We want to be able to select top N rows using a SQL Query. The target database could be Oracle or MySQL. Is there an elegant approach to this? (Needless to say, we\'re dealing w
To get the top 5 scorers from this table:
CREATE TABLE people
(id int,
name string,
score int)
try this SQL:
SELECT id,
name,
score
FROM people p
WHERE (SELECT COUNT(*)
FROM people p2
WHERE p2.score > p.score
) <=4
I believe this should work in most places.
If there is a unique key on the table yes...
Select * From Table O
Where (Select Count(*) From Table I
Where [UniqueKeyValue] < O.UniqueKeyValue) < N
You can substitute your own criteria if you want the "Top" definition to be based on some other logic than on the unique key...
EDIT: If the "sort" that defines the meaning of "Top" is based on a non-unique column, or set of columns, then you can still use this, but you can't guarantee you will be able to get exactly N records out...
Select * From Table O
Where (Select Count(*) From Table I
Where nonUniqueCol < O.nonUniqueCol) < 10
If records 8, 9, 10, 11, and 12 all have the same value in [nonUniqueCol], then the query will either only generate 7 records, (with '<') ... , or 12 (if you use '<=')
NOTE: As this involves a correlated sub-query, the performance can be an issue for very large tables...
No. The syntax is different.
You may, however, create views:
/* Oracle */
CREATE VIEW v_table
AS
SELECT *
FROM (
SELECT *
FROM table
ORDER BY
column
)
WHERE rownum <= n
/* MySQL */
CREATE VIEW v_table
AS
SELECT *
FROM table
ORDER BY
column
LIMIT n
The big problem, after looking this over, is that MySQL isn't ISO SQL:2003 compliant. If it was, you'd have these handy windowing functions:
SELECT * from
( SELECT
RANK() OVER (ORDER BY <blah>) AS ranking,
<rest of columns here>,
FROM <table>
)
WHERE ranking <= <N>
Alas, MySQL (and others that mimic it's behavior, eg SQLite), do not, hence the whole limiting issue.
Check out this snippet from Wikipedia (http://en.wikipedia.org/wiki/Window_function_(SQL)#Limiting_result_rows)
I don't think that's possible even just between mysql and mssql. I do an option for simulating such behaviour though:
SELECT columns FROM viewname WHERE PagingHelperID BETWEEN startindex AND stopindex
This will make ordering difficult, you will need different views for every order in which you intend to retreive data.
You could also "rewrite" your sql on the fly when querying depending on the database and define your own method for the rewriter, but I don't think there is any "good" way to do this.