Introduction to MySQL HAVING clause

The HAVING is use along with the GROUP BY clause to filter the group based on a specified condition.

SELECT 
  column1, 
  column2, 
  column3, 
  aggregate_function(column4)
FROM 
  table_name 
WHERE 
  condition_statement
GROUP BY
 column1, column2, column3
HAVING
  group_conditions;

• Note that the HAVING clause only becomes operational after the the GROUP BY returns a value, it is the value return by the GROUP BY that the HAVING filter according to the instruction. The HAVING clause applies the condition to groups of rows, while the WHERE clause applies the condition to individual rows.
• Note that if you omit the GROUP BY clause in a syntax, the HAVING clause will behaves like a WHERE clause.

MySQL evaluates the HAVING clause after the FROM, WHERE, SELECT and GROUP BYclauses, but before DISTINCT, SELECT, 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

MySQL HAVING COUNT

In MySQL, when you need to group rows and then filter those groups based on an aggregate calculation (like counting the number of rows in each group), you use the combination of the GROUP BY clause with an aggregate function (such as COUNT()) and then filter the resulting groups with the HAVING clause. The key difference between WHERE and HAVING is that WHERE filters rows before grouping, while HAVING filters groups after aggregation.

How It Works

Grouping Data

• GROUP BY: This clause arranges your rows into groups based on one or more columns. For example, if you have a table of sales, you can group the data by product name so that each group contains all sales for a specific product.

Aggregating Data

• COUNT() Function: This function tallies the number of rows within each group. When combined with GROUP BY, it lets you see how many entries exist in each group.

Filtering Groups

• HAVING Clause: After the data is grouped and the aggregate functions (like COUNT()) are calculated, the HAVINGclause is used to filter out groups that do not meet a certain condition. For example, you might want to display only those groups where the count is greater than a certain threshold.

Important Note:

In MySQL, you cannot use an alias defined in the SELECT clause (e.g., an alias for COUNT(c2)) within the HAVING clause because the HAVING clause is executed before the SELECT clause is processed. This means that the alias isn’t recognized when MySQL evaluates the HAVING condition.

Key Takeaways

• GROUP BY and Aggregation: The GROUP BY clause organizes rows into groups, and functions like COUNT()calculate aggregate values for each group.
• HAVING vs. WHERE: The HAVING clause filters groups after aggregation, unlike the WHERE clause which filters individual rows before grouping.
• Alias Limitation: You cannot use an alias from the SELECT clause in the HAVING clause because of the order in which MySQL processes the query.

This detailed explanation and rewritten tutorial should help you understand how to use the HAVING clause along with the COUNT function to filter groups in MySQL.