how to use the MySQL subquery to write complex queries

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A MySQL subquery is a query that is nested inside another query such as SELECT, INSERT, UPDATEor DELETE. In addition, a MySQL subquery can be nested inside another subquery.

A MySQL subquery is also called an inner query while the query that contains the subquery is called an outer query.

You can use a subquery anywhere that you use an expression. In addition, you must enclose a subquery in parentheses.

1��MySQL subquery within a WHERE clause

1.1��MySQL subquery with comparison operators

You can use comparison operators e.g., =, >, <, etc., to compare a single value returned by the subquery with the expression in the WHERE clause.

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

In addition to the equality operator, you can use other comparison operators such as greater than (>), less than(<), etc.

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

1.2��MySQL subquery with IN and NOT IN operators

If a subquery returns more than one value, you can use other operators such as IN or NOT IN operator in the WHERE clause.

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

1.3��MySQL subquery with EXISTS and NOT EXISTS

When a subquery is used with EXISTS or NOT EXISTS operator, a subquery returns a Boolean value of TRUE or FALSE. The subquery acts as an existence check.

SELECT
 customerName
FROM
 customers
WHERE
 EXISTS (
 SELECT
 priceEach * quantityOrdered
 FROM
 orderdetails
 WHERE
 priceEach * quantityOrdered > 10000
 GROUP BY
 orderNumber
 )

2��MySQL subquery in FROM clause

When you use a subquery in the FROM clause, the result set returned from a subquery is used as a table. This table is referred to as a derived table or materialized subquery.

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

3��MySQL correlated subquery

In the previous examples, you notice that the subquery is independent. It means you can execute the subquery as a single query. However, a correlated subquery is a subquery that uses the information from the outer query. In other words, a correlated subquery depends on the outer query. A correlated subquery is evaluated once for each row in the outer query.

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