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How do sub queries work?
A subquery is a query within another query. The outer query is called as main query and inner query is called as subquery. The subquery generally executes first, and its output is used to complete the query condition for the main or outer query. Subquery must be enclosed in parentheses.
What is an example of a subquery?
A subquery can be used anywhere an expression can be used in the SELECT clause. The following example finds the salaries of all employees, their average salary, and the difference between the salary of each employee and the average salary.
How many subqueries can write in SOQL?
A subquery that involves external objects can fetch up to 1,000 rows of data. Each SOQL query can have up to 4 joins across external objects and other types of objects.
How to execute a subquery in a query?
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. You can use a subquery in many places such as:
When to use a subquery with not in operator?
For example, you can use a subquery with NOT IN operator to find the customers who have not placed any orders as follows: When you use a subquery in the FROM clause, the result set returned from a subquery is used as a temporary table. This table is referred to as a derived table or materialized subquery.
What to do with a subquery introduced with exists?
The select list of a subquery introduced with EXISTS, by convention, has an asterisk (*) instead of a single column name. The rules for a subquery introduced with EXISTS are the same as those for a standard select list, because a subquery introduced with EXISTS creates an existence test and returns TRUE or FALSE, instead of data.
What is the difference between subquery and main in SQL?
The outer query is called as main query and inner query is called as subquery. The subquery generally executes first, and its output is used to complete the query condition for the main or outer query. Subquery must be enclosed in parentheses. Subqueries are on the right side of the comparison operator.