mysql 分组查询和过滤

1.分组

GROUP BY 子句将表中数据分成若干小组

语法格式

select column, group_function(column)
    from table
    [where condition]
    [group by group_by_expression]
    [order by column];

2.使用举例

DROP TABLE IF EXISTS `bonus`;
CREATE TABLE `bonus` (
`ENAME` varchar(10) DEFAULT NULL,
`JOB` varchar(9) DEFAULT NULL,
`SAL` int(11) DEFAULT NULL,
`COMM` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of bonus
-- ----------------------------

-- ----------------------------
-- Table structure for `dept`
-- ----------------------------
DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
`DEPTNO` int(2) NOT NULL,
`DNAME` varchar(14) DEFAULT NULL,
`LOC` varchar(13) DEFAULT NULL,
PRIMARY KEY (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of dept
-- ----------------------------
INSERT INTO `dept` VALUES ('10', 'ACCOUNTING', 'NEW YORK');
INSERT INTO `dept` VALUES ('20', 'RESEARCH', 'DALLAS');
INSERT INTO `dept` VALUES ('30', 'SALES', 'CHICAGO');
INSERT INTO `dept` VALUES ('40', 'OPERATIONS', 'BOSTON');

-- ----------------------------
-- Table structure for `emp`
-- ----------------------------
DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (
`EMPNO` int(4) NOT NULL,
`ENAME` varchar(10) DEFAULT NULL,
`JOB` varchar(9) DEFAULT NULL,
`MGR` int(4) DEFAULT NULL,
`HIREDATE` date DEFAULT NULL,
`SAL` int(7) DEFAULT NULL,
`COMM` int(7) DEFAULT NULL,
`DEPTNO` int(2) DEFAULT NULL,
PRIMARY KEY (`EMPNO`),
KEY `FK_DEPTNO` (`DEPTNO`),
CONSTRAINT `FK_DEPTNO` FOREIGN KEY (`DEPTNO`) REFERENCES `dept` (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of emp
-- ----------------------------
INSERT INTO `emp` VALUES ('7369', 'SMITH', 'CLERK', '7902', '1980-12-17', '800', null, '20');
INSERT INTO `emp` VALUES ('7499', 'ALLEN', 'SALESMAN', '7698', '1981-02-20', '1600', '300', '30');
INSERT INTO `emp` VALUES ('7521', 'WARD', 'SALESMAN', '7698', '1981-02-22', '1250', '500', '30');
INSERT INTO `emp` VALUES ('7566', 'JONES', 'MANAGER', '7839', '1981-04-02', '2975', null, '20');
INSERT INTO `emp` VALUES ('7654', 'MARTIN', 'SALESMAN', '7698', '1981-09-28', '1250', '1400', '30');
INSERT INTO `emp` VALUES ('7698', 'BLAKE', 'MANAGER', '7839', '1981-05-01', '2850', null, '30');
INSERT INTO `emp` VALUES ('7782', 'CLARK', 'MANAGER', '7839', '1981-06-09', '2450', null, '10');
INSERT INTO `emp` VALUES ('7788', 'SCOTT', 'ANALYST', '7566', '1987-04-19', '3000', null, '20');
INSERT INTO `emp` VALUES ('7839', 'KING', 'PRESIDENT', null, '1981-11-17', '5000', null, '10');
INSERT INTO `emp` VALUES ('7844', 'TURNER', 'SALESMAN', '7698', '1981-09-08', '1500', '0', '30');
INSERT INTO `emp` VALUES ('7876', 'ADAMS', 'CLERK', '7788', '1987-05-23', '1100', null, '20');
INSERT INTO `emp` VALUES ('7900', 'JAMES', 'CLERK', '7698', '1981-12-03', '950', null, '30');
INSERT INTO `emp` VALUES ('7902', 'FORD', 'ANALYST', '7566', '1981-12-03', '3000', null, '20');
INSERT INTO `emp` VALUES ('7934', 'MILLER', 'CLERK', '7782', '1982-01-23', '1300', null, '10');

-- ----------------------------
-- Table structure for `salgrade`
-- ----------------------------
DROP TABLE IF EXISTS `salgrade`;
CREATE TABLE `salgrade` (
`GRADE` int(11) DEFAULT NULL,
`LOSAL` int(11) DEFAULT NULL,
`HISAL` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of salgrade
-- ----------------------------
INSERT INTO `salgrade` VALUES ('1', '700', '1200');
INSERT INTO `salgrade` VALUES ('2', '1201', '1400');
INSERT INTO `salgrade` VALUES ('3', '1401', '2000');
INSERT INTO `salgrade` VALUES ('4', '2001', '3000');
INSERT INTO `salgrade` VALUES ('5', '3001', '9999');

select deptno, avg(sal) from emp group by deptno;
select  deptno, job, count(*),avg(sal) from emp group by deptno, job；

3.注意事项

• 出现在SELECT列表中的字段，如果不是包含在多行函数中，那么该字段必须同时在GROUP BY子句中出现

  错误：select ename,job,sum(sal) from emp group by job;

• 包含在GROUP BY子句中的字段则不必须出现在SELECT列表中。

• 如果没有GROUP BY子句，SELECT列表中不允许出现字段（单行函数）与多行函数混用的情况

  •select empno, sal from emp; //合法
    •select avg(sal) from emp; //合法
    •select empno, avg(sal) from emp; //非法

• 不允许在WHERE 子句中使用多行函数

  select deptno, avg(sal) from emp
    where avg(sal) > 2000;   //执行where时尚未执行groupby 及其他
    group by deptno;

4.having过滤

对分组查询的结果进行过滤，要使用having从句

having从句过滤分组后的结果，它只能出现在group by从句之后，而where从句要出现在group by从句之前

where过滤行，having过滤分组

having支持所有where操作符

语法格式

select column, group_function(column)
from table
[where condition]
[group by  group_by_expression]
[having group_condition]
[order by column];

执行过程

from--where -- group by– having– select-- order by

用法举例

• 在emp表中，列出工资最小值小于2000的职位

  思考：select job,min(sal) from emp where min(sal)<=2000
        Select job,min(sal) from emp group by job having min(sal)<2000

• 列出平均工资大于1200元的部门和工作搭配组合

  select deptno, job, avg(sal)  
    from emp
    group by deptno,job  having avg(sal) > 1200  order by deptno,job;