production server ( where it tppl 0.016 of a second)
UPDATE:
Yeah I didn't think about that possibility that you might have more than one top results.
SELECT GROUP_CONCAT(id) AS popular_ids, (upvotes - downvotes) AS popular
FROM topics
GROUP BY popular ORDER BY popular DESC LIMIT 1
popular_ids - will contain popular records as a text field, that can be easily parsed out if you need to.
There may be more than 1 record that match that condition. In order to get them all you could do something like this in mysql:
SELECT *
FROM topics
WHERE upvotes - downvotes = (select MAX(upvotes - downvotes) from topics)
SELECT (upvotes-downvotes) AS NetVotes, *
FROM topics
ORDER BY NetVotes DESC LIMIT 1
Does this do it for you?
SELECT *
FROM topics
ORDER BY topic.upvotes - topic.downvotes DESC
LIMIT 1;
Simply put, I have a table with, among other things, a column for timestamps. I want to get the row with the most recent (i.e. greatest value) timestamp. Currently I'm doing this:
SELECT * FROM table ORDER BY timestamp DESC LIMIT 1
But I'd much rather do something like this:
SELECT * FROM table WHERE timestamp=max(timestamp)
However, SQLite rejects this query:
SQL error: misuse of aggregate function max()
The documentation confirms this behavior (bottom of page):
Aggregate functions may only be used in a SELECT statement.
My question is: is it possible to write a query to get the row with the greatest timestamp without ordering the select and limiting the number of returned rows to 1? This seems like it should be possible, but I guess my SQL-fu isn't up to snuff.
SELECT * from foo where timestamp = (select max(timestamp) from foo)
or, if SQLite insists on treating subselects as sets,
SELECT * from foo where timestamp in (select max(timestamp) from foo)
There are many ways to skin a cat.
If you have an Identity Column that has an auto-increment functionality, a faster query would result if you return the last record by ID, due to the indexing of the column, unless of course you wish to put an index on the timestamp column.
SELECT * FROM TABLE ORDER BY ID DESC LIMIT 1
I think I've answered this question 5 times in the past week now, but I'm too tired to find a link to one of those right now, so here it is again...
SELECT
*
FROM
table T1
LEFT OUTER JOIN table T2 ON
T2.timestamp > T1.timestamp
WHERE
T2.timestamp IS NULL
You're basically looking for the row where no other row matches that is later than it.
NOTE: As pointed out in the comments, this method will not perform as well in this kind of situation. It will usually work better (for SQL Server at least) in situations where you want the last row for each customer (as an example).
you can simply do
SELECT *, max(timestamp) FROM table
Edit:
As aggregate function can't be used like this so it gives error. I guess what SquareCog had suggested was the best thing to do
SELECT * FROM table WHERE timestamp = (select max(timestamp) from table)