23.MySQL中的HAVING(分组过滤)

1.准备

CREATE DATABASE mahaiwuji;

USE mahaiwuji;

CREATE TABLE student (
    sid INT (4) PRIMARY KEY,
    sname VARCHAR (36),
    course VARCHAR (36),
    score INT,
    grade INT (4)
) ENGINE = INNODB DEFAULT CHARSET = utf8;

INSERT INTO student VALUES (1,'aa','语文',60,1);
INSERT INTO student VALUES (2,'aa','数学',70,1);
INSERT INTO student VALUES (3,'aa','英语',80,1);

INSERT INTO student VALUES (4,'bb','语文',70,1);
INSERT INTO student VALUES (5,'bb','数学',60,1);
INSERT INTO student VALUES (6,'bb','英语',60,1);

INSERT INTO student VALUES (7,'cc','语文',90,2);
INSERT INTO student VALUES (8,'cc','数学',50,2);
INSERT INTO student VALUES (9,'cc','英语',60,2);

INSERT INTO student VALUES (10,'dd','语文',70,2);
INSERT INTO student VALUES (11,'dd','数学',60,2);
INSERT INTO student VALUES (12,'dd','英语',90,2);

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2.实例演示

功能：根据条件表达式对分组后的内容进行过滤，因此使用having时通常都会先使用group by。

-- 查询平均分大于68分的人
SELECT sname,AVG(score) FROM student GROUP BY sname HAVING AVG(score)>68;

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3.HAVING和WHERE的异同

相同

都是对数据过滤，只保留有效的数据。

不同

WHERE是对原始的记录过滤，HAVING是对分组之后的记录过滤；
WHERE必须的写在HAVING的前面，顺序不可颠倒否则运行出错。