There is a table named STATION.
I want to display half rows of the table.
MYSQL query
SELECT *
FROM STATION
LIMIT (SELECT COUNT(ID) FROM STATION)/2
I tried to perform a query like this but I am getting syntax error.
What is wrong in this query?
How can I perform this query?
One method is to use window functions:
select t.*
from (select t.*,
ntile(2) over (order by id) as tile
from t
) t
where tile = 1;
I have never seen a need of querying exactly half the table.
If you are asking this out of curiosity, that's fair but if there is really a need where you are trying to implement something like this, please revisit the design.
Coming to your question, you can possibly do two things:
Implement a stored procedure and query the count and store in a variable and then pass it on to the actual SELECT query for the LIMIT clause.
From your client code fire 2 different queries - 1 for count and calculate half (non fraction) and then pass it to the limit clause of next select query .
Related
My database is called: (training_session)
I try to print out some information from my data, but I do not want to have any duplicates. I do get it somehow, may someone tell me what I do wrong?
SELECT DISTINCT athlete_id AND duration FROM training_session
SELECT DISTINCT athlete_id, duration FROM training_session
It works perfectly if i use only one column, but when I add another. it does not work.
I think you misunderstood the use of DISTINCT.
There is big difference between using DISTINCT and GROUP BY.
Both have some sort of goal, but they have different purpose.
You use DISTINCT if you want to show a series of columns and never repeat. That means you dont care about calculations or group function aggregates. DISTINCT will show different RESULTS if you keep adding more columns in your SELECT (if the table has many columns)
You use GROUP BY if you want to show "distinctively" on a certain selected columns and you use group function to calculate the data related to it. Therefore you use GROUP BY if you want to use group functions.
Please check group functions you can use in this link.
https://dev.mysql.com/doc/refman/8.0/en/group-by-functions.html
EDIT 1:
It seems like you are trying to get the "latest" of a certain athlete, I'll assume the current scenario if there is no ID.
Here is my alternate solution:
SELECT a.athlete_id ,
( SELECT b.duration
FROM training_session as b
WHERE b.athlete_id = a.athlete_id -- connect
ORDER BY [latest column to sort] DESC
LIMIT 1
) last_duration
FROM training_session as a
GROUP BY a.athlete_id
ORDER BY a.athlete_id
This syntax is called IN-SELECT subquery. With the help of LIMIT 1, it shows the topmost record. In-select subquery must have 1 record to return or else it shows error.
MySQL's DISTINCT clause is used to filter out duplicate recordsets.
If your query was SELECT DISTINCT athlete_id FROM training_session then your output would be:
athlete_id
----------
1
2
3
4
5
6
As soon as you add another column to your query (in your example, the column called duration) then each record resulting from your query are unique, hence the results you're getting. In other words the query is working correctly.
Assume a houses table with lot's of fields, related images tables, and 3 other related tables. I have an expensive query that retrieves all houses data, with all data from the related tables. Do I need to run the same expensive MySql query twice in the case of pagination: once for current result page and once to get the total number of records?
I'm using server-side pagination with Limit 0,10, and need to return the total number of houses along with the data. It doesn't make sense to me to run the same expensive query with the count(*) function, just because I'm limiting the result-set for pagination.
Is there another way to instruct MySQL to count the whole query, but bring back only the current pagination data?
I hope my question is clear...
thanks
I don't know MySql but for many dbs, I think you'll find that the cost of running it twice isn't as high as you'd suspect - if you do it in such a way that the db's optimization engine sees the two queries as having a lot in common.
Running
select count(1) from (
select some_fields, row_number over (order by field) as rownum
from some_table
)
and then
select * from (
select some_fields, row_number over (order by field) as rownum
from some_table
)
where rownum between :startRow and :endRow
order by row_number
This also has the advantage of you being able to maintain the query in just one place with two different wrappers around it, 1 for paging and 1 for getting the total count.
Just as a side note, the best optimization you can do is make sure you send the exact same query to the db every time. In other words, if the user can change the sort or change what fields they can query on, bake it all into the same query. E.g:
select some_fields,
case
when :sortField = 'ID' and :sortType = 'asc'
then row_number over (order by id)
when :sortField = 'ID' and :sortType = 'desc'
then row_number over (order by id desc)
end as rownum
from some_table
where (:searchType = 'name'
and last_name like :lastName and first_name like :firstName)
or (:searchType = 'customerType'
and customer_type = :customer_type)
cfquery has a recordcount variable that might be useful. You can also use the startrow and maxrows attributes of cfoutput to control how many records get displayed. Finally, you can cache the query results in coldfusion so you don't have to run it against the database each time.
I have a scenario. I have say 300 records in my table. I execute a query to get the total count. Then , since i have to implement pagination,
I select the data from the same table using limits according t the count. I was thinking if i can get the count and data in a single query.? .
I tried below code:
Select * ,count(*) as cnt from table;
But this gave me the total count but only 1 record!
Is there a way to save my time exhausted in query and get results in a single query?
something like:
select t1.*,t2.cnt
from table t1
cross join (select count(*) as cnt from table) t2
limit 'your limit for the first page'
or
select *,(select count(*) from table) as cnt
from table
limit 'your limit for the first page'
You can get information in data structure you mentioned, but there is really no reason to do it. There is no performance problem when you do two queries - one for getting rows count and another for data selection. You don't save anything when you try to select all information in one query. Do two simple queries instead, it will be better solution for your app - you will preserve its simplicity and clarity.
Using two queries might not be as bad as you may think, you can read this for more information.
I am making a pagination method, what i did was:
First query will count all results and the second query will do the normal select with LIMIT
Is there technically any way to do this what I've done, but with only one query?
What I have now:
SELECT count(*) from table
SELECT * FROM table LIMIT 0,10
No one really mentions this, but the correct way of using the SQL_CALC_FOUND_ROWS technique is like this:
Perform your query: SELECT SQL_CALC_FOUND_ROWS * FROM `table` LIMIT 0, 10
Then run this query directly afterwards: SELECT FOUND_ROWS(). The result of this query contains the full count of the previous query, i.e. as if you hadn't used the LIMIT clause. This second query is instantly fast, because the result has already been cached.
You can do it with a subquery :
select
*,
(select count(*) from mytable) as total
from mytable LIMIT 0,10
But I don't think this has any kind of advantage.
edit: Like Ilya said, the total count and the rows have a totally different meaning, there's no real point in wanting to retrieve these data in the same query. I'll stick with the two queries. I just gave this answer for showing that this is possible, not that it is a good idea.
While i've seen some bad approaches to this, when i have looked into this previously there were two commonly accepted solutions:
Running your query and then running the same query with a count as you have done in your question.
Run your query and then run it again with the SQL_CALC_FOUND_ROWS keyword.
eg. SELECT SQL_CALC_FOUND_ROWS * FROM table LIMIT 0,10
This second approach is how phpMyAdmin does it.
You can run the first query and then the second query, and so you'll get both the count and the first results.
A query returns a set of records. The count is definitely not one of the records a "SELECT *" query can return, because there is only one count for the entire result set.
Anyway, you didn't say what programming language you run these SQL queries from and what interface you're using. Maybe this option exists in the interface.
SELECT SQL_CALC_FOUND_ROWS
your query here
Limit ...
Without running any other queries or destroying the session then run
SELECT FOUND_ROWS();
And you will get the total count of rows
The problem with 2 queries is consistency. In the time between the 2 queries the data may be changed. If your logic depends on what is counted has to be returned then your only approach would be to use SQL_CALC_FOUND_ROWS.
As of Mysql 8.0.17 SQL_CALC_FOUND_ROWS and FOUND_ROWS() will be deprecated. I don't know of a consistent solution after this functionality has been removed.
I have a simple report sending framework that basically does the following things:
It performs a SELECT query, it makes some text-formatted tables based on the results, it sends an e-mail, and it performs an UPDATE query.
This system is a generalization of an older one, in which all of the operations were hard coded. However, in pushing all of the logic of what I'd like to do into the SELECT query, I've run across a problem.
Before, I could get most of the information for my text tables by saying:
SELECT Name, Address FROM Databas.Tabl WHERE Status='URGENT';
Then, when I needed an extra number for the e-mail, also do:
SELECT COUNT(*) FROM Databas.Tabl WHERE Status='URGENT' AND TimeLogged='Noon';
Now, I no longer have the luxury of multiple SELECT queries. What I'd like to do is something like:
SELECT Tabl.Name, Tabl.Address, COUNT(Results.UID) AS Totals
FROM Databas.Tabl
LEFT JOIN Databas.Tabl Results
ON Tabl.UID = Results.UID
AND Results.TimeLogged='Noon'
WHERE Status='URGENT';
This, at least in my head, says to get a total count of all the rows that were SELECTed and also have some conditional.
In reality, though, this gives me the "1140 - Mixing of GROUP columns with no GROUP columns illegal if no GROUP BY" error. The problem is, I don't want to GROUP BY. I want this COUNT to redundantly repeat the number of results that SELECT found whose TimeLogged='Noon'. Or I want to remove the AND clause and include, as a column in the result of the SELECT statement, the number of results that that SELECT statement found.
GROUP BY is not the answer, because that causes it to get the COUNT of only the rows who have the same value in some column. And COUNT might not even be the way to go about this, although it's what comes to mind. FOUND_ROWS() won't do the trick, since it needs to be part of a secondary query, and I only get one (plus there's no LIMIT involved), and ROW_COUNT() doesn't seem to work since it's a SELECT statement.
I may be approaching it from the wrong angle entirely. But what I want to do is get COUNT-type information about the results of a SELECT query, as well as all the other information that the SELECT query returned, in one single query.
=== Here's what I've got so far ===
SELECT Tabl.Name, Tabl.Address, Results.Totals
FROM Databas.Tabl
LEFT JOIN (SELECT COUNT(*) AS Totals, 0 AS Bonus
FROM Databas.Tabl
WHERE TimeLogged='Noon'
GROUP BY NULL) Results
ON 0 = Results.Bonus
WHERE Status='URGENT';
This does use sub-SELECTs, which I was initially hoping to avoid, but now realize that hope may have been foolish. Plus it seems like the COUNTing SELECT sub-queries will be less costly than the main query since the COUNT conditionals are all on one table, but the real SELECT I'm working with has to join on multiple different tables for derived information.
The key realizations are that I can GROUP BY NULL, which will return a single result so that COUNT(*) will actually catch everything, and that I can force a correlation to this column by just faking a Bonus column with 0 on both tables.
It looks like this is the solution I will be using, but I can't actually accept it as an answer until tomorrow. Thanks for all the help.
SELECT Tabl.Name, Tabl.Address, Results.Totals
FROM Databas.Tabl
LEFT JOIN (SELECT COUNT(*) AS Totals, 0 AS Bonus
FROM Databas.Tabl
WHERE TimeLogged='Noon'
GROUP BY NULL) Results
ON 0 = Results.Bonus
WHERE Status='URGENT';
I figured this out thanks to ideas generated by multiple answers, although it's not actually the direct result of any one. Why this does what I need has been explained in the edit of the original post, but I wanted to be able to resolve the question with the proper answer in case anyone else wants to perform this silly kind of operation. Thanks to all who helped.
You could probably do a union instead. You'd have to add a column to the original query and select 0 in it, then UNION that with your second query, which returns a single column. To do that, the second query must also select empty fields to match the first.
SELECT Cnt = 0, Name, Address FROM Databas.Tabl WHERE Status='URGENT'
UNION ALL
SELECT COUNT(*) as Cnt, Name='', Address='' FROM Databas.Tabl WHERE Status='URGENT' AND TimeLogged='Noon';
It's a bit of a hack, but what you're trying to do isn't ideal...
Does this do what you need?
SELECT Tabl.Name ,
Tabl.Address ,
COUNT(Results.UID) AS GrandTotal,
COUNT(CASE WHEN Results.TimeLogged='Noon' THEN 1 END) AS NoonTotal
FROM Databas.Tabl
LEFT JOIN Databas.Tabl Results
ON Tabl.UID = Results.UID
WHERE Status ='URGENT'
GROUP BY Tabl.Name,
Tabl.Address
WITH ROLLUP;
The API you're using to access the database should be able to report to you how many rows were returned - say, if you're running perl, you could do something like this:
my $sth = $dbh->prepare("SELECT Name, Address FROM Databas.Tabl WHERE Status='URGENT'");
my $rv = $sth->execute();
my $rows = $sth->rows;
Grouping by Tabl.id i dont believe would mess up the results. Give it a try and see if thats what you want.