I am trying to do a simple test where I'm pulling from a table the information of a specific part number as such:
SELECT *
FROM table_name
WHERE part_no IN ('abc123')
This returns 25 rows. Now I want to count the number that meet the "accepted" condition in a specific column but the result is limited to only the 10 most recent. My approach is to write it as follows:
Select Count(*)
FROM table_name
WHERE part_no IN ('abc123') AND lot IN ('accepted')
ORDER BY date DESC
LIMIT 10
I'm having a hard time to get the ORDER BY and LIMIT operations to work. I could use help just getting it to limit appropriately, and I can figure out the rest from there.
Edit: I understand that the operations are happening on the COUNT which only returns one row with a value; but I put the second clip to show where I am stuck in my thought process.
Your query SELECT Count(*) FROM ... will always return exactly one row.
It's not 100% clear what exactly you want to do, but if you want to know how many of the last 10 have been accepted, you could use a subquery - something like:
SELECT COUNT(*) FROM (
SELECT lot
FROM table_name
WHERE part_no IN ('abc123')
ORDER BY date DESC
LIMIT 10
)
WHERE lot IN ('accepted')
The inner query will return the 10 most recent rows for part abc123, then the outer query will count the accepted ones.
There are also other solution (for example, you could have the inner query output a field that is 0 when the part is not accepted and 1 when the part is accepted, then take the sum). Depending on which exact dialect/database you are using, you may also have more elegant options.
Select count returns ONE ROW therefore the ORDER BY and the LIMIT will not work on the results
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.
I have been looking on the web on how to select a random row on big tables, I have found various results, but then I analyzed my data and figured out that the best way for me to go is to count the rows and select a random one of those with LIMIT
While testing I start to wonder why this works:
SET #t = CEIL(RAND()*(SELECT MAX(id) FROM logo));
SELECT id
FROM logo
WHERE
current_status_id=29 AND
logo_type_id=4 AND
active='y' AND
id>=#t
ORDER BY id
LIMIT 1;
and gives random results, but this always returns the same 4 or 5 results ?
SELECT id
FROM logo
WHERE
current_status_id=29 AND
logo_type_id=4 AND
active='y' AND
id>=CEIL(RAND()*(SELECT MAX(id) FROM logo))
ORDER BY id
LIMIT 1;
the table has MANY fields (almost 100) and quite a few indexes. over 14 Million records and counting. When I select a random it is almost NEVER that I have to select it from the table, I always have to select depending on various fields values (all indexed).
Could it be a bug of my MySQL server version (5.6.13-log Source distribution)?
One possibility is that this statement in the documentation:
RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.
is simply not always true. It is true when you do:
where rand() < 0.01
to get an approximate 1% sample of the rows. Perhaps the MySQL optimizer says something like "Oh, I'll evaluate the subquery to get one value back. And, just to be more efficient, I'll multiply that row by rand() before defining the constant."
If I had to guess, that would be the case.
Another possibility is that the data is arranged so the values you are looking for has one row with a large id. Or, it could be that there are lots of rows with small ids at the very beginning, and then a very large gap.
Your method of getting a random row, by the way is not guaranteed to return a result when you are doing filtering. I don't know if that is important to you.
EDIT:
Check to see if this version works as you expect:
SELECT id
FROM logo cross join
(SELECT MAX(id) as maxid FROM logo) c
WHERE current_status_id = 29 AND
logo_type_id = 4 AND
active = 'y' AND
id >= RAND() * maxid
ORDER BY id
LIMIT 1;
If so, the problem is that the max id is being calculated and then there is an extra step of multiplying it by rand() as execution of the query begins.
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 have the following SQL query , it seems to run ok , but i am concerned as my site grows it may not perform as expected ,I would like some feeback as to how effective and efficient this query really is:
select * from articles where category_id=XX AND city_id=XXX GROUP BY user_id ORDER BY created_date DESC LIMIT 10;
Basically what i am trying to achieve - is to get the newest articles by created_date limited to 10 , articles must only be selected if the following criteria are met :
City ID must equal the given value
Category ID must equal the given value
Only one article per user must be returned
Articles must be sorted by date and only the top 10 latest articles must be returned
You've got a GROUP BY clause which only contains one column, but you are pulling all the columns there are without aggregating them. Do you realise that the values returned for the columns not specified in GROUP BY and not aggregated are not guaranteed?
You are also referencing such a column in the ORDER BY clause. Since the values of that column aren't guaranteed, you have no guarantee what rows are going to be returned with subsequent invocations of this script even in the absence of changes to the underlying table.
So, I would at least change the ORDER BY clause to something like this:
ORDER BY MAX(created_date)
or this:
ORDER BY MIN(created_date)
some potential improvements (for best query performance):
make sure you have an index on all columns you querynote: check if you really need an index on all columns because this has a negative performance when the BD has to build the index. -> for more details take a look here: http://dev.mysql.com/doc/refman/5.1/en/optimization-indexes.html
SELECT * would select all columns of the table. SELECT only the ones you really require...
What's the most efficient way to select the last n number of rows in a table using mySQL? The table contains millions of rows, and at any given time I don't know how large the table is (it is constantly growing). The table does have a column that is automatically incremented and used as a unique identifier for each row.
SELECT * FROM table_name ORDER BY auto_incremented_id DESC LIMIT n
Actually the right way to get last n rows in order is to use a subquery:
(SELECT id, title, description FROM my_table ORDER BY id DESC LIMIT 5)
ORDER BY tbl.id ASC
As this way is the only I know that will return them in right order. The accepted answer is actually a solution for "Select first 5 rows from a set ordered by descending ID", but that is most probably what you need.
(Similar to "marco"s answer,)
my fav is the max()-function of MySQL too, in a simple one-liner, but there are other ways of sure:
SELECT whatever FROM mytable WHERE id > (SELECT max(id)-10 FROM mytable);
... and you get "last id minus 10", normally the last 10 entries of that table.
It's a short way, to avoid the a error 1111 ("Invalid use of group function") not only if there is a auto_increment-row (here id).
The max()-function can be used many ways.
Maybe order it by the unique id descending:
SELECT * FROM table ORDER BY id DESC LIMIT n
The only problem with this is that you might want to select in a different order, and this problem has made me have to select the last rows by counting the number of rows and then selecting them using LIMIT, but obviously that's probably not a good solution in your case.
Use ORDER BY to sort by the identifier column in DESC order, and use LIMIT to specify how many results you want.
You would probably also want to add a descending index (or whatever they're called in mysql) as well to make the select fast if it's something you're going to do often.
This is a lot faster when you have big tables because you don't have to order an entire table.
You just use id as a unique row identifier.
This is also more eficient when you have big amounts of data in some colum(s) as images for example (blobs). The order by in this case can be very time and data consuming.
select *
from TableName
where id > ((select max(id) from TableName)-(NumberOfRowsYouWant+1))
order by id desc|asc
The only problem is if you delete rows in the interval you want. In this case you would't get the real "NumberOfRowsYouWant".
You can also easily use this to select n rows for each page just by multiplying (NumberOfRowsYouWant+1) by page number when you need to show the table backwards in multiple web pages.
Here you can change table name and column name according your requirement . if you want to show last 10 row then put n=10,or n=20 ,or n=30 ...etc according your requirement.
select * from
(select * from employee
Order by emp_id desc limit n)
a Order by emp_id asc;