The role of having in mysql

HAVING clause is used to filter aggregation results. The specific functions include: filtering grouped data and excluding groups that do not meet the conditions. Filtering nested aggregation based on multiple conditions

The role of the HAVING clause in MySQL

The HAVING clause is used to filter aggregate results, and it is used after the GROUP BY clause. The HAVING clause filters the conditions on the aggregate columns and selects only the aggregate result rows that meet the conditions.

Specific function:

• Filter the grouped data: GROUP BY After grouping, the HAVING clause can be based on the aggregate function The calculation results are further filtered out to groups that meet the conditions.
• Exclude groups that do not meet the conditions: It can exclude groups that do not meet the specified conditions in the aggregation results, thereby obtaining more fine-grained aggregation results.
• Filter based on multiple conditions: HAVING clause can specify multiple conditions at the same time for more complex filtering of grouped data.
• Nested aggregation: The HAVING clause can also nest other aggregate functions to create more complex filtering conditions.

Usage syntax:

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

Example:

<code>SELECT COUNT(*) AS 总数
FROM 订单
GROUP BY 客户ID
HAVING COUNT(*) > 1;</code>

This query counts the order quantity of each customer, And filter out customers whose order quantity is greater than 1. The difference between

and WHERE clause:

• The WHERE clause is used to filter data before grouping, while the HAVING clause is used to filter data after grouping. .
• The WHERE clause filters individual records, while the HAVING clause filters aggregate results.

The above is the detailed content of The role of having in mysql. For more information, please follow other related articles on the PHP Chinese website!