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When to use a subquery in a SELECT statement?
You can use a subquery in the FROM clause of the SELECT statement as follows: In this syntax, the table alias is mandatory because all tables in the FROM clause must have a name. Note that the subquery specified in the FROM clause is called a derived table in MySQL or inline view in Oracle.
Can a table subquery be flattened in SQL?
In many cases, table subqueries can be “flattened.” That is replaced by equivalent joins; however, there are times, where a subquery in the FROM clause shines. Here is a couple of examples I thought of.
Is the SELECT query always enclosed in parentheses?
The SELECT query of a subquery is always enclosed in parentheses. It cannot include a COMPUTE or FOR BROWSE clause, and may only include an ORDER BY clause when a TOP clause is also specified. A subquery can be nested inside the WHERE or HAVING clause of an outer SELECT, INSERT, UPDATE, or DELETE statement,…
When to use a subquery in a derived table?
You can also join two derived tables together! This can come in handy when you need to aggregate data from two separate tables and combine them together. In the following example we’re going to do a comparison of Territory Sales to Territory Sales Quotas.
Which is the subquery of the outer query?
The query that contains the subquery is called an outer query or an outer select. To execute the query, first, the database system has to execute the subquery and substitute the subquery between the parentheses with its result – a number of department id located at the location 1700 – and then executes the outer query.
How is the greater than operator used in subquery?
In this example, first, the subquery returns the average salary of all employees. Then, the outer query uses the greater than operator to find all employees whose salaries are greater than the average. The EXISTS operator checks for the existence of rows returned from the subquery. It returns true if the subquery contains any rows.