I’m doing some research into databases and I’m looking at some limitations of relational DBs. I’m getting that joins of large tables is very expensive, but I’m not completely sure why. What does the DBMS need to do to execute a join operation, where is the bottleneck? How can denormalization help to overcome this expense? … Read more
Suppose I have a table of customers and a table of purchases. Each purchase belongs to one customer. I want to get a list of all customers along with their last purchase in one SELECT statement. What is the best practice? Any advice on building indexes? Please use these table/column names in your answer: customer: … Read more
Is there any efficiency difference in an explicit vs implicit inner join? For example: SELECT * FROM table a INNER JOIN table b ON a.id = b.id; vs. SELECT a.*, b.* FROM table a, table b WHERE a.id = b.id; 12 Answers 12 Performance wise, they are exactly the same (at least in SQL Server). … Read more
I’m writing a LINQ to SQL statement, and I’m after the standard syntax for a normal inner join with an ON clause in C#. How do you represent the following in LINQ to SQL: select DealerContact.* from Dealer inner join DealerContact on Dealer.DealerID = DealerContact.DealerID 19 Answers 19 It goes something like: from t1 in … Read more
I asked a question and got this reply which helped. UPDATE TABLE_A a JOIN TABLE_B b ON a.join_col = b.join_col AND a.column_a = b.column_b SET a.column_c = a.column_c + 1 Now I am looking to do this if there are three tables involved something like this. UPDATE tableC c JOIN tableB b JOIN tableA a … Read more
Scenario: Let’s say I have two tables, TableA and TableB. TableB’s primary key is a single column (BId), and is a foreign key column in TableA. In my situation, I want to remove all rows in TableA that are linked with specific rows in TableB: Can I do that through joins? Delete all rows that … Read more
I’m having trouble with a query written in LINQ and Lambda. So far, I’m getting a lot of errors here’s my code: int id = 1; var query = database.Posts.Join(database.Post_Metas, post => database.Posts.Where(x => x.ID == id), meta => database.Post_Metas.Where(x => x.Post_ID == id), (post, meta) => new { Post = post, Meta = meta … Read more
How do I perform the SQL Join equivalent in MongoDB? For example say you have two collections (users and comments) and I want to pull all the comments with pid=444 along with the user info for each. comments { uid:12345, pid:444, comment=”blah” } { uid:12345, pid:888, comment=”asdf” } { uid:99999, pid:444, comment=”qwer” } users { … Read more
This question already has answers here: What is the difference between “INNER JOIN” and “OUTER JOIN”? (27 answers) Closed 6 months ago. I am wondering how to differentiate all these different joins … 9 s 9 Simple Example: Let’s say you have a Students table, and a Lockers table. In SQL, the first table you … Read more
How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause? Correct problem: For inner join is easy and I have a solution like this List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key select … Read more
