SELECT 语句中的 HAVING 子句是否“无用”？

Question

我正在阅读一篇名为 Query Optimization Techniques - Tips For Writing 的论文高效快速 SQL 查询.

该文档表明 HAVING 子句在 SELECT 语句中“无用”：

Tip #2: Avoid including a HAVING clause in SELECT statements

The HAVING clause is used to filter the rows after all the rows are selected and it is used like a filter. It is quite useless in a SELECT statement. It works by going through the final result table of the query parsing out the rows that don’t meet the HAVING condition.

Example:

Original query:
SELECT s.cust_id,count(s.cust_id)
FROM SH.sales s
GROUP BY s.cust_id
HAVING s.cust_id != '1660' AND s.cust_id != '2';
Improved query:
SELECT s.cust_id,count(cust_id)
FROM SH.sales s
WHERE s.cust_id != '1660'
AND s.cust_id !='2'
GROUP BY s.cust_id;

问题：

这个说法正确吗？ HAVING 子句在 SELECT 语句中没有用途吗？

Answer 1

如果它没用，它就不会存在。

Use the HAVING clause to restrict the groups of returned rows to those groups for which the specified condition is TRUE

根据您的示例，您可以将其用作示例

SELECT s.cust_id,count(cust_id)
FROM SH.sales s
WHERE s.cust_id != '1660'
AND s.cust_id !='2'
GROUP BY s.cust_id
HAVING count(cust_id) > 5;       --> here

它的目的是什么？您不能将 WHERE 与聚合函数一起使用，例如这是无效的：

FROM ...
WHERE count(cust_id) > 5    --> this
AND ...

SELECT 语句中的 HAVING 子句是否“无用”？

Is the HAVING clause “useless” in a SELECT statement?

sql

oracle

query-optimization

having