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

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

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

问题 While writing in SQL, how would I know if I should use cross product (cross join, Cartesian product) or natural join? 回答1: CROSS JOIN creates all possible pairings of rows from two tables, whether they match or not. You don't use any join condition for a cross product, because the condition would always be true for any pairing. An example of using CROSS JOIN : you have tables of ShoeColors and ShoeSizes, and you want to know how many possible combinations there are. SELECT COUNT(*) FROM

问题 So I am fairly new to SQL and currently I am stuck with the concept of Natural Join. Currently I understand that the Natural Join operator joins tables by matching all columns with the same names and discarding duplicate columns and discarding rows which do not match. So recently I came across a question, pretty basic really, however i couldn't wrap my head around it. So there are 2 relations R(A,B,C) and S(A,B,D) A B C A B D 6 8 7 5 8 7 6 6 7 6 6 7 7 8 6 6 8 6 How many rows would the

问题 I have a query that I've been using in phpMyAdmin and it's been working perfectly however I migrated my database over to different server and I am now using SQL*Plus to perform my queries. The query is now generating ERROR at line 10: ORA-25155: column used in NATURAL join cannot have qualifier Here is my query: SELECT Block FROM ( SELECT CardId, Block FROM Contains GROUP BY Block UNION SELECT CardId, Block FROM Contains NATURAL JOIN (SELECT CardId FROM Costs NATURAL JOIN (SELECT Id FROM Card

问题 I've been trying to understand the concept of sql joins fully, venn diagrams have helped me a lot to do that. I've found them for all kind of joins but not for natural joins. How would a venn diagram for a natural join look like? 回答1: Venn diagrams are not very helpful for understanding natural join or inner join. Most Venn diagrams associated with joins on Stack Overflow are parroted worthless misrepresentations--even in cases where a Venn diagram could be useful. Here are some valid uses of

问题 I am trying to understand what will be the result of performing a natural join between two relations R and S, where they have no common attributes. By following the below definition, I thought the answer might be an empty set: Natural Join definition. My line of thought was because the condition in the 'Select' symbol is not met, the projection of all of the attributes won't take place. When I asked my lecturer about this, he said that the output will be the same as doing a cartezian product

问题 Disclaimer: I'm a developer and not a DBA. I've been a huge fan of the USING clause in Oracle since I accidentally stumbled upon it and have used it in place of the old-fashioned ON clause to join fact tables with dimension tables ever since. To me, it creates a much more succinct SQL and produces a more concise result set with no unnecessary duplicated columns. However, I was asked yesterday by a colleague to convert all my USING clauses into ONs. I will check with him and ask him what his

问题 Suppose I perform A natural join B, where: A's schema is: (aID), where (aID) is a primary key. B's schema is: (aID,bID), where (aID, bID) is a composite primary key. Would performing the natural join work in this case? Or is it necessary for A to have both aID and bID for this to work? 回答1: NATURAL JOIN returns rows with one copy each of the common input table column names and one copy each of the column names that are unique to an input table. It returns a table with all such rows that can