What does having mean in mysql

HAVING clause is used to filter the aggregation results of grouped data. The following example illustrates its usage: after grouping customer sales data, only select customers whose total sales are greater than 10,000. The functions are different: the WHERE clause filters individual rows, and the HAVING clause filters aggregate values. The order of execution is different: WHERE is executed first, HAVING is executed later. Aggregation functions can be used to improve performance and provide more flexible filtering conditions.

HAVING clause in MySQL

What is the HAVING clause?

The HAVING clause is used to filter the results of aggregate functions. It is only used after the GROUP BY clause to perform conditional filtering on grouped data.

Syntax of HAVING clause

<code>SELECT 聚合函数(列名)
FROM 表名
GROUP BY 分组列名
HAVING 条件</code>

Conditions can be comparison operators, logical operators and aggregate functions.

The following example illustrates the use of the HAVING clause:

<code class="sql">SELECT SUM(sales) AS 总销售额
FROM 订单
GROUP BY 客户ID
HAVING 总销售额 > 10000;</code>

This query will select only those whose total sales are greater than 10,000 after grouping the customer sales data customer of.

The difference between HAVING and WHERE

• The functions are different: The WHERE clause filters the conditions of a single row, while The HAVING clause filters the grouped aggregate values.
• The order of execution is different: The WHERE clause is executed before grouping, while the HAVING clause is executed after grouping.
• Usage of aggregate functions: Aggregate functions can be used in the HAVING clause, but not in the WHERE clause.

Advantages of the HAVING clause

• Improving performance:By filtering the data after grouping, the HAVING clause can reduce The amount of data that needs to be calculated and transferred to improve query performance.
• More flexible filtering: The HAVING clause allows complex filtering of grouped data, which may not be possible when using the WHERE clause.
• Reusable: HAVING clause can be reused as needed for the query without modifying the main query.

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