How to use having in sql

HAVING clause is used to filter aggregate results. Its usage is: used after GROUP BY clause. Specify criteria to filter aggregated results. Use aggregate functions like SUM, AVG and grouping columns. It is often used to filter aggregated results that meet specific conditions, find groups that meet conditions, or further segment aggregated results. The difference with the WHERE clause: The HAVING clause acts after the grouping operation, while the WHERE clause acts before the grouping, and the filtering conditions are different for the aggregated results and the underlying data.

Usage of HAVING clause in SQL

HAVING clause is used to filter a set of aggregate results . It is used after the GROUP BY clause to filter the conditions of the aggregate results.

Grammar:

SELECT 聚合函数(列)
FROM 表名
GROUP BY 分组列
HAVING 条件；

Usage:

• Condition: In HAVING clause You can specify one or more conditions to filter the aggregated results.
• Aggregation functions: Aggregation functions that can be used in the HAVING clause include SUM, COUNT, AVG, MAX, MIN, etc.
• Group columns: The columns in the HAVING clause condition must be the columns grouped in the GROUP BY clause.

Example:

SELECT COUNT(*) AS 订单数
FROM 订单表
GROUP BY 客户编号
HAVING COUNT(*) > 10;

This query counts the number of orders placed by each customer and only displays customers with orders greater than 10.

Use:

The HAVING clause is often used in the following scenarios:

• Filter aggregate results that exist under specific conditions.
• Find groups that meet specific criteria.
• Further segment the aggregation results. The difference between

and the WHERE clause:

The main difference between the HAVING clause and the WHERE clause is:

• Action time: The HAVING clause is executed after the grouping operation, while the WHERE clause is executed before the grouping operation.
• Filter conditions: HAVING clause is used to filter aggregate results, while WHERE clause is used to filter basic data.

Note:

• The HAVING clause can only be used with the GROUP BY clause.
• The conditions in the HAVING clause must use aggregate functions or grouping columns.

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