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What are the restriction of subquery in SQL Server?
Subqueries cannot manipulate their results internally, that is, a subquery cannot include the order by clause, the compute clause, or the into keyword. Correlated (repeating) subqueries are not allowed in the select clause of an updatable cursor defined by declare cursor. There is a limit of 50 nesting levels.
What is the requirement for using a subquery in the select clause?
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.
When to run sub query in SQL Server?
Here if the table products contains more than one rows for departmentID = 1 then sub-query is run for each of the time when departmentID = 1. This is procedural way of thinking and is wrong in case of SQL because SQL is a declarative language. You should not think iteratively for each records.
The SQL optimizer determines the best approach for running the query. That said, some optimizers are smarter than others. And in some databases, a correlated subquery will always result in its being run once per row in the outer query. (And even this behavior is significantly mitigated by proper indexing.)
How are subqueries nested in a statement in SQL Server?
SQL Server implicitly qualifies the column in the subquery with the table name in the outer query. A subquery can itself include one or more subqueries. Any number of subqueries can be nested in a statement. The following query finds the names of employees who are also sales persons. Here is the result set.
Is there a performance difference between subquery and join in SQL?
In Transact-SQL, there is usually no performance difference between a statement that includes a subquery and a semantically equivalent version that does not. For architectural information on how SQL Server processes queries, see SQL statement processing .However, in some cases where existence must be checked, a join yields better performance.