I am new to SQL, so I need your help on a query. Basically I have a Database of ZipCodes and I want to get 3 items before the selected ZipCode and 3 items after. The Query that I came up with is pretty bad...
WITH numberedlogtable AS
(
SELECT *
FROM dbo.US
)
SELECT *
FROM numberedlogtable
WHERE ZipCode IN (SELECT ZipCode+i
FROM numberedlogtable
CROSS JOIN (SELECT -1 AS i UNION ALL SELECT 0 UNION ALL SELECT 1) n
WHERE ZipCode='91803')
I picked up a sample Query from somewhere and successfully converted it for my use. The only problem is that this Query returns current item and the next item. Instead, it should returns previous 3 items, current item, and next three items.
Using a common table expression (the WITH part) producing a numbered sequence:
WITH NumberedZipCodes AS
(SELECT SELECT ROW_NUMBER() OVER (ORDER BY ZipCode) AS RowNumber, *
FROM ZipCodes)
SELECT * From NumberedZipCodes
WHERE RowNumber BETWEEN
(SELECT RowNumber FROM NumberedZipCodes WHERE ZipCode=91803) - 3
AND (SELECT RowNumber FROM NumberedPerson WHERE ZipCode=91803) + 3
Normally in SQL there is no such concept as the previous or next items in a match. Actually, unless an order by clause is specified the rows are returned in any order that the sql engine find suitable. To make a query like this, an order has to be applied and index numbers generated. That's done in NumberedZipCodes. The second part is just a query to get the data out of it.
To have the query run efficiently, make sure that there is an index on the ZipCode column.
"Before" and "after" only have meaning in the context of ordering. Assuming you wish to order by ZIP code, selecting the desired ZIP code and 2 rows after it could be done like this:
SELECT TOP(3) *
FROM numberedlogtable
WHERE ZipCode >= '91803'
ORDER BY ZipCode
Selecting 3 rows before:
SELECT TOP(3) *
FROM numberedlogtable
WHERE ZipCode < '91803'
ORDER BY ZipCode DESC
Put UNION ALL between these two queries to make it one, if that's what you wish.
You can play with it in the SQL Fiddle.
3 items before the selected ZipCode and 3 items afte
SQL is set based, it has no defined order UNLESS YOU DEFINE ONE (with order by).
Now, lets not get into the more complex stuff - FURST you have to create an order asking for the central item, then in a second query you can ask for the other 6. Sorry, no other way. x+3 would be doable with a Top 4 statement and a filter etc. - but the 3 before will definitely require a second query.
All assume you creata query / view that has
* THe zipCodes in a defined order
* a row number once you filter it, used to define (a) the "current line" as well as filter for all with smaller lines.
But you need an order manually first.
Related
I would like to display a table of results. The data is sourced from a SQL query on an Oracle database. I would like to show the results one page (say, 10 records) at a time, minimising the actual data being sent to the front-end.
At the same time, I would like to show the total number of possible results (say, showing 1-10 of 123), and to allow for pagination (say, to calculate that 10 per page, 123 results, therefore 13 pages).
I can get the total number of results with a single count query.
SELECT count(*) AS NUM_RESULTS FROM ... etc.
and I can get the desired subset with another query
SELECT * FROM ... etc. WHERE ? <= ROWNUM AND ROWNUM < ?
But, is there a way to get all the relevant details in one single query?
Update
Actually, the above query using ROWNUM seems to work for 0 - 10, but not for 10 - 20, so how can I do that too?
ROWNUM is a bit tricky to use.
The ROWNUM pseudocolumn always starts with 1 for the first result that actually gets fetched. If you filter for ROWNUM>10, you will never fetch any result and therefore will not get any.
If you want to use it for paging (not that you really should), it requires nested subqueries:
select * from
(select rownum n, x.* from
(select * from mytable order by name) x
)
where n between 3 and 5;
Note that you need another nested subquery to get the order by right; if you put the order by one level higher
select * from
(select rownum n, x.* from mytable x order by name)
where n between 3 and 5;
it will pick 3 random(*) rows and sort them, but that is ususally not what you want.
(*) not really random, but probably not what you expect.
See http://use-the-index-luke.com/sql/partial-results/window-functions for more effient ways to implement pagination.
You can use inner join on your table and fetch total number of result in your subquery. The example of an query is as follows:
SELECT E.emp_name, E.emp_age, E.emp_sal, E.emp_count
FROM EMP as E
INNER JOIN (SELECT emp_name, COUNT(*) As emp_count
FROM EMP GROUP BY emp_name) AS T
ON E.emp_name = T.emp_name WHERE E.emp_age < 35;
Not sure exactly what you're after based on your question wording, but it seems like you want to see your specialized table of all records with a row number between two values, and in an adjacent field in each record see the total count of records. If so, you can try selecting everything from your table and joining a subquery of a COUNT value as a field by saying where 1=1 (i.e. everywhere) tack that field onto the record. Example:
SELECT *
FROM table_name LEFT JOIN (SELECT COUNT(*) AS NUM_RESULTS FROM table_name) ON 1=1
WHERE ? <= ROWNUM AND ROWNUM < ?
My situation is that a SQL statement which is not predictable, is given to the program and I need to do pagination on top of it. The final SQL statement would be similar to the following one:
SELECT * FROM (*Given SQL Statement*) b
OFFSET 0 ROWS FETCH NEXT 50 ROWS ONLY;
The problem here is that the *Given SQL Statement* is unpredictable. It may or may not contain order by clause. I am not able to change the query result of this SQL Statement and I need to do pagination on it.
I searched for solution on the Internet, but all of them suggested to use an arbitrary column, like primary key, in order by clause. But it will change the original order.
The short answer is that it can't be done, or at least can't be done properly.
The problem is that SQL Server (or any RDBMS) does not and can not guarantee the order of the records returned from a query without an order by clause.
This means that you can't use paging on such queries.
Further more, if you use an order by clause on a column that appears multiple times in your resultset, the order of the result set is still not guaranteed inside groups of values in said column - quick example:
;WITH cte (a, b)
AS
(
SELECT 1, 'a'
UNION ALL
SELECT 1, 'b'
UNION ALL
SELECT 2, 'a'
UNION ALL
SELECT 2, 'b'
)
SELECT *
FROM cte
ORDER BY a
Both result sets are valid, and you can't know in advance what will you get:
a b
-----
1 b
1 a
2 b
2 a
a b
-----
1 a
1 b
2 a
2 b
(and of course, you might get other sorts)
The problem here is that the *Given SQL Statement" is unpredictable. It may or may not contain order by clause.
your inner query(unpredictable sql statement) should not contain order by,even if it contains,order is not guaranteed.
To get guaranteed order,you have to order by some column.for the results to be deterministic,the ordered column/columns should be unique
Please note: what I'm about to suggest is probably horribly inefficient and should really only be used to help you go back to the project leader and tell them that pagination of an unordered query should not be done. Having said that...
From your comments you say you are able to change the SQL statement before it is executed.
You could write the results of the original query to a temporary table, adding row count field to be used for subsequent pagination ordering.
Therefore any original ordering is preserved and you can now paginate.
But of course the reason for needing pagination in the first place is to avoid sending large amounts of data to the client application. Although this does prevent that, you will still be copying data to a temp table which, depending on the row size and count, could be very slow.
You also have the problem that the page size is coming from the client as part of the SQL statement. Parsing the statement to pick that out could be tricky.
As other notified using anyway without using a sorted query will not be safe, But as you know about it and search about it, I can suggest using a query like this (But not recommended as a good way)
;with cte as (
select *,
row_number() over (order by (select 0)) rn
from (
-- Your query
) t
)
select *
from cte
where rn between (#pageNumber-1)*#pageSize+1 and #pageNumber*#pageSize
[SQL Fiddle Demo]
I finally found a simple way to do it without any order by on a specific column:
declare #start AS INTEGER = 1, #count AS INTEGER = 5;
select * from (SELECT *,ROW_NUMBER() OVER (ORDER BY (SELECT 1)) AS fakeCounter
FROM (select * from mytable) AS t) AS t2 order by fakeCounter OFFSET #start ROWS
FETCH NEXT #count ROWS ONLY
where select * from mytable can be any query
I need to fetch 4 random values from each category. What should be the correct sql syntax for maria db. I have attached one image of table structure.
Please click here to check the structure
Should i write some procedure or i can do it with basic sql syntax?
You can do that with a SQL statement if you only have a few rows:
SELECT id, question, ... FROM x1 ORDER BY rand() LIMIT 1
This works fine if you have only a few rows - as soon as you have thousands of rows the overhead for sorting the rows becomes important, you have to sort all rows for getting only one row.
A trickier but better solution would be:
SELECT id, question from x1 JOIN (SELECT CEIL(RAND() * (SELECT(MAX(id)) FROM x1)) AS id) as id using(id);
Running EXPLAIN on both SELECTS will show you the difference...
If you need random value for different categories combine the selects via union and add a where clause
http://mysql.rjweb.org/doc.php/groupwise_max#top_n_in_each_group
But then ORDER BY category, RAND(). (Your category is the blog's province.)
Notice how it uses #variables to do the counting.
If you have MariaDB 10.2, then use one of its Windowing functions.
SELECT column FROM table WHERE category_id = XXX
ORDER BY RAND()
LIMIT 4
do it for all categories
I have three columns in one table (code, code alt and product). Code column has duplicate data. I want to leave all results without repeating the code column. I try with this
Select code, code alt, product from table
where code in
(
select code from table
group by code
having count (code)=1
)
but do not appear all results.
Thanks
If you want to leave only one row with the particular code from the bunch of rows with the same code you need to decide which one from multiple rows you want to leave.
You need some criteria by which you will be able to rank rows with the same code and select one from them by higher (for example) rank value. The script below will leave only one - random - row with specific code.
This is just an example that shows you the idea and it intended for SQL Server - because you do not pointed your DBMS
with [src] as (
select code, [code alt], product, rank() over(partition by code order by newid()) [rank]
from [table])
select * from [src] where [rank] = 1
Ranking Functions
I'm using sql-server 2005 and ASP.NET with C#.
I have Users table with
userId(int),
userGender(tinyint),
userAge(tinyint),
userCity(tinyint)
(simplified version of course)
I need to select always two fit to userID I pass to query users of opposite gender, in age range of -5 to +10 years and from the same city.
Important fact is it always must be two, so I created condition if ##rowcount<2 re-select without age and city filters.
Now the problem is that I sometimes have two returned result sets because I use first ##rowcount on a table. If I run the query.
Will it be a problem to use the DataReader object to read from always second result set? Is there any other way to check how many results were selected without performing select with results?
Can you simplify it by using SELECT TOP 2 ?
Update: I would perform both selects all the time, union the results, and then select from them based on an order (using SELECT TOP 2) as the union may have added more than two. Its important that this next select selects the rows in order of importance, ie it prefers rows from your first select.
Alternatively, have the reader logic read the next result-set if there is one and leave the SQL alone.
To avoid getting two separate result sets you can do your first SELECT into a table variable and then do your ##ROWCOUNT check. If >= 2 then just select from the table variable on its own otherwise select the results of the table variable UNION ALLed with the results of the second query.
Edit: There is a slight overhead to using table variables so you'd need to balance whether this was cheaper than Adam's suggestion just to perform the 'UNION' as a matter of routine by looking at the execution stats for both approaches
SET STATISTICS IO ON
Would something along the following lines be of use...
SELECT *
FROM (SELECT 1 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender AND
M1.userAge - 5 >= M2.userAge AND
M1.userAge + 15 <= M2.userAge AND
M1.userCity = M2.userCity
LIMIT TO 2 ROWS
UNION
SELECT 2 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender
LIMIT TO 2 ROWS)
ORDER BY prio
LIMIT TO 2 ROWS;
I haven't tried it as I have no SQL Server and there may be dialect issues.