MySQL之复杂查询的实现
1.排序
ORDER BY 子句来设定哪个字段哪种方式来进行排序，再返回搜索结果。
desc：降序
select * from blog order by balance desc;
asc：升序，默认，可不写
select * from blog order by balance asc;
多字段排序
update blog set age = 25 where age < 25;
先根据年龄升序，再根据余额降序
select * from blog order by age asc, balance desc;
2.分组
GROUP BY 语句根据一个或多个列对结果集进行分组。
新建员工表
CREATE TABLE `employee` (
`id` int NOT NULL,
`name` varchar(20) NOT NULL DEFAULT ”,
`identity` varchar(20) NOT NULL DEFAULT ”,
`signin` tinyint NOT NULL DEFAULT ‘0’ COMMENT ‘打卡次数’,
`date` date NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
`id` int NOT NULL,
`name` varchar(20) NOT NULL DEFAULT ”,
`identity` varchar(20) NOT NULL DEFAULT ”,
`signin` tinyint NOT NULL DEFAULT ‘0’ COMMENT ‘打卡次数’,
`date` date NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
插入数据
INSERT INTO `employee` VALUES
(‘1’, ‘小明’, ‘firemen’, 3, ‘2022-02-21’),
(‘2’, ‘小王’, ‘doctor’, 3, ‘2022-02-21’),
(‘3’, ‘小丽’, ‘police’, 3, ‘2022-02-21’),
(‘4’, ‘小王’, ‘doctor’, 2, ‘2022-02-22’),
(‘5’, ‘小明’, ‘firemen’, 1, ‘2022-02-22’),
(‘6’, ‘小丽’, ‘police’, 3, ‘2022-02-22’),
(‘7’, ‘小明’, ‘firemen’, 2, ‘2022-02-23’),
(‘8’, ‘小王’, ‘doctor’, 2, ‘2022-02-23’),
(‘9’, ‘小红’, ‘nurse’, 3, ‘2022-02-23’);
(‘1’, ‘小明’, ‘firemen’, 3, ‘2022-02-21’),
(‘2’, ‘小王’, ‘doctor’, 3, ‘2022-02-21’),
(‘3’, ‘小丽’, ‘police’, 3, ‘2022-02-21’),
(‘4’, ‘小王’, ‘doctor’, 2, ‘2022-02-22’),
(‘5’, ‘小明’, ‘firemen’, 1, ‘2022-02-22’),
(‘6’, ‘小丽’, ‘police’, 3, ‘2022-02-22’),
(‘7’, ‘小明’, ‘firemen’, 2, ‘2022-02-23’),
(‘8’, ‘小王’, ‘doctor’, 2, ‘2022-02-23’),
(‘9’, ‘小红’, ‘nurse’, 3, ‘2022-02-23’);
统计每人打卡记录数
SELECT name, COUNT(*) FROM employee GROUP BY name;
WITH ROLLUP 可以实现在分组统计数据基础上再进行相同的统计（SUM,AVG,COUNT…）
统计每人打卡总数
SELECT name, SUM(signin) as signin_count FROM employee GROUP BY name WITH ROLLUP;
其中记录 NULL 表示所有人的登录次数。
使用 coalesce 来设置一个可以取代 NUll 的名称
coalesce 语法：select coalesce(a,b,c);
SELECT name, SUM(signin) as signin_count FROM employee GROUP BY name WITH ROLLUP;
3.联合查询
UNION 操作符用于连接两个以上的 SELECT 语句的结果组合到一个结果集合中。
UNION ALL：返回所有结果集，包含重复数据。
select author from blog
UNION ALL
select identity from employee;
UNION ALL
select identity from employee;
报错：Illegal mix of collations for operation ‘UNION’
原因：相同字段的编码不一致造成的
解决：修改blog表的author字段
alter table blog modify `author` varchar(40) COLLATE utf8mb4_general_ci NOT NULL DEFAULT ”;
UNION DISTINCT: 删除结果集中重复的数据。默认，可不写
select author from blog
UNION
select identity from employee;
UNION
select identity from employee;
4.多表连接
INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。
LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。
内连接
INNER 可以省略
select b.author, b.age, b.title, e.name, e.signin from blog b
INNER JOIN employee e on b.author = e.identity;
INNER JOIN employee e on b.author = e.identity;
where条件实现内连接效果
select b.author, b.age, b.title, e.name, e.signin from blog b,employee e
where b.author = e.identity;
where b.author = e.identity;
左连接：读取左边数据表的全部数据，即便右边表无对应数据。
select b.author, b.age, b.title, e.name, e.signin from blog b
LEFT JOIN employee e on b.author = e.identity;
LEFT JOIN employee e on b.author = e.identity;
右连接：读取右边数据表的全部数据，即便左边边表无对应数据。
select b.author, b.age, b.title, e.name, e.signin from blog b
RIGHT JOIN employee e on b.author = e.identity;
RIGHT JOIN employee e on b.author = e.identity;
