MySQL- Select at least n rows per group - mysql

Suppose you have
id / value
1 2
1 3
1 6
2 3
3 1
3 3
3 6
And I want to retrieve at least n rows per id group, let's say n = 4. In addition, it would help if a counter is added as a column. So the results should be like:
counter / id / value
1 1 2
2 1 3
3 1 6
4 null null
1 2 3
2 null null
3 null null
4 null null
1 3 1
2 3 3
3 3 6
4 null null
regards


I'm assuming that the combination of id and value is unique. Here's how you can do it without using MySQL variables:
SELECT
a.n AS counter,
b.id,
b.value
FROM
(
SELECT
aa.n,
bb.id
FROM
(
SELECT 1 AS n UNION ALL
SELECT 2 AS n UNION ALL
SELECT 3 AS n UNION ALL
SELECT 4 AS n
) aa
CROSS JOIN
(
SELECT DISTINCT id
FROM tbl
) bb
) a
LEFT JOIN
(
SELECT aa.id, aa.value, COUNT(*) AS rank
FROM tbl aa
LEFT JOIN tbl bb ON aa.id = bb.id AND aa.value >= bb.value
GROUP BY aa.id, aa.value
) b ON a.id = b.id AND a.n = b.rank
ORDER BY
a.id,
a.n


The next blog post describes the solution to your query:
SQL: selecting top N records per group.
It requires an additional small table of numbers, which is utilized to "iterate" the top N values per group via String Walking technique.
It uses GROUP_CONCAT as a way to overcome the fact MySQL does not support Window Functions. This also means it's not a pretty sight!
An advantage of this technique is that it does not require subqueries, and can optimally utilize an index on the table.
To complete the answer to your question, we must add an additional columns: you have requested a counter per item per group.
Here's an example using the world sample database, choosing top 5 largest counties per continent:
CREATE TABLE `tinyint_asc` (
`value` tinyint(3) unsigned NOT NULL default '0',
PRIMARY KEY (value)
) ;
INSERT INTO `tinyint_asc` VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15),(16),(17),(18),(19),(20),(21),(22),(23),(24),(25),(26),(27),(28),(29),(30),(31),(32),(33),(34),(35),(36),(37),(38),(39),(40),(41),(42),(43),(44),(45),(46),(47),(48),(49),(50),(51),(52),(53),(54),(55),(56),(57),(58),(59),(60),(61),(62),(63),(64),(65),(66),(67),(68),(69),(70),(71),(72),(73),(74),(75),(76),(77),(78),(79),(80),(81),(82),(83),(84),(85),(86),(87),(88),(89),(90),(91),(92),(93),(94),(95),(96),(97),(98),(99),(100),(101),(102),(103),(104),(105),(106),(107),(108),(109),(110),(111),(112),(113),(114),(115),(116),(117),(118),(119),(120),(121),(122),(123),(124),(125),(126),(127),(128),(129),(130),(131),(132),(133),(134),(135),(136),(137),(138),(139),(140),(141),(142),(143),(144),(145),(146),(147),(148),(149),(150),(151),(152),(153),(154),(155),(156),(157),(158),(159),(160),(161),(162),(163),(164),(165),(166),(167),(168),(169),(170),(171),(172),(173),(174),(175),(176),(177),(178),(179),(180),(181),(182),(183),(184),(185),(186),(187),(188),(189),(190),(191),(192),(193),(194),(195),(196),(197),(198),(199),(200),(201),(202),(203),(204),(205),(206),(207),(208),(209),(210),(211),(212),(213),(214),(215),(216),(217),(218),(219),(220),(221),(222),(223),(224),(225),(226),(227),(228),(229),(230),(231),(232),(233),(234),(235),(236),(237),(238),(239),(240),(241),(242),(243),(244),(245),(246),(247),(248),(249),(250),(251),(252),(253),(254),(255);
SELECT
Continent,
SUBSTRING_INDEX(
SUBSTRING_INDEX(
GROUP_CONCAT(Name ORDER BY SurfaceArea DESC),
',', value),
',', -1)
AS Name,
CAST(
SUBSTRING_INDEX(
SUBSTRING_INDEX(
GROUP_CONCAT(SurfaceArea ORDER BY SurfaceArea DESC),
',', value),
',', -1)
AS DECIMAL(20,2)
) AS SurfaceArea,
CAST(
SUBSTRING_INDEX(
SUBSTRING_INDEX(
GROUP_CONCAT(Population ORDER BY SurfaceArea DESC),
',', value),
',', -1)
AS UNSIGNED
) AS Population,
tinyint_asc.value AS counter
FROM
Country, tinyint_asc
WHERE
tinyint_asc.value >= 1 AND tinyint_asc.value <= 5
GROUP BY
Continent, value
;

Related

How to select a column value depending if id is even or odd

having a table structure of id and a name:
create table Mytable (
id integer not null,
name varchar(30) not null,
unique(id)
);
insert into Mytable (id,name) values
(1 , 'one'),
(2 , 'two'),
(3 , 'three'),
(4 , 'four'),
(6 , 'six');
How may I get a mix of even and odd rows in a result table like:
even | odd
-----------
null one '0 is not in Mytable so it puts null value
two three
four null '5 and 6 are not in Mytable so it puts null value
six null
I was trying to first get the following as a template and use it later as
a dictionary:
SELECT MIN(id-1) as id,MAX(id-1) as col
FROM Mytable
GROUP BY FLOOR((id+1)/2);
I get:
id col
0 1
2 3
5 5
But I do not know how to continue

For MySQL Version <= 5.7, You can use the below query
Query 1:
SELECT
MAX(CASE WHEN m.id % 2 = 0 THEN name END) AS even,
MAX(CASE WHEN m.id % 2 = 1 THEN name END) AS odd
FROM
(
SELECT
(SELECT MAX(id) FROM mytable) AS maxid,
#rn := #rn + 1 AS rn,
(SELECT IF((#rn * 2) <= maxid, #rn, NULL)) AS rid
FROM
mytable
JOIN
(SELECT #rn := -1) AS var
) AS t
JOIN
mytable m ON FLOOR(m.id/2) = t.rid
GROUP BY rid;
Result 1:
even | odd
:--- | :----
null | one
two | three
four | null
six | null
Demo 1:
db fiddle
Query 2:
After confirmation based on #Madhur Bhaiya comment. If there is no row for id = 8 and 9 then it will show null, null.
SELECT
MAX(CASE WHEN m.id % 2 = 0 THEN name END) AS even,
MAX(CASE WHEN m.id % 2 = 1 THEN name END) AS odd
FROM
(
SELECT
(SELECT MAX(id) FROM mytable) AS maxid,
#rn := #rn + 1 AS rn,
(SELECT IF((#rn * 2) <= maxid, #rn, NULL)) AS rid
FROM
(SELECT 0 UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) t
JOIN
(SELECT 0 UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) t2
JOIN
(SELECT #rn := -1) var -- currently it will return 1..100, if needed more add joins based on your needs
) AS t
LEFT JOIN
mytable m ON FLOOR(m.id/2) = t.rid
GROUP BY rid HAVING rid IS NOT NULL;
Result 2:
even | odd
:------ | :-----
null | one
two | three
four | null
six | null
null | null
null | eleven
null | null
null | null
sixteen | null
Demo 2:
db fiddle
For MySQL Version > 8.0, You can use #Nick query but if you need null, null like Result 2 mentioned for <= v5.7 then add LEFT JOIN with ORDER BY clause.
Query:
with recursive maxid as (
select max(id) as id from Mytable)
, cte as (
select 0 as rid
union all
select rid + 1
from cte
cross join maxid
where (rid + 1) * 2 <= maxid.id)
select max(case when m.id % 2 = 0 then name end) as even,
max(case when m.id % 2 = 1 then name end) as odd
from cte
left join Mytable m on floor(m.id / 2) = cte.rid
group by rid order by rid;
Result:
even | odd
:------ | :-----
null | one
two | three
four | null
six | null
null | null
null | eleven
null | null
null | null
sixteen | null
Demo: db fiddle
Credits: Thanks to #Nick, #Madhur Bhaiya for the fiddle and the logic used to create this query.

Here's a CTE based query that will work in SQL Server and MySQL > v8.0 (with the addition of the keyword recursive before maxid). It generates a list of rows that encompasses the pairs of MyTable values (in the sample, this is 0,1,2,3) and then JOINs that to Mytable to extract the even/odd column values:
with maxid as (
select max(id) as id from Mytable)
, cte as (
select 0 as rid
union all
select rid + 1
from cte
cross join maxid
where (rid + 1) * 2 <= maxid.id)
select max(case when m.id % 2 = 0 then name end) as even,
max(case when m.id % 2 = 1 then name end) as odd
from cte
join Mytable m on m.id / 2 = cte.rid
group by rid
Output:
even odd
one
two three
four
six
Demo on dbfiddle

how find a max value with comma separeted values and ids into a table

i have some problem with this query, to find max item_cost into this table
OrderID | item_ids| item_cost
---------------------------------------------------------
1 1,2,3 22,88,77
2 2,4 83,26
i find this function
SELECT * FROM scb.invoice_out
where FIND_IN_SET('2',item_ids)
but the answer is the complete row and i only need the max (item_cost) of the single item
OrderID | item_ids| item_cost
---------------------------------------------------------
1 1,2,3 22,88,77
2 2,4 83,26
somebody with the same problem ?

A mysql solution for your Question Could be
select
max(SUBSTRING_INDEX(SUBSTRING_INDEX(`item_cost`, ',', numbers.n), ',', -1)) as max_id
from
(select 1 n union all
select 2 union all select 3 union all
select 4 union all select 5) numbers INNER JOIN invoice_out
on CHAR_LENGTH(`item_cost`)
-CHAR_LENGTH(REPLACE(`item_cost`, ',', ''))>=numbers.n-1
where
FIND_IN_SET('2',item_ids)
order by
OrderID, n

SQL query of multiple values in one cell

There is a table(Course Interests) which has all the values in one cell. But those values are just ids and I want to join them with another table(Course) so I can know their names.
Course Interests:
MemberID MemberName CoursesInterested
-------------- --------------------- --------------
1 Al 1,4,5,6
2 A2 3,5,6
Course Table:
CourseId Course
-------------- ---------------------
1 MBA
2 Languages
3 English
4 French
5 Fashion
6 IT
Desired Output:
MemberID MemberName CoursesInterested
-------------- --------------------- --------------
1 Al MBA,French,Fashion,IT
2 A2 English,Fashion,IT
I would like to do a SQL query in MySql that can help me to extract the desired output. I know how to do it in the opposite way(join values to one cell), but I've struggling on seek a way to separate the ids and do a cross-join into another table.
I'll appreciate any help from the community. Thanks

Use FIND_IN_SET to search for something in a comma-delimited list.
SELECT i.MemberID, i.MemberName, GROUP_CONCAT(c.Course) AS CoursesInterested
FROM CourseInterests AS i
JOIN Course AS c ON FIND_IN_SET(c.CourseId, i.CoursesInterested)
However, it would be better to create a relation table instead of storing the courses in a single column. This type of join cannot be optimized using an index, so it will be expensive for a large table.

Try this Out:
SELECT MemberID,MemberName,Group_Concat(C.Course) from
(
SELECT MemberID,MemberName,SUBSTRING_INDEX(SUBSTRING_INDEX(t.CoursesInterested, ',', n.n), ',', -1) value
FROM Table1 t CROSS JOIN
(
SELECT a.N + b.N * 10 + 1 n
FROM
(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) a
,(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) b
ORDER BY n
) n
WHERE n.n <= 1 + (LENGTH(t.CoursesInterested) - LENGTH(REPLACE(t.CoursesInterested, ',', '')))
ORDER BY MemberID,value
) T JOIN course C ON T.value = C.CourseId
Group By MemberID,MemberName
Fiddle Demo
Output:
MemberID MemberName CoursesInterested
-------------- --------------------- --------------
1 Al MBA,French,Fashion,IT
2 A2 English,Fashion,IT

finding second position in mysql

I need to pull the name of the students who stood second positions from grade 1 to grade 12. each grade has separate databases with similar table structure
I have the following data:
Set 1
uid marks
1 10
2 20
3 17
4 17
5 20
6 20
Set 2
uid marks
1 10
2 20
3 17
4 17
5 20
6 17
7 20
I need a query which can say uid 3,4 are second in set 1 and 3,4,6 are second in set 2.
i need it in a single query because there are several set of databases
what could be the possible way?
I tried:
SELECT * FROM TBL WHERE marks ! = SELECT MAX(marks) from tbl
but it fetched all marks except the highest

Try this out:
SELECT uid, marks FROM (
SELECT uid, marks, #rank := #rank + (#prevMarks != marks) rank, #prevMarks := marks
FROM t, (SELECT #rank := 0, #prevMarks := 0) init
ORDER BY marks
) s
WHERE rank = 2
Fiddle here.
Another alternative without User Defined Variables:
SELECT t.uid, t.marks FROM t
JOIN (
SELECT DISTINCT marks FROM t
ORDER BY marks
LIMIT 1, 1
) s
ON t.marks = s.marks
Output:
| UID | MARKS |
|-----|-------|
| 3 | 17 |
| 4 | 17 |

Use LIMIT and ORDER BY
SELECT * FROM TBL ORDER BY marks DESC LIMIT 1,1
There you ordered all students by marks fro hi to low. And then limit return from second (0 is first record) and return only one record.
If need all students with second mark, the use subquery
SELECT * FROM TBL WHERE marks = (
SELECT marks FROM TBL ORDER BY marks DESC GROUP BY marks LIMIT 1,1
)

SELECT *
FROM table
WHERE mark = (
SELECT MAX(mark)
FROM table
WHERE mark <
(
SELECT MAX(mark)
FROM table
)
)

Try this
SELECT t.marks, t.uid, (
SELECT COUNT( marks ) +1
FROM tbl t1
WHERE t.marks < t1.marks
) AS rank
FROM tbl t
LIMIT 0 , 30
now you can use rank column with bit modification below
SELECT * from (
SELECT t.marks, t.uid, (
SELECT COUNT( marks ) +1
FROM tbl t1
WHERE t.marks < t1.marks
) AS rank
FROM tbl t
) alias where rank=n (2 here)

SQL split comma separated row [duplicate]

This question already has answers here:
MySQL: Split comma separated list into multiple rows
(4 answers)
Closed 9 years ago.
I have a column with a variable number of comma seperated values:
somethingA,somethingB,somethingC
somethingElseA, somethingElseB
And I want the result to take each value, and create a row:
somethingA
somethingB
somethingC
somethingElseA
somethingElseB
How can I do this in SQL (MySQL)?
(I've tried googling "implode" and "lateral view", but those don't seem to turn up related questions. All the related SO questions are trying to do much more complicated things)

You can do it with pure SQL like this
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(t.values, ',', n.n), ',', -1) value
FROM table1 t CROSS JOIN
(
SELECT a.N + b.N * 10 + 1 n
FROM
(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) a
,(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) b
ORDER BY n
) n
WHERE n.n <= 1 + (LENGTH(t.values) - LENGTH(REPLACE(t.values, ',', '')))
ORDER BY value
Note: The trick is to leverage tally(numbers) table and a very handy in this case MySQL function SUBSTRING_INDEX(). If you do a lot of such queries (splitting) then you might consider to populate and use a persisted tally table instead of generating it on fly with a subquery like in this example. The subquery in this example generates a sequence of numbers from 1 to 100 effectively allowing you split up to 100 delimited values per row in source table. If you need more or less you can easily adjust it.
Output:
| VALUE |
|----------------|
| somethingA |
| somethingB |
| somethingC |
| somethingElseA |
| somethingElseB |
Here is SQLFiddle demo
This is how the query might look with a persisted tally table
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(t.values, ',', n.n), ',', -1) value
FROM table1 t CROSS JOIN tally n
WHERE n.n <= 1 + (LENGTH(t.values) - LENGTH(REPLACE(t.values, ',', '')))
ORDER BY value
Here is SQLFiddle demo