MySQL HAVING

 MySQL HAVING

Summary: in this tutorial, you will learn how to use MySQL HAVING clause to specify a filter condition for groups of rows or aggregates.

Introduction to MySQL HAVING clause

The  HAVING a clause is used in the SELECT statement to specify filter conditions for a group of rows or aggregates.

The HAVING a clause is often used with the GROUP BY clause to filter groups based on a specified condition. If the GROUP BY a clause is omitted, the HAVING clause behaves like the WHERE clause.

The following illustrates the syntax of the HAVING clause:

SELECT 
    select_list
FROM 
    table_name
WHERE 
    search_condition
GROUP BY 
    group_by_expression
HAVING 
    group_condition;

In this syntax, you specify a condition in the HAVING clause. If a row, which is generated by the group by clause, causes the group_condition to evaluate to true, the query will include it in the result set.

Notice that the HAVING clause applies a filter condition to each group of rows, while the WHERE clause applies the filter condition to each individual row.

MySQL evaluates the HAVING clause after the FROM, WHERE, SELECT and GROUP BY clauses and before ORDER BY, and LIMIT clauses.

Note that the SQL standard specifies that the HAVING is evaluated before SELECT clause and after GROUP BY clause.

MySQL HAVING clause examples

Let’s take some examples of using the  HAVING clause to see how it works. We’ll use the orderdetails the table in the sample database for the demonstration.

The following uses the GROUP BY clause to get order numbers, the number of items sold per order, and total sales for each from the orderdetails table:

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY ordernumber;

Now, you can find which order has total sales greater than 1000 by using the HAVING clause as follows:

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY 
   ordernumber
HAVING 
   total > 1000;

You can construct a complex condition in the HAVING clause using logical operators such as OR and AND.

The following example uses the HAVING clause to find orders that have total amounts greater than 1000 and contain more than 600 items:

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY ordernumber
HAVING 
    total > 1000 AND 
    itemsCount > 600;

Suppose that you want to find all orders that are in shipped status and have a total amount greater than 1500, you can join the orderdetails table with the orders table using the INNER JOIN clause and apply a condition on status column and total aggregate as shown in the following query:

SELECT 
    a.ordernumber, 
    status, 
    SUM(priceeach*quantityOrdered) total
FROM
    orderdetails a
INNER JOIN orders b 
    ON b.ordernumber = a.ordernumber
GROUP BY  
    ordernumber, 
    status
HAVING 
    status = 'Shipped' AND 
    total > 1500;

The HAVING a clause is only useful when you use it with the GROUP BY clause to generate the output of the high-level reports. For example, you can use the HAVING clause to answer questions like finding the number of orders this month, this quarter, or this year that have total sales greater than 10K.

In this tutorial, you have learned how to use the MySQL HAVING clause with the GROUP BY clause to specify filter conditions for groups of rows or aggregates.join