**Correlated queries** are queries in which the subquery references a column from the outer query. The subquery is evaluated for each row processed by the outer query, taking into account the values from the outer query. This allows the subquery to be correlated or connected to the outer query.



Correlated queries are useful when performing operations based on related data between two or more tables. They can help us retrieve data that depends on the values of the current row being processed in the outer query.

Here's an example of a correlated query in MySQL:

SELECT column1
FROM table1 t1
WHERE column1 > (
    SELECT AVG(column2) 
    FROM table2 t2 
    WHERE t2.foreign_key = t1.key
);


In this example, the subquery `(SELECT AVG(column2) FROM table2 t2 WHERE t2.foreign_key = t1.key)` is correlated to the outer query. It calculates the average value of `column2` from `table2` for each row processed in `table1` based on the matching `foreign_key` values. The subquery result is then used as a condition in the `WHERE` clause of the outer query.


Let's write down a correlated query and test the output below:

#!/usr/bin/env bash

mkdir -p /var/run/mysqld
chmod 777 -R /var/run/mysqld
chown -R mysql:mysql /var/lib/mysql /var/run/mysqld

# check if mysql is running before restarting it

/usr/bin/mysqld_safe &

# wait for mysql to start up
echo "Please wait for 3 seconds ..."
sleep 3
mysql -u root -Bse "DROP DATABASE IF EXISTS MovieIndustry;"
clear
# Launch MySQL
mysql < mysql.sql



-- Use the database MySQL
USE mysql;

-- Create the table 1
CREATE TABLE table1 (
  id INT PRIMARY KEY,
  column1 INT
);

-- Create the table 2
CREATE TABLE table2 (
  id INT PRIMARY KEY,
  column2 INT,
  foreign_key INT,
  FOREIGN KEY (foreign_key) REFERENCES table1(id)
);

-- Insert sample data in table 1
INSERT INTO table1 (id, column1) VALUES (1, 10);
INSERT INTO table1 (id, column1) VALUES (2, 20);
INSERT INTO table1 (id, column1) VALUES (3, 30);

-- Insert sample data in table 2
INSERT INTO table2 (id, column2, foreign_key) VALUES (1, 5, 1);
INSERT INTO table2 (id, column2, foreign_key) VALUES (2, 15, 1);
INSERT INTO table2 (id, column2, foreign_key) VALUES (3, 25, 2);
INSERT INTO table2 (id, column2, foreign_key) VALUES (4, 35, 2);
INSERT INTO table2 (id, column2, foreign_key) VALUES (5, 45, 3);

-- Correlated query
SELECT t1.column1
FROM table1 t1
WHERE t1.column1 < (SELECT AVG(t2.column2) FROM table2 t2 WHERE t2.foreign_key = t1.id);



import React from 'react';
import { shallow, configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() });

import HelloWorld from './app';

var TestResult = function() {
    this.succeeded = false;
    this.reason = "";
    this.input = "";
    this.expected_output = "";
    this.actual_output = "";
}

export const executeTests = function() {

  var results = [];

  result = new TestResult();
  result.input = 'HelloWorld Component';
  result.expected_output = "span containing text 'Hello World'"

  let wrapper = shallow(<HelloWorld />);

  // Call your Challenge function here.

  let type = wrapper.type();
  let testSuccessful = true;
  let failureReason;

  if (type !== 'span') {
    testSuccessful = false;
    failureReason = "You need to render exactly one span HTML element";
  } else if (wrapper.props().children != "Hello World") {
     testSuccessful = false;
     failureReason = "You have rendered wrong message in your span element";
  }

  result.actual_output = wrapper.html();

  if (testSuccessful) {
    result.succeeded = true;
    result.reason = "Succeeded"
  } else {
    result.succeeded = false;
    result.reason = failureReason;
  }

  results.push(result);

  return results;
}


mysql

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Typescript React

Vue.js

## Code explanation


* **Line 2:** Use the database `mysql` to create tables.

* **Lines 5–16:** We have two tables, `table1` and `table2`. `table1` has columns `id` and `column1`, while `table2` has columns `id`, `column2`, and `foreign_key` referencing table1's `id` column. 

* **Lines 19-28:** We insert sample data in both tables.

* **Lines 31-33:** The correlated query selects the `column1` values from `table1` where the value of `column1` is less than the average value of `column2` from `table2` for each corresponding `foreign_key`. This should return some results based on the sample data provided.



mysql.tar.gz

What are the correlated queries in MySQL?

Correlated queries use subqueries referencing outer query columns, evaluated per row for related data retrieval.