HAVING 和 WHERE SQL 子句的主要区别是什么？

Question

sqlserver中HAVING和WHERE SQL子句的主要区别，例如，你会如何编写一个查询来分别使用两种方式来识别属于特定部门的员工，如果你知道，请给我留言，我将不胜感激非常喜欢

Answer 1

1. WHERE 子句： WHERE子句用于过滤表中的记录或在连接多个表时使用。只有满足WHERE子句中指定条件的记录才会被提取。它可以与 SELECT、UPDATE、DELETE 语句一起使用。

让我们考虑下表“学生”

Roll_no       S_Name      Age

1                a             17

2                b             20

3                c             21

4                d             18

5                e             20

6                f             17

7                g             21

8                h             17

考虑查询：

从学生中选择 S_Name、年龄 年龄 >=18 岁 上述查询的输出是：

 S_Name      Age

b             20             

c             21             

d             18             

e             20             

g             21  

       

2. HAVING 条款： HAVING 子句用于根据 HAVING 子句中的给定条件过滤组中的记录。满足给定条件的组将出现在最终结果中。 HAVING 子句只能使用 与 SELECT 语句。

让我们考虑上面提到的 Student 表并在其上应用having子句：

选择年龄，COUNT(Roll_No) AS No_of_Students 来自学生按年龄分组 有计数（Roll_No）> 1 上述查询的输出是：

Age     No_of_Students

17              3

20              2

21              2 

SQL 中Where 和Having 子句的区别：

SR.NO.  WHERE Clause    HAVING Clause
1.  WHERE Clause is used to filter the records from the table based on the specified condition. HAVING Clause is used to filter record from the groups based on the specified condition.
2.  WHERE Clause can be used without GROUP BY Clause    HAVING Clause cannot be used without GROUP BY Clause
3.  WHERE Clause implements in row operations   HAVING Clause implements in column operation
4.  WHERE Clause cannot contain aggregate function  HAVING Clause can contain aggregate function
5.  WHERE Clause can be used with SELECT, UPDATE, DELETE statement. HAVING Clause can only be used with SELECT statement.
6.  WHERE Clause is used before GROUP BY Clause HAVING Clause is used after GROUP BY Clause
7.  WHERE Clause is used with single row function like UPPER, LOWER etc.    HAVING Clause is used with multiple row function like SUM, COUNT etc.

来自：https://www.geeksforgeeks.org/difference- Between-where-and-having-clause-in-sql/#