aggregate-functions

问题 I have read much in textbooks and browsed a lot of pages on the internet but I can't understand how functions/operators like min, max, count, ... that aggregate over a relation/table or groups of tuples/rows in a relation/table are built with basic operations such as ∪ (union), ∩ (intersection), x (join), - (minus), π (projection), .... Can anyone show me how to express these functions/operators with relational algebra? 回答1: Computing functions in relation algebra are not fully included yet.

问题 I have read much in textbooks and browsed a lot of pages on the internet but I can't understand how functions/operators like min, max, count, ... that aggregate over a relation/table or groups of tuples/rows in a relation/table are built with basic operations such as ∪ (union), ∩ (intersection), x (join), - (minus), π (projection), .... Can anyone show me how to express these functions/operators with relational algebra? 回答1: Computing functions in relation algebra are not fully included yet.

问题 I need help with a SQL that will convert this table: =================== | Id | FK | Status| =================== | 1 | A | 100 | | 2 | A | 101 | | 3 | B | 100 | | 4 | B | 101 | | 5 | C | 100 | | 6 | C | 101 | | 7 | A | 102 | | 8 | A | 102 | | 9 | B | 102 | | 10 | B | 102 | =================== to this: ========================================== | FK | Count 100 | Count 101 | Count 102 | ========================================== | A | 1 | 1 | 2 | | B | 1 | 1 | 2 | | C | 1 | 1 | 0 | ===========

问题 To be more specific, how can i convert a scala.Iterable to a org.apache.spark.rdd.RDD ? I have an RDD of (String, Iterable[(String, Integer)]) and i want this to be converted into an RDD of (String, RDD[String, Integer]) , so that i can apply a reduceByKey function to the internal RDD . e.g i have an RDD where key is 2-lettered prefix of a person's name and the value is List of pairs of Person name and hours that they spent in an event my RDD is : ("To", List(("Tom",50),("Tod","30"),("Tom",70

问题 This question is about the functionality of first_value(), using another function or workaround. It is also about "little gain in performance" in big tables. To use eg. max() in the explained context below, demands spurious comparisons. Even if fast, it imposes some additional cost. This typical query SELECT x, y, count(*) as n FROM t GROUP BY x, y; needs to repeat all columns in GROUP BY to return more than one column. A syntactic sugar to do this, is to use positional references: SELECT x,

问题 Simply put, I have a table with, among other things, a column for timestamps. I want to get the row with the most recent (i.e. greatest value) timestamp. Currently I'm doing this: SELECT * FROM table ORDER BY timestamp DESC LIMIT 1 But I'd much rather do something like this: SELECT * FROM table WHERE timestamp=max(timestamp) However, SQLite rejects this query: SQL error: misuse of aggregate function max() The documentation confirms this behavior (bottom of page): Aggregate functions may only

问题 I have a table that has user_id, item_id and interaction_type as columns. interaction_type could be 0, 1,2,3,4 or 5. However, for some user_id and item_id pairs, we might have multiple interaction_types. For example, we might have: user_id item_id interaction_type 2 3 1 2 3 0 2 3 5 4 1 0 5 4 4 5 4 2 What I want is to only keep the maximum interaction_type if there are multiples. So I want this: user_id item_id interaction_type 2 3 5 4 1 0 5 4 4 Here is the query I wrote for this purpose:

问题 I have a table that has user_id, item_id and interaction_type as columns. interaction_type could be 0, 1,2,3,4 or 5. However, for some user_id and item_id pairs, we might have multiple interaction_types. For example, we might have: user_id item_id interaction_type 2 3 1 2 3 0 2 3 5 4 1 0 5 4 4 5 4 2 What I want is to only keep the maximum interaction_type if there are multiples. So I want this: user_id item_id interaction_type 2 3 5 4 1 0 5 4 4 Here is the query I wrote for this purpose:

问题 I have a table of revenue as title_id revenue cost 1 10 5 2 10 5 3 10 5 4 10 5 1 20 6 2 20 6 3 20 6 4 20 6 when i execute this query SELECT SUM(revenue),SUM(cost) FROM revenue GROUP BY revenue.title_id it produces result title_id revenue cost 1 30 11 2 30 11 3 30 11 4 30 11 which is ok, now i want to combine sum result with another table which has structure like this title_id interest 1 10 2 10 3 10 4 10 1 20 2 20 3 20 4 20 when i execute join with aggregate function like this SELECT SUM

问题 I have a table of revenue as title_id revenue cost 1 10 5 2 10 5 3 10 5 4 10 5 1 20 6 2 20 6 3 20 6 4 20 6 when i execute this query SELECT SUM(revenue),SUM(cost) FROM revenue GROUP BY revenue.title_id it produces result title_id revenue cost 1 30 11 2 30 11 3 30 11 4 30 11 which is ok, now i want to combine sum result with another table which has structure like this title_id interest 1 10 2 10 3 10 4 10 1 20 2 20 3 20 4 20 when i execute join with aggregate function like this SELECT SUM