join | 易学教程

maximum and minimum number of tuples in natural join

阅读更多关于 maximum and minimum number of tuples in natural join

问题 I came across a question that states Consider the following relation schema pertaining to a students database: Student ( rollno , name, address) Enroll ( rollno, courseno , coursename) where the primary keys are shown underlined. The number of tuples in the Student and Enroll tables are 120 and 8 respectively. What are the maximum and minimum number of tuples that can be present in (Student * Enroll), where '*' denotes natural join ? I have seen several solutions on Internet like this or this

maximum and minimum number of tuples in natural join

阅读更多关于 maximum and minimum number of tuples in natural join

maximum and minimum number of tuples in natural join

阅读更多关于 maximum and minimum number of tuples in natural join

linux线程基本编程

阅读更多关于 linux线程基本编程

索引： 1.创建线程pthread_create 2.等待线程结束pthread_join 3.分离线程pthread_detach 4.创建线程键pthread_key_create 5.删除线程键pthread_key_delete 6.设置线程数据pthread_setspecific 7.获取线程数据pthread_getspecific 8.获取线程标示符pthread_self 9.比较线程pthread_equal 10.一次执行pthread_once 11.出让执行权sched_yield 12.修改优先级pthread_setschedparam 13.获取优先级pthread_getschedparam 14.发送信号pthread_kill 15.设置线程掩码pthread_sigmask 16.终止线程pthread_exit 17.退出线程pthread_cancel 18.允许/禁止退出线程pthread_setcancelstate 19.设置退出类型pthread_setcanceltype 20.创建退出点pthread_testcancel 21.压入善后处理函数 22.弹出善后处理函数 -------------------------------------------------------------------------------

SQL help - exception report

阅读更多关于 SQL help - exception report

问题 I have a company table (CompanyID, CompanyName), Date Table (Datekey int, date, isTradingHoliday 0/1), Fact table (id, datekey, companyID, StockClosePrice) I need an exception report. Expectation is for all the companies, for all trading days, there will be a StockClosePrice. I need to find for which date and for which companies on the trading days I don't have data. So basically, I need help to write a query to find for year 2019 for which days and for which companies I don't have data in

Pandas DataFrame merge, ends up with more rows

阅读更多关于 Pandas DataFrame merge, ends up with more rows

问题 I am doing a_df = a_df.merge(b_df, how='left', on=['col1', col2]) After this, a_df actually has more rows than before the operation. How is this possible? They both have millions of rows, so it's hard for me to narrow down the problem. Probably I am missing something about how left merge works. 回答1: Problem is with duplicates, so instead left join merge return all combination of dupplicates pairs of both DataFrame s, check sample below: a_df = pd.DataFrame({'A':list('abcdef'), 'B':[4,5,4,5,5

Power Query - Merged Query Returning Incorrect Result

阅读更多关于 Power Query - Merged Query Returning Incorrect Result

问题 I have 2 tables that are merged on multiple fields, when I preview the result, it is correct (Rank = 712) as can be seen from this image: However when I expand the resultant table, an incorrect value (Rank = 1) appears: Has anyone ever seen something like this before? 回答1: If you sorted Ranking table,find step:"Sorted Rows",and add Table.Buffer outside Table.Sort,like Table.Buffer(Table.Sort()) ,then it will be correct. 来源： https://stackoverflow.com/questions/50321954/power-query-merged-query

Conditional order by clause

阅读更多关于 Conditional order by clause

问题 Below query returning error. Help me under the error and a way to achieve the conditional order by. I am trying to order by grade and name when grade >=8 and grade and marks when grade <8. SELECT name, grade, marks FROM students, grades WHERE min_mark <= marks AND marks <= max_mark AND marks >= 70 UNION SELECT TO_CHAR('NULL') AS name, grade, marks FROM students, grades WHERE min_mark <= marks AND marks <= max_mark AND marks <= 69 order by grade desc,(case when grade >= 8 then name when grade

ef core 3.0 query with join table from DB and list from memory fails

阅读更多关于 ef core 3.0 query with join table from DB and list from memory fails

问题 I wrote simple query, which should find images with hashes already presented in database. So "images" is List from memory and _context.AdPics is table from DB var picsThatAlreadyInDb = (from pic in _context.AdPics join image in images on pic.ImgHash equals image.ImgHash select new {DbPicId=pic.ID, newImage = image}).ToList(); I get this error: System.InvalidOperationException : Processing of the LINQ expression 'DbSet<AdPic> .Join( outer: __p_0, inner: pic => pic.ImgHash, outerKeySelector:

Getting wrong values after merging two dataframe on datetime

阅读更多关于 Getting wrong values after merging two dataframe on datetime

问题 I want to merge a time serie of % humidity with a range of datetime created as expected, to fill missing records (or rows) with NaN and obtain a time serie based on 15min records (as long as the sensor is designed for). Data of humidity following recorded datetime : humdt = pd.DataFrame(data = data["la-salade"][["datetime","humidite"]]) datetime humidite 0 2019-07-09 08:30:00 87 1 2019-07-09 11:00:00 87 2 2019-07-09 17:30:00 82 3 2019-07-09 23:30:00 80 4 2019-07-11 06:15:00 79 5 2019-07-19 14