Assuming the query
SELECT * FROM table WHERE a='1' and b='2' and c>'3' and d>'4' and e!='5' and f='6'
returns 10000 results.
My question is, let's say I limit the search to the first 10 results like this:
SELECT * FROM table WHERE a='1' and b='2' and c>'3' and d>'4' and e!='5' and f='6' LIMIT 10
Will mysql search through all the 10000 results or it will stop at the 10th result?
LIMIT will only display the rows specified, based on their position within the resultset. Without an ORDER BY, you're relying on the order the records were inserted.
You'll probably be interested to read about MySQL's ORDER BY/LIMIT performance...
Since there is no ORDER BY, it will stop at the 10th result (after going through as many non-matching rows as necessary). As OMG Ponies says, which 10 rows you get is unspecified.
Related
If I have a mysql limited query:
SELECT * FROM my_table WHERE date > '2020-12-12' LIMIT 1,16;
Is there a faster way to check and see how many results are left after my limit?
I was trying to do a count with limit, but that wasn't working, i.e.
SELECT count(ID) AS count FROM my_table WHERE date > '2020-12-12' LIMIT 16,32;
The ultimate goal here is just to determine if there ARE any other rows to be had beyond the current result set, so if there is another faster way to do this that would be fine too.
It's best to do this by counting the rows:
SELECT count(*) AS count FROM my_table WHERE date > '2020-12-12'
That tells you how many total rows match the condition. Then you can compare that to the size of the result you got with your query using LIMIT. It's just arithmetic.
Past versions of MySQL had a function FOUND_ROWS() which would report how many rows would have matched if you didn't use LIMIT. But it turns out this had worse performance than running two queries, one to count rows and one to do your limit. So they deprecated this feature.
For details read:
https://www.percona.com/blog/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/
https://dev.mysql.com/worklog/task/?id=12615
(You probably want OFFSET 0, not 1.)
It's simple to test whether there ARE more rows. Assuming you want 16 rows, use 1 more:
SELECT ... WHERE ... ORDER BY ... LIMIT 0,17
Then programmatically see whether it returned only 16 rows (no more available) or 17 (there ARE more).
Because it is piggybacking on the fetch you are already doing and not doing much extra work, it is very efficient.
The second 'page' would use LIMIT 16, 17; 3rd: LIMIT 32,17, etc. Each time, you are potentially getting and tossing an extra row.
I discuss this and other tricks where I point out the evils of OFFSET: Pagination
COUNT(x) checks x for being NOT NULL. This is [usually] unnecessary. The pattern COUNT(*) (or COUNT(1)) simply counts rows; the * or 1 has no significance.
SELECT COUNT(*) FROM t is not free. It will actually do a full index scan, which is slow for a large table. WHERE and ORDER BY are likely to add to that slowness. LIMIT is useless since the result is always 1 row. (That is, the LIMIT is applied to the result, not to the counting.)
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
I have 4000 rows for example, and I define X limit.
The query stops after it finds X rows? or the query finds all the rows and then takes X rows from the found rows?
Thank you.
From MySQL Reference Manual:
If you use LIMIT row_count with ORDER BY, MySQL ends the sorting as
soon as it has found the first row_count rows of the sorted result,
rather than sorting the entire result. If ordering is done by using an
index, this is very fast. If a filesort must be done, all rows that
match the query without the LIMIT clause must be selected, and most or
all of them must be sorted, before it can be ascertained that the
first row_count rows have been found. In either case, after the
initial rows have been found, there is no need to sort any remainder
of the result set, and MySQL does not do so.
So it looks like it's possible that the entire result set is known before the LIMIT is applied. But MySQL will try everything it can not to do so. And you can help it by providing useful indexes that match your queries.
EDIT: Furthermore, if the set is not sorted it terminates the SELECT operation as soon as it's streamed enough rows to the result set.
SELECT * FROM your_table LIMIT 0, 10
This will display the first 10 results from the database.
SELECT * FROM your_table LIMIT 5, 5
This will show records 6, 7, 8, 9, and 10
It's like telling MySql; I want you to start counting from 5+1 or the 6th record, but Select only upto 5 records
I'm assuming you're thinking about MySQL, in which according to the documentation, the answer is it depends. If you're using a LIMIT (without a HAVING), then:
If you are selecting only a few rows with LIMIT, MySQL uses indexes
in some cases when normally it would prefer to do a full table scan.
As soon as MySQL has sent the required number of rows to the client, it aborts the query unless you are using SQL_CALC_FOUND_ROWS.
There are a few other cases which you should read about in the documentation.
Introduction to MySQL LIMIT clause
The following illustrates the LIMIT clause syntax with two arguments:
SELECT
select_list
FROM
table_name
LIMIT [offset,] row_count;
The offset specifies the offset of the first row to return. The offset of the first row is 0, not 1.
The row_count specifies the maximum number of rows to return.
The following picture illustrates the LIMIT clause:
Therefore, these two clauses are equivalent:
> LIMIT row_count;
> LIMIT 0 , row_count;
The following picture illustrates the evaluation order of the LIMIT clause in the SELECT statement:
It stops after it found the number of rows specified in the LIMIT clause. This can be verified with a large amount of data. It retrieves the result in a time that is not possible if it is getting all the rows of the table and filtering after that.
If you are using MS SQL Server, then you can write it as given below.
Select TOP [x]
*
From MyTable
Hope it helps.
Vamyip
I have a query that looks like this:
SELECT article FROM table1 ORDER BY publish_date LIMIT 20
How does ORDER BY work? Will it order all records, then get the first 20, or will it get 20 records and order them by the publish_date field?
If it's the last one, you're not guaranteed to really get the most recent 20 articles.
It will order first, then get the first 20. A database will also process anything in the WHERE clause before ORDER BY.
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows
In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.
All details on: http://dev.mysql.com/doc/refman/5.0/en/select.html
Just as #James says, it will order all records, then get the first 20 rows.
As it is so, you are guaranteed to get the 20 first published articles, the newer ones will not be shown.
In your situation, I recommend that you add desc to order by publish_date, if you want the newest articles, then the newest article will be first.
If you need to keep the result in ascending order, and still only want the 10 newest articles you can ask mysql to sort your result two times.
This query below will sort the result descending and limit the result to 10 (that is the query inside the parenthesis). It will still be sorted in descending order, and we are not satisfied with that, so we ask mysql to sort it one more time. Now we have the newest result on the last row.
select t.article
from
(select article, publish_date
from table1
order by publish_date desc limit 10) t
order by t.publish_date asc;
If you need all columns, it is done this way:
select t.*
from
(select *
from table1
order by publish_date desc limit 10) t
order by t.publish_date asc;
I use this technique when I manually write queries to examine the database for various things. I have not used it in a production environment, but now when I bench marked it, the extra sorting does not impact the performance.
You could add [asc] or [desc] at the end of the order by to get the earliest or latest records
For example, this will give you the latest records first
ORDER BY stamp DESC
Append the LIMIT clause after ORDER BY
If there is a suitable index, in this case on the publish_date field, then MySQL need not scan the whole index to get the 20 records requested - the 20 records will be found at the start of the index. But if there is no suitable index, then a full scan of the table will be needed.
There is a MySQL Performance Blog article from 2009 on this.
You can use this code
SELECT article FROM table1 ORDER BY publish_date LIMIT 0,10
where 0 is a start limit of record & 10 number of record
LIMIT is usually applied as the last operation, so the result will first be sorted and then limited to 20. In fact, sorting will stop as soon as first 20 sorted results are found.
Could be simplified to this:
SELECT article FROM table1 ORDER BY publish_date DESC FETCH FIRST 20 ROWS ONLY;
You could also add many argument in the ORDER BY that is just comma separated like: ORDER BY publish_date, tab2, tab3 DESC etc...
I have a query like this where I want to display sorted records in a paginated format.
This is my query.
SELECT * FROM answers
WHERE userid = '10'
ORDER BY votes LIMIT 10, 20
The two arguments to LIMIT will be used for generating page-by-page display of records. Now, my problem is ORDER BY. How does MySQL execute this query?
1st way
Select the records according to filters
Sort them
Limit them
2nd way
Select the records according to filters
Limit them
Sort them
If I think like MySQL engine, I would want to implement 2nd, since I would then have to sort lesser records?
Can somebody throw some light on this?
Regards
A SQL LIMIT will in all databases always work on the result of a query, so it will run the query with the ORDER BY and that result will then be limited. This is a functional requirement, the database not perse needs to execute it like that.
Thus the 1st way.
Mysql follows the 1st way.
If you want your 2nd way, try using a subquery with the limit, then order.
Something like:
SELECT * FROM
(
SELECT * FROM answers
WHERE userid = '10'
LIMIT 10
)
ORDER BY votes
The 1st way as mentioned by the last two answers. From a user perspective, the 2nd way won't be very useful anyways.