Q&A

How do you use a subquery?

How do you use a subquery?

Subqueries can be used with SELECT, UPDATE, INSERT, DELETE statements along with expression operator. It could be equality operator or comparison operator such as =, >, =, <= and Like operator. A subquery is a query within another query. The outer query is called as main query and inner query is called as subquery.

Can a subquery be used in a WHERE clause?

A subquery in a WHERE clause can be used to qualify a column against a set of rows. For example, the following subquery returns the department numbers for departments on the third floor. The outer query retrieves the names of employees who work on the third floor.

How do you structure a subquery?

Subqueries must be enclosed within parentheses. A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns. An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY.

WHERE can you place a subquery?

A subquery is usually added within the WHERE Clause of another SQL SELECT statement. You can use the comparison operators, such as >, <, or =. The comparison operator can also be a multiple-row operator, such as IN, ANY, or ALL.

Can we use JOIN IN subquery?

A subquery can be used with JOIN operation. The temporary table from the subquery is given an alias so that we can refer to it in the outer select statement. Note that the left and right table of the join keyword must both return a common key that can be used for the join.

What is subquery explain with example?

A subquery is a query that is nested inside a SELECT , INSERT , UPDATE , or DELETE statement, or inside another subquery. In this example a subquery is used as a column expression named MaxUnitPrice in a SELECT statement.

Which clauses is mandatory in a subquery?

“SELECT” clause is mandatorily used in a sub-query .

Can subqueries contain group by and order by clauses?

Subqueries cannot contain GROUP BY and ORDER BY clauses. Subqueries can contain ORDER BY but not the GROUP BY clause.

What is the correct subquery?

Answer: A. A subquery is a complete query nested in the SELECT, FROM, HAVING, or WHERE clause of another query. The subquery must be enclosed in parentheses and have a SELECT and a FROM clause, at a minimum. A single-row subquery can return a maximum of one value.

How do you avoid subquery in select statement?

Change the EXISTS statement to a JOIN statement to avoid nested subqueries and reduce the execution time from 1.93 seconds to 1 millisecond.

Is subquery faster than join?

The advantage of a join includes that it executes faster. The retrieval time of the query using joins almost always will be faster than that of a subquery. By using joins, you can maximize the calculation burden on the database i.e., instead of multiple queries using one join query.

Why inner join is faster?

Well, in general INNER JOIN will be faster because it only returns the rows matched in all joined tables based on the joined column. So even though they both return the same number of rows, INNER JOIN is still faster.

Which is the first option in creating subqueries in LINQ?

Which leads to your first option in creating subqueries in LINQ. LINQ doesn’t execute your query until you “realize” it. You can realize a query by calling some method on it (ToList is a good example) or by working with individual objects in the result (by using a foreach loop to process each object in the query’s result).

Can you do joins and subqueries in LINQ?

You can always do joins in Linq, so there is no real limitation. Conceptually, yes, it might be sometimes more understandable to use subqueries, but from the actual underlying syntax (which really is SQL) they are the same.

How is a subquery run against a group?

This subquery is run against each new group that is created by the outer query. Note that in this particular example the final output is not a group, but a flat sequence of anonymous types.

How does LINQ execute the result of a query?

LINQ doesn’t execute your query until you “realize” it. You can realize a query by calling some method on it (ToList is a good example) or by working with individual objects in the result (by using a foreach loop to process each object in the query’s result). This means that, like SQL, you can assemble complex LINQ queries out of simpler queries.