MySQL的基本查询(二)
3. Update
语法:
UPDATE table_name SET column = expr [, column = expr ...]
[WHERE ...] [ORDER BY ...] [LIMIT ...]
对查询到的结果进行列值更新
案例:
3.1 将孙悟空同学的数学成绩变更为 80 分
:
-- 更新值为具体值
-- 查看原数据
SELECT name, shuxue FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name | shuxue |
+-----------+--------+
| 孙悟空 | 78 |
+-----------+--------+
1 row in set (0.00 sec)
-- 数据更新
UPDATE exam_result SET shuxue = 80 WHERE name = '孙悟空';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
-- 查看更新后数据
SELECT name, shuxue FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name | shuxue |
+-----------+--------+
| 孙悟空 | 80 |
+-----------+--------+
1 row in set (0.00 sec)
3.2 将曹孟德同学的数学成绩变更为 60 分,语文成绩变更为 70 分
:
-- 一次更新多个列
-- 查看原数据
SELECT name, shuxue, yuwen FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
| name | shuxue | yuwen |
+-----------+--------+-------+
| 曹孟德 | 84 | 82 |
+-----------+--------+-------+
1 row in set (0.00 sec)
-- 数据更新
UPDATE exam_result SET shuxue = 60, yuwen = 70 WHERE name = '曹孟德';
Query OK, 1 row affected (0.14 sec)
Rows matched: 1 Changed: 1 Warnings: 0
-- 查看更新后数据
SELECT name, shuxue, yuwen FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
| name | shuxue | yuwen |
+-----------+--------+-------+
| 曹孟德 | 60 | 70 |
+-----------+--------+-------+
1 row in set (0.00 sec)
3.3 将总成绩倒数前三的 3 位同学的数学成绩加上 30 分
:
-- 更新值为原值基础上变更
-- 查看原数据
SELECT name, shuxue, yuwen + shuxue + yingyu 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name | shuxue | 总分 |
+-----------+--------+--------+
| 宋公明 | 65 | 170 |
| 刘玄德 | 85 | 185 |
| 曹孟德 | 60 | 197 |
+-----------+--------+--------+
3 rows in set (0.00 sec)
-- 数据更新,不支持 shuxue += 30 这种语法
UPDATE exam_result SET shuxue = shuxue + 30
ORDER BY yuwen + shuxue + yingyu LIMIT 3;
-- 查看更新后数据
-- 思考:这里还可以按总分升序排序取前 3 个么?
SELECT name, shuxue, yuwen + shuxue + yingyu 总分 FROM exam_result
WHERE name IN ('宋公明', '刘玄德', '曹孟德');
+-----------+--------+--------+
| name | shuxue | 总分 |
+-----------+--------+--------+
| 曹孟德 | 90 | 227 |
| 刘玄德 | 115 | 215 |
| 宋公明 | 95 | 200 |
+-----------+--------+--------+
3 rows in set (0.00 sec)
-- 按总成绩排序后查询结果
SELECT name, shuxue, yuwen + shuxue + yingyu 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name | shuxue | 总分 |
+-----------+--------+--------+
| 宋公明 | 95 | 200 |
| 刘玄德 | 115 | 215 |
| 唐三藏 | 98 | 221 |
+-----------+--------+--------+
3 rows in set (0.00 sec)
3.4 将所有同学的语文成绩更新为原来的 2 倍
:
注意:更新全表的语句慎用!
4. Delete
4.1 删除数据
:
语法:
DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...]
案例:
4.1.1 删除孙悟空同学的考试成绩
:
-- 查看原数据
SELECT * FROM exam_result WHERE name = '孙悟空';
+----+-----------+-------+--------+--------+
| id | name | yuwen | shuxue | yingyu |
+----+-----------+-------+--------+--------+
| 2 | 孙悟空 | 174 | 80 | 77 |
+----+-----------+-------+--------+--------+
1 row in set (0.00 sec)
-- 删除数据
DELETE FROM exam_result WHERE name = '孙悟空';
Query OK, 1 row affected (0.17 sec)
-- 查看删除结果
SELECT * FROM exam_result WHERE name = '孙悟空';
Empty set (0.00 sec)
4.1.2 删除整张表数据
:
注意:删除整表操作要慎用!
-- 准备测试表
CREATE TABLE for_delete (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
Query OK, 0 rows affected (0.16 sec)
-- 插入测试数据
INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0
-- 查看测试数据
SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 3 | C |
+----+------+
3 rows in set (0.00 sec)
-- 删除整表数据
DELETE FROM for_delete;
Query OK, 3 rows affected (0.00 sec)
-- 查看删除结果
SELECT * FROM for_delete;
Empty set (0.00 sec)
-- 再插入一条数据,自增 id 在原值上增长
INSERT INTO for_delete (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)
-- 查看数据
SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 4 | D |
+----+------+
1 row in set (0.00 sec)
-- 查看表结构,会有 AUTO_INCREMENT=n 项
SHOW CREATE TABLE for_delete\G
*************************** 1. row ***************************
Table: for_delete
Create Table: CREATE TABLE `for_delete` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.2 截断表
:
语法:
TRUNCATE [TABLE] table_name
注意:这个操作慎用
- 只能对整表操作,不能像 DELETE 一样针对部分数据操作;
- 实际上 MySQL 不对数据操作,所以比 DELETE 更快
- 会重置 AUTO_INCREMENT 项
-- 准备测试表
CREATE TABLE for_truncate (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
Query OK, 0 rows affected (0.16 sec)
-- 插入测试数据
INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0
-- 查看测试数据
SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 3 | C |
+----+------+
3 rows in set (0.00 sec)
-- 截断整表数据,注意影响行数是 0,所以实际上没有对数据真正操作
TRUNCATE for_truncate;
Query OK, 0 rows affected (0.10 sec)
-- 查看删除结果
SELECT * FROM for_truncate;
Empty set (0.00 sec)
-- 再插入一条数据,自增 id 在重新增长
INSERT INTO for_truncate (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)
-- 查看数据
SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1 | D |
+----+------+
1 row in set (0.00 sec)
-- 查看表结构,会有 AUTO_INCREMENT=2 项
SHOW CREATE TABLE for_truncate\G
*************************** 1. row ***************************
Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
5. 插入查询结果
语法:
INSERT INTO table_name [(column [, column ...])] SELECT ...
案例:删除表中的的重复复记录,重复的数据只能有一份
-- 创建原数据表
CREATE TABLE duplicate_table (id int, name varchar(20));
Query OK, 0 rows affected (0.01 sec)
-- 插入测试数据
INSERT INTO duplicate_table VALUES
(100, 'aaa'),
(100, 'aaa'),
(200, 'bbb'),
(200, 'bbb'),
(200, 'bbb'),
(300, 'ccc');
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0
思路:
-- 创建一张空表 no_duplicate_table,结构和 duplicate_table 一样
CREATE TABLE no_duplicate_table LIKE duplicate_table;
Query OK, 0 rows affected (0.00 sec)
-- 将 duplicate_table 的去重数据插入到 no_duplicate_table
INSERT INTO no_duplicate_table SELECT DISTINCT * FROM duplicate_table;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
-- 通过重命名表,实现原子的去重操作
RENAME TABLE duplicate_table TO old_duplicate_table,
no_duplicate_table TO duplicate_table;
Query OK, 0 rows affected (0.00 sec)
-- 查看最终结果
SELECT * FROM duplicate_table;
+------+------+
| id | name |
+------+------+
| 100 | aaa |
| 200 | bbb |
| 300 | ccc |
+------+------+
3 rows in set (0.00 sec)
6. 聚合函数
案例:
6.1 统计班级共有多少同学
:
-- 使用 * 做统计,不受 NULL 影响
SELECT COUNT(*) FROM students;
+----------+
| COUNT(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
-- 使用表达式做统计
SELECT COUNT(1) FROM students;
+----------+
| COUNT(1) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
6.2 统计班级收集的 qq 号有多少
:
-- NULL 不会计入结果
SELECT COUNT(qq) FROM students;
+-----------+
| COUNT(qq) |
+-----------+
| 1 |
+-----------+
1 row in set (0.00 sec)
6.3 统计本次考试的数学成绩分数个数
:
-- COUNT(shuxue) 统计的是全部成绩
SELECT COUNT(shuxue) FROM exam_result;
+---------------+
| COUNT(shuxue) |
+---------------+
| 6 |
+---------------+
1 row in set (0.00 sec)
-- COUNT(DISTINCT shuxue) 统计的是去重成绩数量
SELECT COUNT(DISTINCT shuxue) FROM exam_result;
+------------------------+
| COUNT(DISTINCT shuxue) |
+------------------------+
| 5 |
+------------------------+
1 row in set (0.00 sec)
6.4 统计数学成绩总分
:
SELECT SUM(shuxue) FROM exam_result;
+-------------+
| SUM(shuxue) |
+-------------+
| 569 |
+-------------+
1 row in set (0.00 sec)
-- 不及格 < 60 的总分,没有结果,返回 NULL
SELECT SUM(shuxue) FROM exam_result WHERE shuxue < 60;
+-------------+
| SUM(shuxue) |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)
6.4 统计平均总分
:
SELECT AVG(yuwen + shuxue + yingyu) 平均总分 FROM exam_result;
+--------------+
| 平均总分 |
+--------------+
| 297.5 |
+--------------+
6.5 返回英语最高分
:
SELECT MAX(yingyu) FROM exam_result;
+-------------+
| MAX(yingyu) |
+-------------+
| 90 |
+-------------+
1 row in set (0.00 sec)
6.6 返回 > 70 分以上的数学最低分
:
SELECT MIN(shuxue) FROM exam_result WHERE shuxue > 70;
+-------------+
| MIN(shuxue) |
+-------------+
| 73 |
+-------------+
1 row in set (0.00 sec)
7. group by子句的使用
在select中使用group by 子句可以对指定列进行分组查询
select column1, column2, .. from table group by column;
案例:
- 准备工作,创建一个雇员信息表
- EMP员工表
- DEPT部门表
- SALGRADE工资等级表
- 如何显示每个部门的平均工资和最高工资???
select deptno,avg(sal),max(sal) from EMP group by deptno;
- 显示每个部门的每种岗位的平均工资和最低工资
select avg(sal),min(sal),job, deptno from EMP group by deptno, job;
- 显示平均工资低于2000的部门和它的平均工资
- 统计各个部门的平均工资
select avg(sal) from EMP group by deptno
- having和group by配合使用,对group by结果进行过滤
select avg(sal) as myavg from EMP group by deptno having myavg<2000;
SQL查询中各个关键字的执行先后顺序 from > on> join > where > group by > with > having >select > distinct > order by > limit