Is it possible to order the GROUP BY chosen results of a MySQL query w/out using a subquery? I'm finding that, with my large dataset, the subquery adds a significant amount of load time to my query.
Here is a similar situation: how to sort order of LEFT JOIN in SQL query?
This is my code that works, but it takes way too long to load:
SELECT tags.contact_id, n.last
FROM tags
LEFT JOIN ( SELECT * FROM names ORDER BY timestamp DESC ) n
ON (n.contact_id=tags.contact_id)
WHERE tags.tag='$tag'
GROUP BY tags.contact_id
ORDER BY n.last ASC;
I can get a fast result doing a simple join w/ a table name, but the "group by" command gives me the first row of the joined table, not the last row.
I'm not really sure what you're trying to do. Here are some of the problems with your query:
selecting n.last, although it is neither in the group by clause, nor an aggregate value. Although MySQL allows this, it's really not a good idea to take advantage of.
needlessly sorting a table before joining, instead of just joining
the subquery isn't really doing anything
I would suggest carefully writing down the desired query results, i.e. "I want the contact id and latest date for each tag" or something similar. It's possible that will lead to a natural, easy-to-write and semantically correct query that is also more efficient than what you showed in the OP.
To answer the question "is it possible to order a GROUP BY query": yes, it's quite easy, and here's an example:
select a, b, sum(c) as `c sum`
from <table_name>
group by a,b
order by `c sum`
You are doing a LEFT JOIN on contact ID which implies you want all tag contacts REGARDLESS of finding a match in the names table. Is that really the case, or will the tags table ALWAYS have a "Names" contact ID record. Additionally, your column "n.Last". Is this the person's last name, or last time something done (which I would assume is actually the timestamp)...
So, that being said, I would just do a simple direct join
SELECT DISTINCT
t.contact_id,
n.last
FROM
tags t
JOIN names n
ON t.contact_id = n.contact_id
WHERE
t.tag = '$tag'
ORDER BY
n.last ASC
Related
I'm selecting a set of account records from a large table (millions of rows) with integer id values. As basic of a query as one gets, in a sense. What I'm doing us building a large comma separated list, and passing that into the query as an "in" clause. Right now the result is completely unordered. What I'd like to do is get the results back in the order of the values in the "in" clause.
I assume instead I'll have to build a temporary table and do a join instead, which I'd like to avoid, but may not be able to.
Thoughts? The size of the query right now is capped at about 60k each, as we're trying to limit the output size, but it could be arbitrarily large, which might rule out an "in" query anyway from a practical standpoint, if not a physical one.
Thanks in advance.
Actually, this is better:
SELECT * FROM your_table
WHERE id IN (5,2,6,8,12,1)
ORDER BY FIELD(id,5,2,6,8,12,1);
heres the FIELD documentation:
http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_field
A bit of a trick....
SELECT * FROM your_table
WHERE id IN (5,2,6,8,12,1)
ORDER BY FIND_IN_SET(id,'5,2,6,8,12,1') DESC;
note that the list of ID's in the find_in_set is a string, so its quoted.
Also note that without DESC, they results are returned in REVERSE order to what the list specified.
If your query is 60K, that's a sign that you're doing it the wrong way.
There is no other way to order the result set than by using an ORDER BY clause. You could have a complicated CASE clause in your order by listing all the elements in your IN clause again, but then your query would probably be 120K.
I know you don't want to, but you should put the values in the IN clause in a table or a temporary table and join with it. You can also include a SortOrder column in the temporary table, and order by that. Databases like joins. Doing it this way will help your query to perform well.
This is what I get for mysql 8.0. It seems opposite to above answers.
sort in same order as list specified:
SELECT * FROM your_table
WHERE id IN (5,2,6,8,12,1)
ORDER BY FIND_IN_SET(id,'5,2,6,8,12,1');
sort in reverse order as list specified:
SELECT * FROM your_table
WHERE id IN (5,2,6,8,12,1)
ORDER BY FIND_IN_SET(id,'5,2,6,8,12,1') DESC;
You're first query surely uses an order by clause. So, you could just do a join, and use the same order by clause.
For example, if this was your first query
SELECT customer_id
FROM customer
WHERE customer_id BETWEEN 1 AND 100
ORDER
BY last_name
And this was your second query
SELECT inventory_id
FROM rental
WHERE customer_id in (...the ordered list...)
Combined would be
SELECT r.inventory_id
FROM rental r
INNER
JOIN customer c
ON r.customer_id = c.customer_id
WHERE c.customer_id BETWEEN 1 AND 100
ORDER
BY c.last_name
This is what worked for me
SELECT * FROM your_table
WHERE id IN ('5','2','6','8','12','1')
ORDER BY FIELD(id,'5','2','6','8','12','1');
I added the ids in quotes
I am running the below query to retrive the unique latest result based on a date field within a same table. But this query takes too much time when the table is growing. Any suggestion to improve this is welcome.
select
t2.*
from
(
select
(
select
id
from
ctc_pre_assets ti
where
ti.ctcassettag = t1.ctcassettag
order by
ti.createddate desc limit 1
) lid
from
(
select
distinct ctcassettag
from
ctc_pre_assets
) t1
) ro,
ctc_pre_assets t2
where
t2.id = ro.lid
order by
id
Our able may contain same row multiple times, but each row with different time stamp. My object is based on a single column for example assettag I want to retrieve single row for each assettag with latest timestamp.
It's simpler, and probably faster, to find the newest date for each ctcassettag and then join back to find the whole row that matches.
This does assume that no ctcassettag has multiple rows with the same createddate, in which case you can get back more than one row per ctcassettag.
SELECT
ctc_pre_assets.*
FROM
ctc_pre_assets
INNER JOIN
(
SELECT
ctcassettag,
MAX(createddate) AS createddate
FROM
ctc_pre_assets
GROUP BY
ctcassettag
)
newest
ON newest.ctcassettag = ctc_pre_assets.ctcassettag
AND newest.createddate = ctc_pre_assets.createddate
ORDER BY
ctc_pre_assets.id
EDIT: To deal with multiple rows with the same date.
You haven't actually said how to pick which row you want in the event that multiple rows are for the same ctcassettag on the same createddate. So, this solution just chooses the row with the lowest id from amongst those duplicates.
SELECT
ctc_pre_assets.*
FROM
ctc_pre_assets
WHERE
ctc_pre_assets.id
=
(
SELECT
lookup.id
FROM
ctc_pre_assets lookup
WHERE
lookup.ctcassettag = ctc_pre_assets.ctcassettag
ORDER BY
lookup.createddate DESC,
lookup.id ASC
LIMIT
1
)
This does still use a correlated sub-query, which is slower than a simple nested-sub-query (such as my first answer), but it does deal with the "duplicates".
You can change the rules on which row to pick by changing the ORDER BY in the correlated sub-query.
It's also very similar to your own query, but with one less join.
Nested queries are always known to take longer time than a conventional query since. Can you append 'explain' at the start of the query and put your results here? That will help us analyse the exact query/table which is taking longer to response.
Check if the table has indexes. Unindented tables are not advisable(until unless obviously required to be unindented) and are alarmingly slow in executing queries.
On the contrary, I think the best case is to avoid writing nested queries altogether. Bette, run each of the queries separately and then use the results(in array or list format) in the second query.
First some questions that you should at least ask yourself, but maybe also give us an answer to improve the accuracy of our responses:
Is your data normalized? If yes, maybe you should make an exception to avoid this brutal subquery problem
Are you using indexes? If yes, which ones, and are you using them to the fullest?
Some suggestions to improve the readability and maybe performance of the query:
- Use joins
- Use group by
- Use aggregators
Example (untested, so might not work, but should give an impression):
SELECT t2.*
FROM (
SELECT id
FROM ctc_pre_assets
GROUP BY ctcassettag
HAVING createddate = max(createddate)
ORDER BY ctcassettag DESC
) ro
INNER JOIN ctc_pre_assets t2 ON t2.id = ro.lid
ORDER BY id
Using normalization is great, but there are a few caveats where normalization causes more harm than good. This seems like a situation like this, but without your tables infront of me, I can't tell for sure.
Using distinct the way you are doing, I can't help but get the feeling you might not get all relevant results - maybe someone else can confirm or deny this?
It's not that subqueries are all bad, but they tend to create massive scaleability issues if written incorrectly. Make sure you use them the right way (google it?)
Indexes can potentially save you for a bunch of time - if you actually use them. It's not enough to set them up, you have to create queries that actually uses your indexes. Google this as well.
SELECT alert,
(select created_at from alerts
WHERE alert = #ALERT ORDER BY created_at desc LIMIT 1)
AS latest FROM alerts GROUP BY alert;
I am having an issue with the above query where I would like to pass in each alert into the subquery so that I have a column called latest which displays the latest alert for each group of alerts. How should I do this?
This is called a correlated subquery. To make it work, you need table aliases:
SELECT a1.alert,
(select a2.created_at
from alerts a2
WHERE a2.alert = a1.alert
ORDER BY a2.created_at desc
LIMIT 1
) AS latest
FROM alerts a1
GROUP BY a1.alert;
Table aliases are a good habit to get into, because they often make the SQL more readable. It is also a good idea to use table aliases with column references, so you easily know where the column is coming from.
EDIT:
If you really want the latest, you can get it by simply doing:
select alert, max(created_at)
from alerts
group by alert;
If you are trying to get the latest created_at date for each group of alerts, there is a simpler way.
SELECT
alert,
max (created_at) AS latest
FROM
alerts
GROUP BY
alert;
I would do the following
SELECT
alert_group_name,
MAX(created_at) AS latest
FROM
alerts A
GROUP BY
alert_group_name;
For a correlated subquery, you need to reference an expression from the outer query.
The best way to do that is to assign an alias to the table on the outer query, and then reference that in the inner query. Best practice is to assign an alias to EVERY table reference, and qualify EVERY column reference.
All that needs to be done to "fix" your query is to replace the reference to "#ALERT" with a reference to the alert column from the table on the outer query.
In our shop, that statement would be formatted something like this:
SELECT a.alert
, (SELECT l.created_at
FROM alerts l
WHERE l.alert = a.alert
ORDER BY l.created_at DESC
LIMIT 1
) AS latest
FROM alerts a
GROUP
BY a.alert
Not because that's easier to write that way, but more importantly it's easier to read and understand what the statement is doing.
The correlated subquery approach can be efficient for a small number of rows returned (a very restrictive WHERE clause on the outermost query.) But in general, correlated subqueries in the SELECT list can make for a (what we refer to in our shop) an LDQ "light dimming query".
In our shop, if we needed the resultset returned by that query, that statement would likely be rewritten as:
SELECT a.alert
, MAX(a.created_at) AS latest
FROM alerts a
GROUP
BY a.alert
And we'd definitely have an index defined ON alerts(alert,created_at) (or an index with additional columns after those first two.)
size, we
(I don't anticipate any cases where this statement would return a different result.)
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
I got this query which is counting the total entries. The query is generic generated and therefore I can't make any comprehensive changes like subqueries etc.
In some specific cases a group by is needed to retrieve the correct rows and because of this the group by can't be removed
SELECT count(dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
GROUP BY dv.id
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
well, the answer to your question is simply you can't have an aggregate that works on all the results, while having a group by statement. That's the whole purpose of the group by to create groups that change the behaviour of aggregates:
The GROUP BY clause causes aggregations to occur in groups (naturally) for the columns you name.
cf this blog post which is only the first result I found on google on this topic.
You'd need to redesign your query, the easiest way being to create a subquery, or a hell of a jointure. But without the schema and a little context on what you want this query to do, I can't give you an alternative that works.
I just can tell you that you're trying to use a hammer to tighten a screw...
Have found an alternative where COUNT DISTINCT is used
SELECT count(distinct dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
I have followed the tutorial over at tizag for the MAX() mysql function and have written the query below, which does exactly what I need. The only trouble is I need to JOIN it to two more tables so I can work with all the rows I need.
$query = "SELECT idproducts, MAX(date) FROM results GROUP BY idproducts ORDER BY MAX(date) DESC";
I have this query below, which has the JOIN I need and works:
$query = ("SELECT *
FROM operators
JOIN products
ON operators.idoperators = products.idoperator JOIN results
ON products.idProducts = results.idproducts
ORDER BY drawndate DESC
LIMIT 20");
Could someone show me how to merge the top query with the JOIN element from my second query? I am new to php and mysql, this being my first adventure into a computer language I have read and tried real hard to get those two queries to work, but I am at a brick wall. I cannot work out how to add the JOIN element to the first query :(
Could some kind person take pity on a newb and help me?
Try this query.
SELECT
*
FROM
operators
JOIN products
ON operators.idoperators = products.idoperator
JOIN
(
SELECT
idproducts,
MAX(date)
FROM results
GROUP BY idproducts
) AS t
ON products.idproducts = t.idproducts
ORDER BY drawndate DESC
LIMIT 20
JOINs function somewhat independently of aggregation functions, they just change the intermediate result-set upon which the aggregate functions operate. I like to point to the way the MySQL documentation is written, which hints uses the term 'table_reference' in the SELECT syntax, and expands on what that means in JOIN syntax. Basically, any simple query which has a table specified can simply expand that table to a complete JOIN clause and the query will operate the same basic way, just with a modified intermediate result-set.
I say "intermediate result-set" to hint at the mindset which helped me understand JOINS and aggregation. Understanding the order in which MySQL builds your final result is critical to knowing how to reliably get the results you want. Generally, it starts by looking at the first row of the first table you specify after 'FROM', and decides if it might match by looking at 'WHERE' clauses. If it is not immediately discardable, it attempts to JOIN that row to the first JOIN specified, and repeats the "will this be discarded by WHERE?". This repeats for all JOINs, which either add rows to your results set, or remove them, or leaves just the one, as appropriate for your JOINs, WHEREs and data. This process builds what I am referring to when I say "intermediate result-set". Somewhere between starting and finishing your complete query, MySQL has in it's memory a potentially massive table-like structure of data which it built using the process I just described. Only then does it begin to aggregate (GROUP) the results according to your criteria.
So for your query, it depends on what specifically you are going for (not entirely clear in OP). If you simply want the MAX(date) from the second query, you can simply add that expression to the SELECT clause and then add an aggregation spec to the end:
SELECT *, MAX(date)
FROM operators
...
GROUP BY idproducts
ORDER BY ...
Alternatively, you can add the JOIN section of the second query to the first.