Is there a simple way in Mysql to convert a multi-column resultset into a single-column resultset where each row in the single column contains a single value from each cell in the multi-column resultset?
For instance, say I have a table like:
id | fk1 | fk2 | fk3
1 2 3 4
5 6 7 8
Ideally I'd like to be able to run a query along the lines of:
SELECT <some_function>(fk1, fk2, fk3) AS value FROM myTable;
...and then get an output like:
value
2
3
4
6
7
8
Is there a straightforward way of doing so, or is the only real option to walk the multi-column resultset in code and extract the values into the format that I want?
The end goal is to be able to use the first query as a subquery in a context where the input can only be a single column of values, like:
SELECT * FROM myOtherTable WHERE id IN
(SELECT <some_function>(fk1, fk2, fk3) AS value FROM myTable);
Thus a pure SQL solution is preferable, if one exists.
The simplest way is to use union all:
select fk1 from mytable union all
select fk2 from mytable union all
select fk3 from mytable;
There are other methods, but this is simplest.
For your particular query, you can use exists and or or in:
SELECT mo.*
FROM myOtherTable mo
WHERE EXISTS (SELECT 1
FROM MyTable m
WHERE mo.id IN (m.fk1, m.fk2, m.fk3)
);
EDIT:
For performance, but an index on each fk column and use multiple conditions in the where:
SELECT mo.*
FROM myOtherTable mo
WHERE EXISTS (SELECT 1 FROM MyTable m WHERE mo.id = m.fk1) OR
EXISTS (SELECT 1 FROM MyTable m WHERE mo.id = m.fk2) OR
EXISTS (SELECT 1 FROM MyTable m WHERE mo.id = m.fk3);
This will do index lookups, which should be much faster than the original form.
Consider you table name tab1.
Now you can do query like
select fk1 from (
(select fk1 from tab1)
union all
(select fk2 from tab1)) as rt
group by fk1;
This will combine your two column in to one. Contains will appear one after another.
Related
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Possible Duplicate:
SQL: find missing IDs in a table
getting values which dont exist in mysql table
Just wondering, is it possible to have a query that somehow tells you the values it did not find in a table?
So if I had a query SELECT * FROM mytable WHERE id IN (1,2,3,4,5,6,7,8,9) and only 2,3,6,7,9 was returned. I wouldd like to know that 1,4,5,8 were not found.
It will be a little hard to do a manual comparision, because this is going to be run over apx 2,000+ rows in a table (the id's are going to be provided via a csv file which can be copied into the query)
Thanks in advance
This is probably silly, but what about creating a temporary table containing all your IDs from which you'll substract the result of your SELECT query ?
Untested, but in theory:
Table 1:
+----+-----+
| id | num |
+----+-----+
Table 2:
+----+
| id |
+----+
Table 1 contains the data you're looking for (and num is any field containing any data)
Table 2 contains the IDs from the CSV
SQL:
SELECT COUNT(`Table1`.`num`) AS `count`
FROM `Table1`
LEFT JOIN `Table2` ON `Table1`.`id` = `Table2`.`id`
WHERE `count` = 0
Quick solution, open your csv file, replace all comma's with " union select " put select in front of that line and use it as the first line of the query at the bottom query.
So 1,2,3 becomes
Select 1 union select 2 union select 3
Use this in the query below
Select 1 union select 2 union select x -- replace this line with the line generated from your csv
Except
(
Select id from mytable
)
What about:
SELECT *
FROM (select 1 as f
UNION
SELECT 2 as f
UNION
SELECT 3 as f
UNION
SELECT 4 as f
UNION
SELECT 5 as f
UNION
SELECT 6 as f
UNION
SELECT 7 as f
UNION
SELECT 8 as f
UNION
SELECT 9 ) as s1
WHERE f NOT IN (SELECT id FROM mytable);
Lets say I have a MySQL table that has the following entries:
1
2
3
2
5
6
7
6
6
8
When I do an "SELECT * ..." I get back all the entries. But I want to get back only these entries, that exist only once within the table. Means the rows with the values 2 (exists two times) and 6 (exists three times) have to be dropped completely out of my result.
I found a keyword DISTINCT but as far as I understood it only avoids entries are shown twice, it does not filters them completely.
I think it can be done somehow with COUNT, but all I tried was not really successful. So what is the correct SQL statement here?
Edit: to clarify that, the result I want to get back is
1
3
5
7
8
You can use COUNT() in combination with a GROUP BY and a HAVING clause like this:
SELECT yourCol
FROM yourTable
GROUP BY yourCol
HAVING COUNT(*) < 2
Example fiddle.
You want to mix GROUP BY and COUNT().
Assuming the column is called 'id' and the table is called 'table', the following statement will work:
SELECT * FROM `table` GROUP BY id HAVING COUNT(id) = 1
This will filter out duplicate results entirely (e.g. it'll take out your 2's and 6's)
Three ways. One with GROUP BY and HAVING:
SELECT columnX
FROM tableX
GROUP BY columnX
HAVING COUNT(*) = 1 ;
one with a correlated NOT EXISTS subquery:
SELECT columnX
FROM tableX AS t
WHERE NOT EXISTS
( SELECT *
FROM tableX AS t2
WHERE t2.columnX = t.columnX
AND t2.pk <> t.pk -- pk is the primary key of the table
) ;
and an improvement on the first way (if you have a primary key pk column and an index on (columnX, pk):
SELECT columnX
FROM tableX
GROUP BY columnX
HAVING MIN(pk) = MAX(pk) ;
select id from foo group by id having count(*) < 2;
I had a problem in database. I have to insert duplicate records of a particular record on a another table based on a value.
First i used cursor to fetch each records and get the number of duplication i wants and after that used another cursor for duplication. Everything worked fine. But if the records in more than 500, i went dead slow. Then i did some research and found a way to insert without cursor.
INSERT INTO report(id, Name)
SELECT i.id,i.Name FROM (SELECT 1 AS id
UNION SELECT 2
UNION SELECT 3
UNION SELECT 4
UNION SELECT 5
UNION SELECT 6
UNION SELECT 7
UNION SELECT 8
UNION SELECT 9
UNION SELECT 10) AS o
INNER JOIN table i WHERE o.id<=i.frequence;
where frequence is the number of duplication. Please drop your idea to improve your query.
You could try creating a table with a record for each value from 1 to 10 and then join to that. I'm not sure it would be any faster though. You would have to experiment with it.
In this example the table with the values from 1 to 10 is called "dup" and the field containing these values is called "id".
INSERT INTO report(id, Name)
SELECT i.id, i.Name
FROM table i
JOIN dup d
ON d.id <= i.frequence;
If you have any table that contains a row number that goes at least as high as the maximum frequence, you could to this:
INSERT INTO report(id, Name)
SELECT i.id,i.Name FROM table i
inner join (
select distinct some_row_number_column from some_table
) o on o.some_row_number_column <= i.frequence;
This is basically the same as what you were doing, but it avoids the messy union all statements.
Or you could make a cursor that inserts numbers from 1 to the maximum frequence into a temporary table, then use that in your join. Or you could use a row numbering variable to generate the necessary sequence. Basically, do anything that will generate a list of consecutive numbers from 1 to the maximum that you need.
I would normally use recursion for this (DB2 syntax):
INSERT INTO report(id, Name)
with num_list (num) as (
values (1)
union all
select num + 1 from num_list
where num < (select max(frequence) from table)
)
SELECT i.id,i.Name FROM table i
inner join num_list on num_list.num <= i.frequence;
However, MySQL doesn't support recursion, apparently.
I have a MySQL table like this
id Name count
1 ABC 1
2 CDF 3
3 FGH 4
using simply select query I get the values as
1 ABC 1
2 CDF 3
3 FGH 4
How I can get the result like this
1 ABC 1
2 CDF 3
3 FGH 4
4 NULL 0
You can see Last row. When Records are finished an extra row in this format
last_id+1, Null ,0 should be added. You can see above. Even I have no such row in my original table. There may be N rows not fixed 3,4
The answer is very simple
select (select max(id) from mytable)+1 as id, NULL as Name, 0 as count union all select id,Name,count from mytable;
This looks a little messy but it should work.
SELECT a.id, b.name, coalesce(b.`count`) as `count`
FROM
(
SELECT 1 as ID
UNION
SELECT 2 as ID
UNION
SELECT 3 as ID
UNION
SELECT 4 as ID
) a LEFT JOIN table1 b
ON a.id = b.id
WHERE a.ID IN (1,2,3,4)
UPDATE 1
You could simply generate a table that have 1 column preferably with name (ID) that has records maybe up 10,000 or more. Then you could simply join it with your table that has the original record. For Example, assuming that you have a table named DummyRecord with 1 column and has 10,000 rows on it
SELECT a.id, b.name, coalesce(b.`count`) as `count`
FROM DummyRecord a LEFT JOIN table1 b
ON a.id = b.id
WHERE a.ID >= 1 AND
a.ID <= 4
that's it. Or if you want to have from 10 to 100, then you could use this condition
...
WHERE a.ID >= 10 AND
a.ID <= 100
To clarify this is how one can append an extra row to the result set
select * from table union select 123 as id,'abc' as name
results
id | name
------------
*** | ***
*** | ***
123 | abc
Simply use mysql ROLLUP.
SELECT * FROM your_table
GROUP BY Name WITH ROLLUP;
select
x.id,
t.name,
ifnull(t.count, 0) as count
from
(SELECT 1 AS id
-- Part of the query below, you will need to generate dynamically,
-- just as you would otherwise need to generate 'in (1,2,3,4)'
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
UNION ALL SELECT 5
) x
LEFT JOIN YourTable t
ON t.id = x.id
If the id does not exist in the table you're selecting from, you'll need to LEFT JOIN against a list of every id you want returned - this way, it will return the null values for ones that don't exist and the true values for those that do.
I would suggest creating a numbers table that is a single-columned table filled with numbers:
CREATE TABLE `numbers` (
id int(11) unsigned NOT NULL
);
And then inserting a large amount of numbers, starting at 1 and going up to what you think the highest id you'll ever see plus a thousand or so. Maybe go from 1 to 1000000 to be on the safe side. Regardless, you just need to make sure it's more-than-high enough to cover any possible id you'll run into.
After that, your query can look like:
SELECT n.id, a.*
FROM
`numbers` n
LEFT JOIN table t
ON t.id = n.id
WHERE n.id IN (1,2,3,4);
This solution will allow for a dynamically growing list of ids without the need for a sub-query with a list of unions; though, the other solutions provided will equally work for a small known list too (and could also be dynamically generated).
just a quick question:
i have to have one single query that has multiple rows - some rows are identicle - and the order of rows must be preserved in the result -
some idea of what im refering to:
SELECT id,date
FROM items
WHERE id IN (1,2,1,3)
ORDER BY id=1 DESC,id=2 DESC,id=1 DESC,id=3 DESC;
unfortunately mysql result is this:
1,2,3
not 1,2,1,3
it removes the duplicate which i have to have in my result to display in multiple panels on the same webpage -
i really dont want to loop thru each id one by one to get them the way i want to display -
is there a way to actually have one single query that will preserve the order and pull out rows based on request whether its unique or not -
Your query as it stands will never work, because duplicate values in a list of values of an IN clause are ignored. The only way to make this work is by using UNION ALL:
SELECT id, date FROM items where id = 1
UNION ALL
SELECT id, date FROM items where id = 2
UNION ALL
SELECT id, date FROM items where id = 1
UNION ALL
SELECT id, date FROM items where id = 3;
But to be frank, I suspect your data model so far past screwed it's unusable.
try
SELECT
id,
date
FROM items
WHERE id IN (1,2,1,3)
ORDER BY FIND_IN_SET(id, '1,2,1,3')
Another scrupulous way to answer a suspicious question:
SELECT
items.id,
items.date
FROM
items
JOIN
( SELECT 1 AS id, 1 AS ordering
UNION ALL
SELECT 2, 2
UNION ALL
SELECT 1, 3
UNION ALL
SELECT 3, 4
) AS auxilary
ON
auxilary.id = items.id
ORDER BY
auxilary.ordering
Another approach (untested, but should give you the idea):
CREATE TEMPORARY TABLE tt (id INT, ai int unsigned auto_increment primary key);
INSERT INTO tt (id) VALUES (1), (2), (1), (3);
SELECT
id,
date
FROM items JOIN tt USING (id)
ORDER BY tt.ai;
keeps the given order.
If you want to include the records with id=1 and the order doesn't matter as long as you get them, you can split your query into two queries, one for (1,2,3) union all the other query for id=1 or just do:
... In (1,2)
Union all
... In (1,3)
Example:
Select * from
(Select case id when 1 then 1 when 2 then 2 as pseudocol, othercolumns
From table where Id in (1,2)
Union all
Select case id when 1 then 3 when 3 then 4 as pseudocol, othercolumns
From table where Id in (1,3)) t order by pseudocol
Instead of doing what you are trying to, just select the unique rows you need. In the frontend code, store each unique row once in a key=>value structure, where key is the item ID and value is whatever data you need about that item.
Once you have that you can use frontend logic to output them in the desired order including duplicates. This will reduce the amount of redundant data you are trying to select.
For example This is not usable code - exact syntax required depends on your scripting language
-- setup a display order
displayOrder= [1,2,1,3];
-- select data from database, order doesn't matter here
SELECT id,date
FROM items
WHERE id IN (displayOrder);
-- cache the results in a key=> value array
arrCachedRows = {};
for (.... each db row returned ...) {
arrCachedRows[id] = date;
}
-- Now output in desired order
for (listIndex in displayOrder) {
-- Make sure the index is cached
if (listIndex exists in arrCachedRow) {
echo arrCachedRows[listIndex ];
}
}
If you must persist in using UNION despite my warnings
If you go against the above recommendation and absolutely MUST have them back in 1 query in that order then add on an additional row which will enforce the row order. See below query where I use variable #subIndex to add an incrementing value as subIndex. This in turn lets you reorder by that and it'll be in the requested order.
SELECT
i.*
FROM (
SELECT #subIndex:=#subIndex+1 AS subIndex, id, date FROM items where id = 1
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, id, date FROM items where id = 2
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, id, date FROM items where id = 1
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, id, date FROM items where id = 3
) AS i,(SELECT #subIndex:=0) v
ORDER BY i.subIndex
Or a slightly cleaner version that keeps item selection until the outside and hides the subindex
SELECT
items.*
FROM items
-- initialise variable
INNER JOIN (SELECT #subIndex:=0) v
-- create a meta-table with the ids desired in the order desired
INNER JOIN (
SELECT #subIndex:=#subIndex+1 AS subIndex, 1 AS id
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, 2 AS id
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, 1 AS id
UNION
SELECT #subIndex:=#subIndex+1 AS subIndex, 3 AS id
) AS i
ON i.id = items.id
-- order by the subindex from i
ORDER BY i.`subIndex` ASC