Correlated subqueries are used for row-by-row processing. Each subquery is executed once for every heat of the external query.
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A correlated subquery is evaluated when for each row processed through the parental statement. The parental statement have the right to be a SELECT, UPDATE, or DELETE statement.SELECT column1, column2, ....FROM table1 outerWHERE column1 operator (SELECT column1, column2 from table2 whereby expr1 = outer.expr2);A associated subquery is one means of reading every row in a table and also comparing worths in every row against related data. The is supplied whenever a subquery have to return a different an outcome or set of results for each candidate row thought about by the key query. In various other words, you can use a associated subquery come answer a multipart question whose answer relies on the worth in each heat processed through the parent statement.
Nested Subqueries Versus associated Subqueries :With a regular nested subquery, the within SELECT questions runs an initial and executes once, returning worths to be offered by the key query. A associated subquery, however, executes when for every candidate row thought about by the outer query. In other words, the inner ask is thrust by the outer query.NOTE : girlfriend can likewise use the ANY and ALL operator in a correlated subquery.EXAMPLE of correlated Subqueries : uncover all the employees that earn more than the typical salary in your department.SELECT last_name, salary, department_id indigenous employees external WHERE value > (SELECT AVG(salary) indigenous employees where department_id = outer.department_id);Other usage of correlation are in UPDATE and DELETE
CORRELATED upgrade :UPDATE table1 alias1 collection column = (SELECT expression indigenous table2 alias2 whereby alias1.column = alias2.column);Use a associated subquery to update rows in one table based on rows from one more table.
CORRELATED DELETE :DELETE indigenous table1 alias1 wherein column1 operator (SELECT expression indigenous table2 alias2 wherein alias1.column = alias2.column);Use a correlated subquery to delete rows in one table based on the rows from another table.
Using the exist Operator :The exist operator tests for presence of rows in the results set of the subquery. If a subquery row value is uncovered the problem is flagged TRUE and the search does not continue in the inner query, and also if that is not discovered then the condition is flagged FALSE and also the search continues in the inside query.
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EXAMPLE of utilizing EXIST operator :Find employees who have actually at least one human being reporting to them.SELECT employee_id, last_name, job_id, department_idFROM employee outerWHERE exist ( select ’X’FROM employeesWHERE manager_id =outer.employee_id);OUTPUT :EXAMPLE of utilizing NOT EXIST operator :Find every departments that execute not have any employees.SELECT department_id, department_nameFROM department dWHERE not EXISTS (SELECT ’X’FROM employeesWHERE department_id= d.department_id);OUTPUT :Attention reader! Don’t stop finding out now. Discover SQL because that interviews usingSQL Course by 2175forals.com.