group-by

问题 I am new to SQL and after writing some queries I wanted to understand how SQL "internally" processes the queries. I take one query from another post in stackoverflow: select name from contacts group by name having count(*) > 1 My question is: group by name merges all rows with the same name into one row, how does then count know how many rows with the same name were merged. I am trying to split all steps in the processing of the query in order to understand how it is exactly working, but in

问题 I have table like this: d group 1 a 2 b 3 a 4 c 5 f and I like to iterate over values of d and count number of rows that have group=a . Here is what I am doing now, but It does not work: for index,row in df.iterrows(): for x in (1,5): if row['d'] > x: row['tp'] = df.groupby('group').agg(lambda x:x.manual_type=='a') Can anybody help? 回答1: try: df['group'].value_counts()['a'] in general, you should NEVER use for loops in pandas. it's inefficient and usually recreating some existing

问题 I'm looking for some help in writing an Oracle SQL statement to accomplish the following ... Let's say I have the following data: YEAR | PLACE 1984 | somewhere 1983 | work 1985 | somewhere 1982 | home 1984 | work 1983 | home 1984 | somewhere How can I get a result that keeps all the PLACE column values together and orders it by the YEAR column ... so the result I'm looking for is: YEAR | PLACE 1982 | home 1983 | home 1983 | work 1984 | work 1984 | somewhere 1984 | somewhere 1985 | somewhere

问题 I have this MySql query : SELECT forum_categories.title, forum_messages.author, forum_messages.date AS last_message FROM forum_categories JOIN forum_topics ON forum_topics.category_id=forum_categories.id JOIN forum_messages ON forum_messages.topic_id=forum_topics.id WHERE forum_categories.id=6 ORDER BY forum_categories.date ASC And the output is the follow : Welcome daniel 2010-07-09 22:14:49 Welcome daniel 2010-06-29 22:14:49 Welcome luke 2010-08-10 20:12:20 Welcome skywalker 2010-08-19 22

问题 I have the following database table Here is my sample data I have for it. What I am trying to figure out how to do is how to write a query to select all apntoken for userid='20' grouping by deviceid and then by apntoken as well (except that it should show the most recent apntoken). Some queries I have tried are this. SELECT DISTINCT apntoken,deviceid,created FROM `distribution_mobiletokens` WHERE userid='20' GROUP BY deviceid This returns the following result. Notice the date is not the

问题 I have a table in Oracle database, which have 40 columns. I know that if I want to do a group by query, all the columns in select must be in group by. I simply just want to do: select col1, col2, col3, col4, col5 from table group by col3 If I try: select col1, col2, col3, col4, col5 from table group by col1, col2, col3, col4, col5 It does not give the required output. I have searched this, but did not find any solution. All the queries that I found using some kind of Add() or count(*)

问题 I have an online survey for my users, and every time user answers a survey, I would capture their details in a "survey_stats" table like this - | id | user_id | survey_id | key | value | |---------|---------|-----------|--------------|---------| | 1 | 10 | 99 | gender | male | | 2 | 10 | 99 | age | 32 | | 3 | 10 | 99 | relationship | married | | 4 | 11 | 99 | gender | female | | 5 | 11 | 99 | age | 27 | | 6 | 11 | 99 | relationship | single | In other words, when a user answers a survey, I

问题 I am trying to use Spark (Scala) dataframes to do groupby aggregates for mode and the corresponding count. For example, Suppose we have the following dataframe: Category Color Number Letter 1 Red 4 A 1 Yellow Null B 3 Green 8 C 2 Blue Null A 1 Green 9 A 3 Green 8 B 3 Yellow Null C 2 Blue 9 B 3 Blue 8 B 1 Blue Null Null 1 Red 7 C 2 Green Null C 1 Yellow 7 Null 3 Red Null B Now we want to group by Category, then Color, and then find the size of the grouping, count of number non-nulls, the total

问题 Below is the data in TestingTable1 sorted by date in descending order always BUYER_ID | ITEM_ID | CREATED_TIME ----------+-----------------+---------------------- 1345653 110909316904 2012-07-09 21:29:06 1345653 151851771618 2012-07-09 19:57:33 1345653 221065796761 2012-07-09 19:31:48 1345653 400307563710 2012-07-09 18:57:33 And if this is the below data in TestingTable2 sorted by date in descending order always USER_ID | PRODUCT_ID | LAST_TIME ---------+----------------+---------------------

问题 I want to write a query using Postgres and PostGIS. I'm also using Rails with rgeo , rgeo-activerecord and activerecord-postgis-adapter , but the Rails stuff is rather unimportant. The table structure: measurement - int id - int anchor_id - Point groundtruth - data (not important for the query) Example data: id | anchor_id | groundtruth | data ----------------------------------- 1 | 1 | POINT(1 4) | ... 2 | 3 | POINT(1 4) | ... 3 | 2 | POINT(1 4) | ... 4 | 3 | POINT(1 4) | ... ---------------