I have a table with some primary IDs inserted.
In anoter post i have already done i was provided half of the answer i requested and i am thankful for this. (MySQL select statement returning results in circle mode)
I tried to accomplish the other half with no luck. What i want to achieve is a select statement that will get me the opposite of the below example.
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table (id SERIAL PRIMARY KEY);
INSERT INTO my_table VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9);
The select statement i was provided with:
SELECT * FROM my_table ORDER BY id > 5 DESC, id;
Returns 6 - 7 - 8 - 9 - 1 - 2 - 3 - 4 - 5
I also need a select statement to return:
5 - 4 - 3 - 2 - 1 - 9 - 8 - 7 - 6
Thank you in advance!
You need conditional sorting:
SELECT * FROM my_table
ORDER BY id < 6 DESC, id DESC;
See the demo.
Results:
| id |
| --- |
| 5 |
| 4 |
| 3 |
| 2 |
| 1 |
| 9 |
| 8 |
| 7 |
| 6 |
From your earlier question, it seems you need ways to find the next and previous item in a database. This assumes
you know the id of the current item
the id is a primary key or other unique value
next gets the next higher id value
previous gets the next lower id value
when you get to either end of the range of id values, you get nothing back... there isn't any next or previous.
Here's how to get the next id.
SELECT id FROM tbl WHERE id > [[[current_id]]] ORDER BY id LIMIT 1
And, similarly, here's how to get the previous value.
SELECT id FROM tbl WHERE id < [[[current_id]]] ORDER BY ID desc LIMIT 1
These queries return just one row, or no rows if there's no next or previous
There's not much to be gained from a conditional ordering scheme when you only need a single value
Related
I have to get result from column which name is generated by the data from another column. I will explain with the following example:
column names in the database:
months | am1 | am2 | am3 | am4 | am5 | am6 | am7 | am8 | am9 | am10 |am11 | am12
I want in my query to get the value from column starting with am + months value
my current query is:
$query = 'select id, iid, contractnumber, concat("am",`months`) as amount from credits where iid > 0';
but this instead of returning the value (71) of the specific am.. column it returns the column name, for example am5
How can I directly access the value of column am5
Thank you for your time !
That is a bad design and you should change it, but if that's what you got, you can use CASE
select id, iid, contractnumber,
case months
when 1 then am1
when 2 then am2
when 3 then am3
when 4 then am4
when 5 then am5
when 6 then am6
when 7 then am7
when 8 then am8
when 9 then am9
when 10 then am10
when 11 then am11
when 12 then am12
end as amount
from credits
where iid > 0
I want to return all rows that have a certain value in a column and have more than 5 instances in which a number is that certain value. For example, I would like to return all rows of the condition in which if the value in the column M has the number 1 in it and there are 5 or more instances of M having the number 1 in it, then it will return all rows with that condition.
select *
from tab
where M = 1
group by id --ID is the primary key of the table
having count(M) > 5;
EDIT: Here is my table:
id | M | price
--------+-------------+-------
1 | | 100
2 | 1 | 50
3 | 1 | 30
4 | 2 | 20
5 | 2 | 10
6 | 3 | 20
7 | 1 | 1
8 | 1 | 1
9 | 1 | 1
10 | 1 | 1
11 | 1 | 1
Originally I just want to insert into a trigger so that if the number of M = 1's is greater than 5, then I want to create an exception. The query I asked for would be inserted into the trigger. END EDIT.
But my table is always empty. Can anyone help me out? Thanks!
Try this :
select *
from tab
where M in (select M from tab where M = 1 group by M having count(id) > 5);
SQL Fiddle Demo
please try
select *,count(M) from table where M=1 group by id having count(M)>5
Since you group on your PK (which seems a futile excercise), you are counting per ID, whicg will indeed always return 1.
As i explain after this code, this query is NOT good, it is NOT the answer, and i also explain WHY. Please do not expect this query to run correctly!
select *
from tab
where M = 1
group by M
having count(*) > 5;
Like this, you group on what you are counting, which makes a lot more sense. At the same time, this will have unexpected behaviour, as you are selecting all kinds of columns that are not in the group by or in any aggregate. I know mySQL is lenient on that, but I don;t even want to know what it will produce.
Try indeed a subquery along these lines:
select *
from tab
where M in
(SELECT M
from tab
group by M
having count(*) > 5)
I've built a SQLFiddle demo (i used 'Test' as table name out of habit) accomplishing this (I don't have a mySQL at hand now to test it).
-- Made up a structure for testing
CREATE TABLE Test (
id INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY(id),
M int
);
SELECT id, M FROM tab
WHERE M IN (
SELECT M
FROM Test
WHERE M = 1
GROUP BY M
HAVING COUNT(M) > 5
)
The sub-query is a common "find the duplicates" kind of query, with the added condition of a specific value for the column M, also stating that there must be at least 5 dupes.
It will spit out a series of values of M which you can use to query the table against, ending with the rows you need.
You shouldn't use SELECT * , it's a bad practice in general: don't retrieve data you aren't actually using, and if you are using it then take the little time needed to type in a list of field, you'll likely see faster querying and on the other hand the code will be way more readable.
I need a single query. Delete all rows from the table except the top N rows. The table has only one column. Like,
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
| etc... |
This column may contain repeated names as well.
Contains repeated names
Only one column.
If you can order your records by friends_name, and if there are no duplicates, you could use this:
DELETE FROM names
WHERE
friends_name NOT IN (
SELECT * FROM (
SELECT friends_name
FROM names
ORDER BY friends_name
LIMIT 10) s
)
Please see fiddle here.
Or you can use this:
DELETE FROM names ORDER BY friends_name DESC
LIMIT total_records-10
where total_records is (SELECT COUNT(*) FROM names), but you have to do this by code, you can't put a count in the LIMIT clause of your query.
If you don't have an id field, i suppose you use an alphabetic order.
MYSQL
DELETE FROM friends
WHERE friends_name
NOT IN (
SELECT * FROM (
SELECT friends_name
FROM friends
ORDER BY friends_name ASC
LIMIT 10) r
)
You delete all rows exept the 10 firsts (alphabetic order)
I just wanted to follow up on this relatively old question because the existing answers don't capture the requirement and/or are incorrect. The question states the names can be repeated, but only the top N must be preserved. Other answers will delete incorrect rows and/or incorrect number of them.
For example, if we have this table:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
| Misbahu |
| Roshit |
| Misbahu |
| Rohan |
And we want to delete all but top 3 rows (N = 3), the expected result would be:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
The DELETE statement from the currently selected answer will result in:
|friends_name|
==============
| Arunji |
| Misbahu |
| Misbahu |
| Misbahu |
See this sqlfiddle. The reason for this is that it first sorts names alphabetically, then takes top 3, then deletes all that don't equal that. But since they are sorted by name they may not be the top 3 we want, and there's no guarantee that we'll end up with only 3.
In the absence of unique indexes and other fields to determine what "top N" means, we go by the order returned by the database. We could be tempted to do something like this (substitute 99999 with however high number):
DELETE FROM names LIMIT 99999 OFFSET 3
But according to MySQL docs, while the DELETE supports the LIMIT clause, it does not support OFFSET. So, doing this in a single query, as requested, does not seem to be possible; we must perform the steps manually.
Solution 1 - temporary table to hold top 3
CREATE TEMPORARY TABLE temp_names LIKE names;
INSERT INTO temp_names SELECT * FROM names LIMIT 3;
DELETE FROM names;
INSERT INTO names SELECT * FROM temp_names;
Here's the sqlfiddle for reference.
Solution 2 - new table with rename
CREATE TABLE new_names LIKE names;
INSERT INTO new_names SELECT * FROM names LIMIT 3;
RENAME TABLE names TO old_names, new_names TO names;
DROP TABLE old_names;
Here's the sqlfiddle for this one.
In either case, we end up with top 3 rows in our original table:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
I have a table with multiple rows which have a same data. I used SELECT DISTINCT to get a unique row and it works fine. But when i use ORDER BY with SELECT DISTINCT it gives me unsorted data.
Can anyone tell me how distinct works?
Based on what criteria it selects the row?
From your comment earlier, the query you are trying to run is
Select distinct id from table where id2 =12312 order by time desc.
As I expected, here is your problem. Your select column and order by column are different. Your output rows are ordered by time, but that order doesn't necessarily need to preserved in the id column. Here is an example.
id | id2 | time
-------------------
1 | 12312 | 34
2 | 12312 | 12
3 | 12312 | 48
If you run
SELECT * FROM table WHERE id2=12312 ORDER BY time DESC
you will get the following result
id | id2 | time
-------------------
2 | 12312 | 12
1 | 12312 | 34
3 | 12312 | 48
Now if you select only the id column from this, you will get
id
--
2
1
3
This is why your results are not sorted.
When you specify SELECT DISTINCT it will give you all the rows, eliminating duplicates from the result set. By "duplicates" I mean rows where all fields have the same values. For example, say you have a table that looks like:
id | num
--------------
1 | 1
2 | 3
3 | 3
SELECT DISTINCT * would return all rows above, whereas SELECT DISTINCT num would return two rows:
num
-----
1
3
Note that which row actual row (eg: whether it's row 2 or row 3) it selects is irrelevant, as the result would be indistinguishable.
Finally, DISTINCT should not affect how ORDER BY works.
Reference: MySQL SELECT statement
The behaviour you describe happens when you ORDER BY an expression that is not present in the SELECT clause. The SQL standard does not allow such a query but MySQL is less strict and allows it.
Let's try an example:
SELECT DISTINCT colum1, column2
FROM table1
WHERE ...
ORDER BY column3
Let's say the content of the table table1 is:
id | column1 | column2 | column3
----+---------+---------+---------
1 | A | B | 1
2 | A | B | 5
3 | X | Y | 3
Without the ORDER BY clause, the above query returns following two records (without ORDER BY the order is not guaranteed):
column1 | column2
---------+---------
A | B
X | Y
But with ORDER BY column3 the order is also not guaranteed.
The DISTINCT clause operates on the values of the expressions present in the SELECT clause. If row #1 is processed first then (A, B) is placed in the result set and it is associated with row #1. Then, when row #2 is processed, the values of the SELECT expressions produce the record (A, B) that is already in the result set. Because of DISTINCT it is dropped. Row #3 produces (X, Y) that is also put in the result set. Then, the ORDER BY column3 clause makes the records be sorted in the result set as (A, B), (X, Y).
But if row #2 is processed before row #1 then, following the same logic exposed in the previous paragraph, the records in the result set are sorted as (X, Y), (A, B).
There is no rule imposed on the database engine about the order it processes the rows when it runs a query. The database is free to process the rows in any order it consider it's better for performance.
Your query is invalid SQL and the fact that it can return different results using the same input data proves it.
I have a table:
+--------+-------------------+-----------+
| ID | Name | Order |
+--------+-------------------+-----------+
| 1 | John | 1 |
| 2 | Mike | 3 |
| 3 | Daniel | 4 |
| 4 | Lisa | 2 |
| 5 | Joe | 5 |
+--------+-------------------+-----------+
The order can be changed by admin hence the order column. On the admin side I have a form with a select box Insert After: to entries to the database. What query should I use to order+1 after the inserted column.
I want to do this in a such way that keeps server load to a minimum because this table has 1200 rows at present. Is this the correct way to save an order of the table or is there a better way?
Any help appreciated
EDIT:
Here's what I want to do, thanks to itsmatt:
want to reorder row number 1 to be after row 1100, you plan to leave 2-1100 the same and then modify 1 to be 1101 and increment 1101-1200
You need to do this in two steps:
UPDATE MyTable
SET `Order` = `Order` + 1
WHERE `Order` > (SELECT `Order`
FROM MyTable
WHERE ID = <insert-after-id>);
...which will shift the order number of every row further down the list than the person you're inserting after.
Then:
INSERT INTO MyTable (Name, `Order`)
VALUES (Name, (SELECT `Order` + 1 FROM MyTable WHERE ID = <insert-after-id>));
To insert the new row (assuming ID is auto increment), with an order number of one more than the person you're inserting after.
Just add the new row in any normal way and let a later SELECT use ORDER BY to sort. 1200 rows is infinitesimally small by MySQL standards. You really don't have to (and don't want to) keep the physical table sorted. Instead, use keys and indexes to access the table in a way that will give you what you want.
you can
insert into tablename (name, `order`)
values( 'name', select `order`+1 from tablename where name='name')
you can also you id=id_val in your inner select.
Hopefully this is what you're after, the question isn't altogether clear.