![]() ![]() What are the pseudocolumns in SQL? Give some examples?Ī pseudocolumn is a function which returns a system generated value. NOT NULL is the only constraint that works at the column level. Which is the only constraint that enforces rules at column level? ![]() No, because ALTER is a DDL command and Oracle server performs an automatic COMMIT when the DDL statements are executed. Can we perform a rollback after using ALTER command? Here it’ll be an error because we can’t perform a DML operation on a view. What is the result of the following command? DROP VIEW view_name ON Customers.CustomerID=Orders.CustomerID SELECT Customers.CustomerName, Orders.OrderID As well as if there are rows in the right table that do not have matches in the left table, those will be included. So if there are rows in the left table that do not have matches in the right table, those will be included. SELECT * FROM A x RIGHT JOIN B y ON y.aId = x.IdĪ full join returns all rows for which there is a match in either of the tables. Rows in the right table that have no match in the left table will have null values for left table columns. Opposite of a left join, this will return all rows from the right table even where there is no match in the left table. SELECT * FROM A x LEFT JOIN B y ON y.aId = x.IdĪ right join returns all rows from the right table, and the matched rows from the left table. The rows from the left table with no match in the right table will have null for right table values. Rows in the left table will be returned even if there was no match in the right table. SELECT * FROM A x JOIN B y ON y.aId = x.IdĪ left join returns all rows from the left table, and the matched rows from the right table. It returns all rows in which there is at least one match in both tables. This is the default type of join if no join is specified. Common SQL Interview Questions What is an inner join in SQL? Please see the manual for your database manager and have fun trying different options yourself. I hope this at least gives you enough to get started. select Candidate, Election_year, sum(Total_$), count(*)Īs with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide. ![]() Ordering this data set in a descending (DESC) order places the candidates with the largest number of contributions at the top of the list. This SQL statement answers the question: “which candidates recieved the largest number of contributions (not $ amount, but count (*)) in 2016, but only those who had more than 80 contributions?” This data is from the campaign contributions data we’ve been using in some of these guides. The Having clause is like the above Where clause, except that it acts on the grouped data. Group By gives us a way to combine rows and aggregate data. The default sort order is ascending (ASC), but to sort in the opposite order (descending) you use DESC, as in the example below: select studentID, FullName, sat_score Here is the same list as above, but sorted by the students Full Name. Order By gives us a way to sort the result set by one or more of the items in the SELECT section. The following example is similar, but it further specifies that if any of the students have certain SAT scores (1000, 1400), they will not be presented: select studentID, FullName, sat_score, recordUpdated
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