distinct

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 由 翻译 强力驱动 问题: The question is pretty much in the title: Is there an efficient way to count the distinct values in every column in a DataFrame? The describe method provides only the count but not the distinct count, and I wonder if there is a a way to get the distinct count for all (or some selected) columns. 回答1: Multiple aggregations would be quite expensive to compute, thus I'd advise you to use approximation distinct count: val df = Seq (( 1 , 3 , 4 ),( 1 , 2 , 3 ),( 2 , 3 , 4 ),( 2 , 3 , 5 )). toDF ( "col1" , "col2" , "col3" ) val exprs = df

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have received a SQL query that makes use of the distinct keyword. When I tried running the query it took at least a minute to join two tables with hundreds of thousands of records and actually return something. I then took out the distinct and it came back in 0.2 seconds. Does the distinct keyword really make things that bad? EDIT: here's the query SELECT Distinct c.username, o.orderno, o.totalcredits, o.totalrefunds, o.recstatus, o.reason from management.contacts c join management.orders o on (c.custID = o.custID) where o.recDate > to

I have a table: abc_test with columns n_num, k_str. This query doesnt work: select distinct(n_num) from abc_test order by(k_str) But this one works: select n_num from abc_test order by(k_str) How do DISTINCT and ORDER BY keywords work internally that output of both the queries is changed? Abhishek Bhandari As far as i understood from your question . distinct :- means select a distinct(all selected values should be unique). order By :- simply means to order the selected rows as per your requirement . The problem in your first query is For example : I have a table ID name 01 a 02 b 03 c 04 d 04

I have two MySQL tables (product and price history) that I would like to join: Product table: Id = int Name = varchar Manufacturer = varchar UPC = varchar Date_added = datetime Price_h table: Id = int Product_id = int Price = int Date = datetime I can perform a simple LEFT JOIN: SELECT Product.UPC, Product.Name, Price_h.Price, Price_h.Date FROM Product LEFT JOIN Price_h ON Product.Id = Price_h.Product_id; But as expected if I have more than one entry for a product in the price history table, I get one result for each historical price. How can a structure a join that will only return one