If my table looks like the following, how can I select classmates of a given student?
student_class_assn
student_id class_id
1 1
2 1
3 1
4 2
5 2
6 3
For example, Student 1 has students 2 and 3 as classmates.
Please help me write a query that pivots on the known student_id and selects only the classmates (and not including the given student).
Here is an example using a sub-select. But can you help me write it with joins instead?
SET #KNOWN_STUDENT=1;
SELECT
student_id
FROM
student_class_assn
WHERE
class_id IN (
SELECT class_id FROM student_class_assn WHERE student_id = #KNOWN_STUDENT
)
AND student_id != #KNOWN_STUDENT;
Given my table above I expect the following results for each given student_id:
1 returns 2,3
2 returns 1,3
3 returns 1,2
4 returns 5
5 returns 4
6 returns NULL
all you need to do is just join the table and pass a conditional
SELECT t1.student_id
FROM student_class_assn t
JOIN student_class_assn t1
ON t1.class_id = t.student_id
AND t1.student_id <> 1
WHERE t.student_id = 1
FIDDLE
This will not show students with no classmates:
select s1.sid,group_concat(s2.sid order by s2.sid,',')
from
stud_class s1 inner join
stud_class s2 on s1.cid=s2.cid
where s1.sid <> s2.sid
group by s1.sid
order by 1
I have data as follows:
Product Quantity
A 3
B 2
This is data that as been previously rolled up at the product level. Assume there are only two columns as of now.
I want an output as follows:
Product Quantity
A 1
A 1
A 1
B 1
B 1
You could use a trick like this:
SELECT Product, 1 AS Quantity
FROM
Products INNER JOIN (
SELECT 1 AS q UNION ALL
SELECT 2 UNION ALL SELECT 2 UNION ALL
SELECT 3 UNION ALL SELECT 3 UNION ALL SELECT 3
) quantities
ON Products.Quantity = quantities.q
of course, this query is limited to a quantity of 3, but you can add more quantities to your subquery if they are of a limited amount.
As I am not good with MySQL query's so I wish someone help me for creating this kind of sql query.
I having two MySQL tables which is describe bellow:
Table Name: rating
-------------------
property_id user_id area_rate_count safety_rate_count friendly_rate_count walkability_rate_count
4 28 1 1 1 2
5 38 2 3 4 1
5 40 2 2 3 1
6 40 2 3 1 4
10 43 2 2 3 1
Table Name: listing
-------------------
property_id title
4 Sample 1
5 Sample 2
6 Sample 3
10 Sample 4
11 Sample 5
12 Sample 6
Now first I want to sum each column and divide. (area_rate_count, safety_rate_count, friendly_rate_count, walkability_rate_count). For example In property_id:5 having two times so first calculate column sum and divide by 2.
After calculation we will get this output:
Table Name: rating (After Calculation)
--------------------------------------
property_id rate
4 5
5 9 (Divided by 2 because this property_id is two times in table)
6 10
10 8
And Finally I want join this result to my listing table and result looks something like this:
Table Name: listing
-------------------
property_id title rate
4 Sample 1 5
5 Sample 2 9 (Divided by 2 becouse property_id is two times in table)
6 Sample 3 10
10 Sample 4 8
11 Sample 5 0
12 Sample 6 0
Thanks.
I think you want the avg() aggregation function along with a join:
select l.property_id, l.title,
coalesce(avg(area_rate_count + safety_rate_count + friendly_rate_count + walkability_rate_count
), 0) as rate
from listing l left outer join
property_id p
on l.property_id = p.property_id
group by l.property_id, l.title ;
If I understood it right I think you need this:
select l.property_id, l.title, coalesce(r.ssum/if(r.ct=0,1,r.ct), 0) as rate
from listing l LEFT JOIN
(select property_id,
sum(area_rate_count+safety_rate_count
+friendly_rate_count+walkability_rate_count) ssum,
count(*) ct
from rating
group by property_id ) r
ON l.property_id = r.property_id
order by l.property_id
See it here on fiddle: http://sqlfiddle.com/#!2/589d6/5
Edit
As OP asked on the comments that he wants all columns from listing here is what he want:
select l.*, coalesce(r.ssum/if(r.ct=0,1,r.ct), 0) as rate
from listing l LEFT JOIN
(select property_id,
sum(area_rate_count+safety_rate_count
+friendly_rate_count+walkability_rate_count) ssum,
count(*) ct
from rating
group by property_id ) r
ON l.property_id = r.property_id
order by l.property_id
CREATE TEMPORARY TABLE IF NOT EXISTS
temp_table ( INDEX(col_2) )
ENGINE=MyISAM
AS (
SELECT
property_id,
AVG(area_rate_count) as area_rate_count,
AVG(safety_rate_count) as safety_rate_count,
AVG(friendly_rate_count) as friendly_rate_count,
AVG(walkability_rate_count) as walkability_rate_count
FROM rating
GROUP BY property_id
)
SELECT * FROM listing L
JOIN temp_table T
ON L.property_id = T.property_id
Use the below statement to get distinct property_id with its own rate
select property_id, sum(separaterating)/count(property_id) from (
select property_id,sum(area_rate_count , safety_rate_count , friendly_rate_count , walkability_rate_count) as separaterating from rating group by property_id AS temp ) group by
property_id
you can then join with the other table to get the final result as below
select * from ( select property_id, sum(separaterating)/count(property_id) from (
select property_id,sum(area_rate_count , safety_rate_count , friendly_rate_count , walkability_rate_count) as separaterating from rating group by property_id AS temp ) group by
property_id) AS A inner join listing AS B on A.property_id = B.property_id
try this:
select a.prop_id as property_id, l.title, a.allratings / b.numberofreviews as rate
from
(
select property_id as prop_id, SUM(coalesce(area_rate_count,0) + coalesce(safety_rate_count,0) + coalesce(friendly_rate_count,0) + coalesce(walkability_rate_count,0)) as allratings
from rating
group by property_id
) a inner join
(
select property_id, count(distinct user_id) as numberofreviews
from rating
group by property_id
) b on a.property_id = b.property_id
inner join listing l on a.property_id = l.property_id
Try This Query
select ls.property_id,ls.title,inr.rate from listing as ls
left join
(select r.property_id as pid,r.rate/r.cnt as rate from
(select property_id,user_id,(area_rate_count+safefty_rate_count+friendly_rate_count+walkability_rate_count) as rate,count(*) as cnt from rating group by property_id) as r) as inr on inr.pid=ls.property_id
I am aggregating data and I cannot sum certain columns so I would like to take the most frequent observation from that column, or the median value. Example follows, thanks in advance.
ID site
1 3
1 3
1 2
1 3
2 4
2 5
2 5
2 5
I want it to look like
ID Site
1 3
2 5
WITH temp AS(
SELECT ID, Site, COUNT(*) As counts
FROM id_table
GROUP BY ID, Site
)
SELECT temp.ID, temp.Site
FROM temp
JOIN (SELECT ID, MAX(counts) max_counts
FROM temp
GROUP BY ID
)b
ON temp.ID = b.ID
AND temp.counts = b.max_counts
ORDER BY ID ASC
SQL Fiddle
I am writing a query to grab the items that a specific user_id was the first to use. Here is some sample data -
item_id used_user_id date_used
1 1 2012-08-25
1 2 2012-08-26
1 3 2012-08-27
2 2 2012-08-27
3 1 2012-08-27
4 1 2012-08-21
4 3 2012-08-24
5 3 2012-08-23
query
select item_id as inner_item_id, ( select used_user_id
from test
where test.item_id = inner_item_id
order by date_used asc
limit 1 ) as first_to_use_it
from test
where used_user_id = 1
group by item_id
It returns the correct values
inner_item_id first_to_use_it
1 1
3 1
4 1
but the query is VERY slow on a giant table. Is there a certain index that I can use or a better query that I can write?
i can't get exactly what you mean because in your inner query you have sorted it by their used_user_id and and on your outer query you have filtered it also by their userid. Why not do this directly?
SELECT DISTINCT item_id AS inner_item_id,
used_user_id AS first_to_use_it
FROM test
WHERE used_user_id = 1
UPDATE 1
SELECT b.item_id,
b.used_user_id AS first_to_use_it
FROM
(
SELECT item_ID, MIN(date_used) minDate
FROM tableName
GROUP BY item_ID
) a
INNER JOIN tableName b
ON a.item_ID = b.item_ID AND
a.minDate = b.date_used
WHERE b.used_user_id = 1