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How to get top n rows per each group?
When we have a rank assigned to each city within its country, we can retrieve the required range: In Oracle, SQL Server and PostgreSQL you can achieve the same functionality using ROW_NUMBER function: This query works in Oracle, SQL Server and PostgreSQL without any changes and returns:
When to optimize for n rows in agilist?
OPTIMIZE FOR n ROWS and FETCH FIRST n ROWS ONLY have no impact on operations which require a sort, like ORDER BY, GROUP BY, DISTINCT, UNION, and merge join. For ORDER BY, however, it does make it more likely that an index will be used, even one with a low cluster ratio, to avoid the sort if n is small (1 or 12 for example).
When to use fetch first n rows only?
FETCH FIRST n ROWS ONLY clause is used for fetching a limited number of rows. In some applications, you execute queries that can return a large number of rows, but you need only a small subset of those rows. Retrieving the entire result table from the query can be inefficient.
When to ignore the optimize for n rows clause?
If the query causes DB2 to gather the whole result set before returning the first row, DB2 ignores the OPTIMIZE FOR n ROWS clause, as in the following situations: The query uses SELECT DISTINCT or a set function distinct, such as COUNT (DISTINCT C1). Either GROUP BY or ORDER BY is used, and no index can give the necessary ordering.
How to get groupnumber of two groups in MySQL?
This works with two groups, if you have more than two groups, then you would need to specify the groupnumber and add queries for each group: ( select * from mytable where `group` = 1 order by age desc LIMIT 2 ) UNION ALL ( select * from mytable where `group` = 2 order by age desc LIMIT 2 )
How to get the rank of each row?
For the first row @current_country is NULL, so rank is also set to 1. For correct ranking, we need to have ORDER BY country, population DESC So if we just execute the subquery: We get the list of cities ranked by population within the country: When we have a rank assigned to each city within its country, we can retrieve the required range:
How to get the results of two groups in SQL?
98 Here is one way to do this, using UNION ALL(See SQL Fiddle with Demo). This works with two groups, if you have more than two groups, then you would need to specify the groupnumber and add queries for each group: