id value
---------
1 a
2 b
3 c
4 a
5 t
6 y
7 a
I want to select all rows where the value is 'a' and the row before it
id value
---------
1 a
3 c
4 a
6 y
7 a
I looked into
but I want to get all such rows in one query.
Please help me start
Thank you
I think the easiest way might be to use variables:
select t.*
from (select t.*,
(rn := if(value = 'a', 1, #rn + 1) as rn
from table t cross join
(select #rn := 0) params
order by id desc
) t
where rn in (1, 2)
order by id;
An alternative method uses a correlated subquery to get the previous value and then uses this in the where clause:
select t.*
from (select t.*,
(select t2.value
from table t2
where t2.id < t.id
order by t2.id desc
limit 1
) as prev_value
from table t
) t
where value = 'a' or prev_value = 'a';
With an index on id, this might even be faster than the method using variables.
Related
I have a list of value in my column. And want to query the range.
Eg. If values are 1,2,3,4,5,9,11,12,13,14,17,18,19
I want to display
1-5,9,11-14,17-19
Assuming that each value is stored on a separate row, you can use some gaps-and-island technique here:
select case when min(val) <> max(val)
then concat(min(val), '-', max(val))
else min(val)
end val_range
from (select val, row_number() over(order by val) rn from mytable) t
group by val - rn
order by min(val)
The idea is to build groups of consecutive values by taking the difference between the value and an incrementing rank, which is computed using row_number() (available in MySQL 8.0):
Demo on DB Fiddle:
| val_range |
| :-------- |
| 1-5 |
| 9 |
| 11-14 |
| 17-19 |
In earlier versions, you can emulate row_number() with a correlated subquery, or a user variable. The second option goes like:
select case when min(val) <> max(val)
then concat(min(val), '-', max(val))
else min(val)
end val_range
from (select #rn := 0) x
cross join (
select val, #rn := #rn + 1 rn
from (select val from mytable order by val) t
) t
group by val - rn
order by min(val)
As a complement to other answers:
select dn.val as dnval, min(up.val) as upval
from mytable up
join mytable dn
on dn.val <= up.val
where not exists (select 1 from mytable a where a.val = up.val + 1)
and not exists (select 1 from mytable b where b.val = dn.val - 1)
group by dn.val
order by dn.val;
1 5
9 9
11 14
17 19
Needless to say, but using an OLAP function like #GNB does, is orders of magnitude more efficient.
A short article on how to mimic OLAP functions in MySQL < 8 can be found at:
mysql-row_number
Fiddle
EDIT:
If another dimension is introduced (in this case p), something like:
select dn.p, dn.val as dnval, min(up.val) as upval
from mytable up
join mytable dn
on dn.val <= up.val
and dn.p = up.p
where not exists (select 1 from mytable a where a.val = up.val + 1 and a.p = up.p)
and not exists (select 1 from mytable b where b.val = dn.val - 1 and b.p = dn.p)
group by dn.p, dn.val
order by dn.p, dn.val;
can be used, see Fiddle2
I have a table like below. I want to extract the latest(based on time) 2 rows having same id. If no rows are same do not return anything. Then subtract the values of the latest row with the second latest and return a table with the ID and the value result.
Below is the table. 1st column is the id. Second is the value, third is the time. Id is not primary or unique
Id value time
3 2 2019-01-11 18:59:07.403
2 7 2019-01-10 18:58:40.400
4 5 2019-01-12 18:58:42.400
2 2 2019-01-11 18:59:23.147
5 -5 2019-01-12 18:58:42.400
3 8 2019-01-12 18:59:27.670
2 5 2019-01-12 18:59:43.777
The result should be
id value
2 3
3 6
One possible solution uses aggregation to get the IDs which occur more than once and correlated subqueries with ORDER BY and LIMIT to get the latest and second latest value.
SELECT x.id,
(SELECT t.value
FROM elbat t
WHERE t.id = x.id
ORDER BY t.time DESC
LIMIT 0, 1)
-
(SELECT t.value
FROM elbat t
WHERE t.id = x.id
ORDER BY t.time DESC
LIMIT 1, 1) value
FROM (SELECT t.id
FROM elbat t
GROUP BY t.id
HAVING count(*) > 1) x;
db<>fiddle
In MySQL 8+, you can use window functions and conditional aggregation
select t.id,
sum(case when seqnum = 1 then value else - value end) as diff
from (select t.*,
row_number() over (partition by id order by time desc) as seqnum
from elbat t
) t
where seqnum in (1, 2)
group by id
order by max(time) desc
limit 2;
The same idea can be adapted to earlier versions, using variables:
select t.id,
sum(case when seqnum = 1 then value else - value end) as diff
from (select t.*,
(#rn := if(#i = id, #rn + 1,
if(#i := id, 1, 1)
)
) as seqnum
from (select t.* from elbat t order by id, time desc) t cross join
(select #i := -1, #rn := 0) params
) t
where seqnum in (1, 2)
group by id
order by max(time) desc
limit 2;
I'm trying to select first row then skip X next rows then select rest in one query. For example if I have (a,b,c,d,e) in table I need to select "a" (first row) then skip X=2 rows ("b", "c") and then select rest which is "d" and "e", all in one query. So the result would be a,d,e
Try
select *
from
(
select *, #rank := #rank + 1 as rank
from your_table
cross join (select #rank := 0) r
order by colA
) tmp
where rank = 1
or rank > 3
or
select * from your_table
order by colA
limit 1
union all
select * from your_table
order by colA
limit 4, 9999999
You can use a variable to generate a row number:
select
YourField,
YourOtherField
from
(
select id,
YourField,
YourOtherField,
#row := #row + 1 as rownum
from YourTable
cross join (select #row:=0) c
order by YourField -- The field you want to sort by when you say 'first' and 'fourth'
) d
where
rownum = 1 or rownum >= 4
This question already has answers here:
How to get next/previous record in MySQL?
(23 answers)
Closed 4 years ago.
I have the following table, named Example:
id(int 11) //not autoincriment
value (varchar 100)
It has the following rows of data:
0 100
2 150
3 200
6 250
7 300
Note that id values are not contiguous.
I've written this SQL so far:
SELECT * FROM Example WHERE id = 3
However, I don't know how to get the value of previous id and value of the next id...
Please help me get previous value and next value if id = 3 ?
P.S.: in my example it will be: previous - 150, next - 250.
Select the next row below:
SELECT * FROM Example WHERE id < 3 ORDER BY id DESC LIMIT 1
Select the next row above:
SELECT * FROM Example WHERE id > 3 ORDER BY id LIMIT 1
Select both in one query, e.g. use UNION:
(SELECT * FROM Example WHERE id < 3 ORDER BY id DESC LIMIT 1)
UNION
(SELECT * FROM Example WHERE id > 3 ORDER BY id LIMIT 1)
That what you mean?
A solution would be to use temporary variables:
select
#prev as previous,
e.id,
#prev := e.value as current
from
(
select
#prev := null
) as i,
example as e
order by
e.id
To get the "next" value, repeat the procedure. Here is an example:
select
id, previous, current, next
from
(
select
#next as next,
#next := current as current,
previous,
id
from
(
select #next := null
) as init,
(
select
#prev as previous,
#prev := e.value as current,
e.id
from
(
select #prev := null
) as init,
example as e
order by e.id
) as a
order by
a.id desc
) as b
order by
id
Check the example on SQL Fiddle
May be overkill, but it may help you
please try this sqlFiddle
SELECT value,
(SELECT value FROM example e2
WHERE e2.value < e1.value
ORDER BY value DESC LIMIT 1) as previous_value,
(SELECT value FROM example e3
WHERE e3.value > e1.value
ORDER BY value ASC LIMIT 1) as next_value
FROM example e1
WHERE id = 3
Edit: OP mentioned to grab value of previous id and value of next id in one of the comments so the code is here SQLFiddle
SELECT value,
(SELECT value FROM example e2
WHERE e2.id < e1.id
ORDER BY id DESC LIMIT 1) as previous_value,
(SELECT value FROM example e3
WHERE e3.id > e1.id
ORDER BY id ASC LIMIT 1) as next_value
FROM example e1
WHERE id = 3
SELECT *,
(SELECT value FROM example e1 WHERE e1.id < e.id ORDER BY id DESC LIMIT 1 OFFSET 0) as prev_value,
(SELECT value FROM example e2 WHERE e2.id > e.id ORDER BY id ASC LIMIT 1 OFFSET 0) as next_value
FROM example e
WHERE id=3;
And you can place your own offset after OFFSET keyword if you want to select records with higher offsets for next and previous values from the selected record.
Here's my solution may suit you:
SELECT * FROM Example
WHERE id IN (
(SELECT MIN(id) FROM Example WHERE id > 3),(SELECT MAX(id) FROM Example WHERE id < 3)
)
Demo: http://sqlfiddle.com/#!9/36c1d/2
A possible solution if you need it all in one row
SELECT t.id, t.value, prev_id, p.value prev_value, next_id, n.value next_value
FROM
(
SELECT t.id, t.value,
(
SELECT id
FROM table1
WHERE id < t.id
ORDER BY id DESC
LIMIT 1
) prev_id,
(
SELECT id
FROM table1
WHERE id > t.id
ORDER BY id
LIMIT 1
) next_id
FROM table1 t
WHERE t.id = 3
) t LEFT JOIN table1 p
ON t.prev_id = p.id LEFT JOIN table1 n
ON t.next_id = n.id
Sample output:
| ID | VALUE | PREV_ID | PREV_VALUE | NEXT_ID | NEXT_VALUE |
|----|-------|---------|------------|---------|------------|
| 3 | 200 | 2 | 150 | 4 | 250 |
Here is SQLFiddle demo
This query uses a user defined variable to calculate the distance from the target id, and a series of wrapper queries to get the results you want. Only one pass is made over the table, so it should perform well.
select * from (
select id, value from (
select *, (#x := ifnull(#x, 0) + if(id > 3, -1, 1)) row from (
select * from mytable order by id
) x
) y
order by row desc
limit 3
) z
order by id
See an SQLFiddle
If you don't care about the final row order you can omit the outer-most wrapper query.
If you do not have an ID this has worked for me.
Next:
SELECT * FROM table_name
WHERE column_name > current_column_data
ORDER BY column_name ASC
LIMIT 1
Previous:
SELECT * FROM table_name
WHERE column_name < current_column_data
ORDER BY column_name DESC
LIMIT 1
I use this for a membership list where the search is on the last name of the member. As long as you have the data from the current record it works fine.
Considering following table that doesn't have any primary key, can I select every other row?
col1 col2
2 a
1 b
3 c
12 g
first select must find: 2, 3
second select must find: 1, 12
is that possible?
In unique MySQL fashion:
select *
from (
select *
, #rn := #rn + 1 as rn
from Table1
join (select #rn := 0) i
) s
where rn mod 2 = 0 -- Use = 1 for the other set
Example at SQL Fiddle.
Try this. I've adapted it from the answer linked below.
I tested it on SQLFiddle and it appears to work.
http://sqlfiddle.com/#!2/0bccf/28
http://sqlfiddle.com/#!2/0bccf/29
Odd Rows:
SELECT x.*
FROM (
SELECT #rownum:=#rownum+1 rownum, t.*
FROM (SELECT #rownum:=0) r, table t
) x
WHERE MOD(x.rownum, 2) = 1
Even Rows:
SELECT x.*
FROM (
SELECT #rownum:=#rownum+1 rownum, t.*
FROM (SELECT #rownum:=0) r, table t
) x
WHERE MOD(x.rownum, 2) = 0
Adapted from:
MySQL row number
yes possible using temp variable
Example :
set #a := 0;
select * from car_m_city WHERE mod((#a:=#a+1), 2) = 1
Explanation :
here in sql we declare #a( set #a := 0;) temp variable.(#a:=#a+1) now #a increment by 1.jsut like simple way to check odd or even
mod((#a:=#a+1), 2) = 1 for odd data
mod((#a:=#a+1), 2) = 0 for even data
This works for me.
SET #row_number = 0;
select* from (
SELECT
(#row_number:=#row_number + 1) AS num, col1,col2
FROM
TABLE1
) as t WHERE num%2=0
You can use mod 1 for odd or mod 0 for even rows
This should work for MySQL:
SELECT col1, col2
FROM (
SELECT col1, col2, #rowNumber:=#rowNumber+ 1 rn
FROM YourTable
JOIN (SELECT #rowNumber:= 0) r
) t
WHERE rn % 2 = 1
This uses % which is the MOD operator.
And here is the sample fiddle: http://sqlfiddle.com/#!2/cd31b/2