MySQL subquery

Sample database

download ：https://download.csdn.net/download/weiguang102/80930478

MySQL Subquery write complex query statements and explain related subquery concepts

MySQL A subquery is nested within another query ( Such as SELECT,INSERT,UPDATE or DELETE) Query in . in addition ,MySQL A subquery can be nested in another subquery .

MySQL Subqueries are called internal queries , Queries that contain subqueries are called external queries . Subqueries can be used anywhere expressions are used , And must be closed in parentheses .

The following query is returned in the United States (USA) Employees working in the office .

SELECT lastName, firstName
FROM employees
WHERE officeCode IN (
	SELECT officeCode
	FROM offices
	WHERE country = 'USA'
);

In this case ：

The subquery returns all office codes for offices located in the United States .

The external query selects the last name and first name of the employee working in the office whose office code is in the result set returned by the sub query .

MySQL Subquery in WHERE clause

We will use the example payments Table .

payments The table structure of the table is as follows

MySQL Subquery and comparison operator

You can use the comparison operator , for example =,>,< Compare the single value returned by the subquery with WHERE Clause .

for example , The following query returns the customer with the maximum payment amount .

SELECT customerNumber, checkNumber, amount
FROM payments
WHERE amount = (
	SELECT MAX(amount)
	FROM payments
);

Except for equality operators , You can also use greater than (>), Less than (<) And so on .

for example , You can use a subquery to find customers whose payments are greater than the average . First , Use subqueries to calculate usage AVG Average payment for aggregate function . then , In an external query , Query the payment that is greater than the average payment returned by the sub query . Refer to the following query statement

SELECT customerNumber, checkNumber, amount
FROM payments
WHERE amount > (
	SELECT AVG(amount)
	FROM payments
);

have IN and NOT IN The operator of the MySQL Subquery

If a subquery returns multiple values , Can be in WHERE Used in clauses IN or NOT IN Operator and other operators .

View the... Of the following customer and order forms ER chart

for example , You can use the NOT IN Operator to find customers who have not placed any orders , As shown below ：

SELECT customerName
FROM customers
WHERE customerNumber NOT IN (
	SELECT DISTINCT customerNumber
	FROM orders
);

FROM In Clause MySQL Subquery

stay FROM When using subqueries in clause , The result set returned from the subquery will be used as a temporary table . This table is called a derived table or materialized subquery .

The following sub query will find the largest... In the order table , Minimum and mean ：

SELECT 
	MAX(items), 
	MIN(items), 
	FLOOR(AVG(items))
FROM (
	SELECT orderNumber, COUNT(orderNumber) AS items
	FROM orderdetails
	GROUP BY orderNumber
) lineitems;

MySQL Subqueries use subqueries of data in external queries

In the previous example , Notice that a subquery is independent . This means that you can execute subqueries as standalone queries , for example ：

SELECT 
    orderNumber, 
    COUNT(orderNumber) AS items
FROM
    orderdetails
GROUP BY orderNumber;

Unlike independent subqueries , Related subqueries are subqueries that use data from external queries .
let me put it another way , The associated subquery depends on the external query .
Evaluate the relevant sub query for each row in the external query .

In the following query , We query and select products whose purchase price is higher than the average purchase price of products in each product line .

SELECT productname, buyprice
FROM products p1
WHERE buyprice > (
	SELECT AVG(buyprice)
	FROM products
	WHERE productline = p1.productline
);

For each line of product line that changes , Each product line performs internal queries .

therefore , The average purchase price will also change . The external query only filters the products whose purchase price is greater than the average purchase price of each product line in the sub query .

MySQL Subquery and EXISTS and NOT EXISTS

When subquery and EXISTS or NOT EXISTS When operators are used together , The subquery returns a Boolean value of TRUE or FALSE Value .

The following query describes the relationship with EXISTS Subqueries used with the ：

SELECT 
    *
FROM
    table_name
WHERE EXISTS(subquery);

In the query above , If the subquery (subquery) There are any rows returned , be EXISTS Subquery returns TRUE, Otherwise return to FALSE.

Usually used in related subqueries EXISTS and NOT EXISTS.

Let's take a look at an example orders and orderDetails surface ：

The total amount of the following query selections is greater than 60000 Sales orders for .

SELECT orderNumber, SUM(priceEach * quantityOrdered) AS total
FROM orderdetails
	INNER JOIN orders USING (orderNumber)
GROUP BY orderNumber
HAVING SUM(priceEach * quantityOrdered) > 60000;

As shown above , return 3 Row data , This means that there are 3 The total amount of sales orders is greater than 60000.

You can use the above query as a related subquery , By using EXISTS Operator to find at least one total greater than 60000 Customer information of the sales order ：

SELECT customerNumber, customerName
FROM customers
WHERE EXISTS (
	SELECT orderNumber, SUM(priceEach * quantityOrdered)
	FROM orderdetails
		INNER JOIN orders USING (orderNumber)
	WHERE customerNumber = customers.customerNumber
	GROUP BY orderNumber
	HAVING SUM(priceEach * quantityOrdered) > 60000
);

Execute the above query , The following results are obtained :