A correlated subquery is where the outer query depends on data from the inner query
Correlated subqueries are used for row-by-row processing. Each subquery is executed once for every row of the outer
query. A correlated subquery is evaluated once for each row processed by the parent statement. The parent statement can be a SELECT, UPDATE, or DELETE statement. Show SELECT column1, column2, .... FROM table1 outer WHERE column1 operator (SELECT column1, column2 FROM table2 WHERE expr1 = outer.expr2); A correlated subquery is one way of reading every row in a table and comparing values in each row against related data. It is used whenever a subquery must return a different result or set of results for each candidate row considered by the main query. In other words, you can use a correlated subquery to answer a multipart question whose answer depends on the value in each row processed by the parent statement. Nested Subqueries Versus Correlated Subqueries :With a normal nested subquery, the inner SELECT query runs first and executes once, returning values to be used by the main query. A correlated subquery, however, executes once for
each candidate row considered by the outer query. In other words, the inner query is driven by the outer query. SELECT last_name, salary, department_id FROM employees outer WHERE salary > (SELECT AVG(salary) FROM employees WHERE department_id = outer.department_id); Other use of correlation are in UPDATE and DELETE CORRELATED UPDATE :UPDATE table1 alias1 SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column); Use a correlated subquery to update rows in one table based on rows from another table. CORRELATED DELETE :DELETE FROM table1 alias1 WHERE column1 operator (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column); Use a correlated subquery to delete rows in one table based on the rows from another table. Using the EXISTS Operator :The EXISTS operator tests for existence of rows in the results set of the subquery. If a subquery row value is found the condition is flagged TRUE and the search does not continue in the inner query, and if it is not found then the condition is flagged FALSE and the
search continues in the inner query. SELECT employee_id, last_name, job_id, department_id FROM employees outer WHERE EXISTS ( SELECT ’X’ FROM employees WHERE manager_id = outer.employee_id); OUTPUT : EXAMPLE of using NOT EXIST operator : SELECT department_id, department_name FROM departments d WHERE NOT EXISTS (SELECT ’X’ FROM employees WHERE department_id = d.department_id); OUTPUT
: 13.2.13.7 Correlated SubqueriesA correlated subquery is a subquery that contains a reference to a table that also appears in the outer query. For example:
Notice that the subquery contains a reference to a column of Suppose that table Scoping rule: MySQL evaluates from inside to outside. For example:
In this
statement, Beginning with MySQL 8.0.24, the optimizer can transform a correlated scalar subquery to a derived table when the
To avoid materializing several times for a given derived table, we can instead materialize—once—a derived table which adds a grouping on the join column from the table referenced in the inner query (
In the rewritten query, This type of transformation can be performed, provided the following conditions are met:
See also Section 13.2.13.8, “Derived Tables”. How does a correlated subquery relate to its outer query?The subquery is correlated because the number that it produces depends on main. ship_date, a value that the outer SELECT produces. Thus, the subquery must be re-executed for every row that the outer query considers. The query uses the COUNT function to return a value to the main query.
What is meant by a correlated subquery?In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. Because the subquery may be evaluated once for each row processed by the outer query, it can be slow.
What is true about correlated subquery?Correlated subqueries are used for row-by-row processing. Each subquery is executed once for every row of the outer query. A correlated subquery is evaluated once for each row processed by the parent statement. The parent statement can be a SELECT, UPDATE, or DELETE statement.
What is a correlated subquery Mcq?Answer: Option B. Explanation: Correlated subquery references a column in the outer query and executes the subquery once for every row in the outer query while Uncorrelated subquery executes the subquery first and passes the value to the outer query.
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