Mysql學習筆記（1） -- Mysql 日常開發中最實用的語句

1. 創建

1.1 創建數據庫

語法：create database db_name

create database `awesome_app`

1.2 創建表格

語法：create table table_name ( … columns )

示例：創建用戶表 users

create table `users`
(
    `id` int,
    `name` char(10),
    `avatar` varchar(300),
    `regtime` date
)

1.3 創建索引

語法：create index index_name on table_name (column_name)

示例：爲用戶 id 創建索引 idx_id
create index `idx_id` on `users` (`id`)

/* 創建唯一索引 */
create unique index `idx_id` on `users` (`id`)

1.4 爲已存在的列創建主鍵

更常用的方式是在創建表語句所有列定義的後面添加一行 primary key (column_name)。

語法：alter table table_name add primary key (column_name)

示例：將用戶 id 設爲主鍵
alter table users add primary key (`id`)

1.5 爲已存在的列創建自增約束

更常用的方式是在創建表語句中添加自增列 id int not null auto_increment。

alter table `users` modify `id` int not null auto_increment

2. 插入

語法：

• insert into table_name values (value1, value2, …) insert into
• table_name (column1, column2, …) values (value1, value2, …)

示例：新增註冊用戶

insert into `users` values (1, 'ken', 'http://cdn.awesome_app.com/path/to/xxx/avatar1.jpg', curdate())
/* 指定列插入 */
insert into `users` (`name`, `avatar`) values ('bill', 'http://cdn.awesome_app.com/path/to/xxx/avatar2.jpg')

3. 修改(更新)

3.1 修改數據記錄

語法：

update table_name set column=new_value where condition
update table_name set column1=new_value1,column2=new_value2,... where condition

示例：

update `users` set `regtime`=curdate() where `regtime` is null
/* 一次修改多列 */
update `users` set `name`='steven',`avatar`='http://cdn.awesome_app.com/path/to/xxx/steven.jpg' where `id`=1

3.2 修改數據庫字符集爲 utf8

alter database `awesome_app` default character set utf8

3.3 修改表字符集爲 utf8

alter table `users` convert to character set utf8

3.4 修改表字段字符集爲 utf8

alter table `users` modify `name` char(10) character set utf8

3.5 修改字段類型

alter table `users` modify `regtime` datetime not null

3.5 修改字段默認值

alter table `users` alter `regtime` set default '2019-10-12 00:00:00'
/* 設置默認爲當前時間 current_timestamp，需要重新定義整個列 */
alter table `users` modify `regtime` datetime not null default current_timestamp

3.6 修改字段註釋

alter table `users` modify `id` int not null auto_increment comment '用戶ID';
alter table `users` modify `name` char(10) comment '用戶名';
alter table `users` modify `avatar` varchar(300) comment '用戶頭像';
alter table `users` modify `regtime` datetime not null default current_timestamp comment '註冊時間';

修改後，查看改動後的列：

mysql> show full columns from users;
+---------+--------------+-----------------+------+-----+-------------------+----------------+---------------------------------+--------------+
| Field   | Type         | Collation       | Null | Key | Default           | Extra          | Privileges                      | Comment      |
+---------+--------------+-----------------+------+-----+-------------------+----------------+---------------------------------+--------------+
| id      | int(11)      | NULL            | NO   | PRI | NULL              | auto_increment | select,insert,update,references | 用戶ID       |
| name    | char(10)     | utf8_general_ci | YES  |     | NULL              |                | select,insert,update,references | 用戶名       |
| avatar  | varchar(300) | utf8_general_ci | YES  |     | NULL              |                | select,insert,update,references | 用戶頭像     |
| regtime | datetime     | NULL            | NO   |     | CURRENT_TIMESTAMP |                | select,insert,update,references | 註冊時間     |
+---------+--------------+-----------------+------+-----+-------------------+----------------+---------------------------------+--------------+

4. 刪除

4.1 刪除數據記錄

• 語法：delete from table_name where condition
• 示例：刪除用戶名未填寫的用戶

#先增加一條用戶名爲空的用戶

mysql> insert into `users` (`regtime`) values (curdate());
mysql> select * from users;
+----+--------+----------------------------------------------------+------------+
| id | name   | avatar                                             | regtime    |
+----+--------+----------------------------------------------------+------------+
|  1 | steven | http://cdn.awesome_app.com/path/to/xxx/steven.jpg  | 2019-10-12 |
|  2 | bill   | http://cdn.awesome_app.com/path/to/xxx/avatar2.jpg | 2019-10-12 |
|  3 | NULL   | NULL                                               | 2019-10-12 |
+----+--------+----------------------------------------------------+------------+

#刪除用戶名爲空的行
mysql> delete from `users` where `name` is null;
mysql> select * from users;
+----+--------+----------------------------------------------------+------------+
| id | name   | avatar                                             | regtime    |
+----+--------+----------------------------------------------------+------------+
|  1 | steven | http://cdn.awesome_app.com/path/to/xxx/steven.jpg  | 2019-10-12 |
|  2 | bill   | http://cdn.awesome_app.com/path/to/xxx/avatar2.jpg | 2019-10-12 |
+----+--------+----------------------------------------------------+------------+

4.2 刪除數據庫

drop database if exists `awesome_app`

4.3 刪除表

drop table if exists `users`

4.4 清空表中所有數據

這個操作相當於先 drop table 再 create table ，因此需要有 drop 權限。
truncate table `users`

4.5 刪除索引

drop index `idx_id` on `users`

5. 查詢

5.1 語法

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
      [PARTITION partition_list]
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
        export_options
      | INTO DUMPFILE 'file_name'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]

5.2 單表查詢

5.2.1 準備數據：

insert into users (`name`, `avatar`) values
('張三', 'http://cdn.awesome_app.com/path/to/xxx/3.jpg'),
('李四', 'http://cdn.awesome_app.com/path/to/xxx/4.jpg'),
('王五', 'http://cdn.awesome_app.com/path/to/xxx/5.jpg'),
('馬六', 'http://cdn.awesome_app.com/path/to/xxx/6.jpg'),
('肖七', 'http://cdn.awesome_app.com/path/to/xxx/7.jpg'),
('劉八', 'http://cdn.awesome_app.com/path/to/xxx/8.jpg'),
('楊九', 'http://cdn.awesome_app.com/path/to/xxx/9.jpg'),
('鄭十', 'http://cdn.awesome_app.com/path/to/xxx/10.jpg');

/* 增加重複行 */
insert into users (`name`, `avatar`) values
('張三', 'http://cdn.awesome_app.com/path/to/xxx/3.jpg'),
('李四', 'http://cdn.awesome_app.com/path/to/xxx/4.jpg'),
('王五', 'http://cdn.awesome_app.com/path/to/xxx/5.jpg');

5.2.2 查詢所有列

mysql> select * from users;
+----+--------+----------------------------------------------------+---------------------+
| id | name   | avatar                                             | regtime             |
+----+--------+----------------------------------------------------+---------------------+
|  1 | steven | http://cdn.awesome_app.com/path/to/xxx/steven.jpg  | 2019-10-12 00:00:00 |
|  2 | bill   | http://cdn.awesome_app.com/path/to/xxx/avatar2.jpg | 2019-10-12 00:00:00 |
|  3 | 張三   | http://cdn.awesome_app.com/path/to/xxx/3.jpg       | 2019-10-13 10:58:37 |
|  4 | 李四   | http://cdn.awesome_app.com/path/to/xxx/4.jpg       | 2019-10-13 10:58:37 |
|  5 | 王五   | http://cdn.awesome_app.com/path/to/xxx/5.jpg       | 2019-10-13 10:58:37 |
|  6 | 馬六   | http://cdn.awesome_app.com/path/to/xxx/6.jpg       | 2019-10-13 10:58:37 |
|  7 | 肖七   | http://cdn.awesome_app.com/path/to/xxx/7.jpg       | 2019-10-13 10:58:37 |
|  8 | 劉八   | http://cdn.awesome_app.com/path/to/xxx/8.jpg       | 2019-10-13 10:58:37 |
|  9 | 楊九   | http://cdn.awesome_app.com/path/to/xxx/9.jpg       | 2019-10-13 10:58:37 |
| 10 | 鄭十   | http://cdn.awesome_app.com/path/to/xxx/10.jpg      | 2019-10-13 10:58:37 |
| 11 | 張三   | http://cdn.awesome_app.com/path/to/xxx/3.jpg       | 2019-10-13 11:20:17 |
| 12 | 李四   | http://cdn.awesome_app.com/path/to/xxx/4.jpg       | 2019-10-13 11:20:17 |
| 13 | 王五   | http://cdn.awesome_app.com/path/to/xxx/5.jpg       | 2019-10-13 11:20:17 |
+----+--------+----------------------------------------------------+---------------------+

5.2.3 查詢指定列

mysql> select id,name from users;
+----+--------+
| id | name   |
+----+--------+
|  1 | steven |
|  2 | bill   |
|  3 | 張三   |
|  4 | 李四   |
|  5 | 王五   |
|  6 | 馬六   |
|  7 | 肖七   |
|  8 | 劉八   |
|  9 | 楊九   |
| 10 | 鄭十   |
| 11 | 張三   |
| 12 | 李四   |
| 13 | 王五   |
+----+--------+

5.2.4 查詢不重複記錄

mysql> select distinct name,avatar  from users;
+--------+----------------------------------------------------+
| name   | avatar                                             |
+--------+----------------------------------------------------+
| steven | http://cdn.awesome_app.com/path/to/xxx/steven.jpg  |
| bill   | http://cdn.awesome_app.com/path/to/xxx/avatar2.jpg |
| 張三   | http://cdn.awesome_app.com/path/to/xxx/3.jpg       |
| 李四   | http://cdn.awesome_app.com/path/to/xxx/4.jpg       |
| 王五   | http://cdn.awesome_app.com/path/to/xxx/5.jpg       |
| 馬六   | http://cdn.awesome_app.com/path/to/xxx/6.jpg       |
| 肖七   | http://cdn.awesome_app.com/path/to/xxx/7.jpg       |
| 劉八   | http://cdn.awesome_app.com/path/to/xxx/8.jpg       |
| 楊九   | http://cdn.awesome_app.com/path/to/xxx/9.jpg       |
| 鄭十   | http://cdn.awesome_app.com/path/to/xxx/10.jpg      |
+--------+----------------------------------------------------+

5.2.5 限制查詢行數

查詢前幾行
mysql> select id,name from users limit 2;
+----+--------+
| id | name   |
+----+--------+
|  1 | steven |
|  2 | bill   |
+----+--------+

查詢從指定偏移(第一行爲偏移爲0)開始的幾行
mysql> select id,name from users limit 2,3;
+----+--------+
| id | name   |
+----+--------+
|  3 | 張三   |
|  4 | 李四   |
|  5 | 王五   |
+----+--------+

5.2.6 排序

# 正序
mysql> select distinct name from users order by name asc limit 3;
+--------+
| name   |
+--------+
| bill   |
| steven |
| 劉八   |
+--------+

# 倒序
mysql> select id,name from users order by id desc limit 3;
+----+--------+
| id | name   |
+----+--------+
| 13 | 王五   |
| 12 | 李四   |
| 11 | 張三   |
+----+--------+

5.2.7 分組

增加城市字段
alter table `users` add `city` varchar(10) comment '用戶所在城市' after `name`;
update `users` set `city`='舊金山' where `id`=1;
update `users` set `city`='西雅圖' where `id`=2;
update `users` set `city`='北京' where `id` in (3,5,7);
update `users` set `city`='上海' where `id` in (4,6,8);
update `users` set `city`='廣州' where `id` between 9 and 10;
update `users` set `city`='深圳' where `id` between 11 and 13;

按城市分組統計用戶數
mysql> select city, count(name) as num_of_user from users group by city;
+-----------+-------------+
| city      | num_of_user |
+-----------+-------------+
| 上海      |           3 |
| 北京      |           3 |
| 廣州      |           2 |
| 舊金山    |           1 |
| 深圳      |           3 |
| 西雅圖    |           1 |
+-----------+-------------+
mysql> select city, count(name) as num_of_user from users group by city having num_of_user=1;
+-----------+-------------+
| city      | num_of_user |
+-----------+-------------+
| 舊金山    |           1 |
| 西雅圖    |           1 |
+-----------+-------------+
mysql> select city, count(name) as num_of_user from users group by city having num_of_user>2;
+--------+-------------+
| city   | num_of_user |
+--------+-------------+
| 上海   |           3 |
| 北京   |           3 |
| 深圳   |           3 |
+--------+-------------+

5.3 多表關聯查詢

5.3.1 準備數據

create table if not exists `orders`
(
    `id` int not null primary key auto_increment comment '訂單ID',
    `title` varchar(50) not null comment '訂單標題',
    `user_id` int not null comment '用戶ID',
    `cretime` timestamp not null default current_timestamp comment '創建時間'
);
create table if not exists `groups`
(
    `id` int not null primary key auto_increment comment '用戶組ID',
    `title` varchar(50) not null comment '用戶組標題',
    `cretime` timestamp not null default current_timestamp comment '創建時間'
);
alter table `users` add `group_id` int comment '用戶分組' after `city`;

insert into `groups` (`title`) values ('大佬'), ('萌新'), ('菜雞');
insert into `orders` (`title`, `user_id`) values ('《大佬是怎樣煉成的？》', 3), ('《MySQL 從萌新到刪庫跑路》', 6), ('《菜雞踩坑記》', 9);
update `users` set `group_id`=1 where `id` between 1 and 2;
update `users` set `group_id`=2 where `id` in (4, 6, 8, 10, 12);
update `users` set `group_id`=3 where `id` in (3, 5, 13);

5.3.2 join

join
用於在多個表中查詢相互匹配的數據。
mysql> select `users`.`name` as `user_name`, `orders`.`title` as `order_title` from `users`, `orders` where `orders`.`user_id`=`users`.`id`;
+-----------+--------------------------------------+
| user_name | order_title                          |
+-----------+--------------------------------------+
| 張三      | 《大佬是怎樣煉成的？》               |
| 馬六      | 《MySQL 從萌新到刪庫跑路》           |
| 楊九      | 《菜雞踩坑記》                       |
+-----------+--------------------------------------+

inner join
內部連接。效果與 join 一樣 , 但用法不同，join 使用 where ，inner join 使用 on 。
mysql> select `users`.`name` as `user_name`, `orders`.`title` as `order_title` from `users` inner join `orders` on `orders`.`user_id`=`users`.`id`;
+-----------+--------------------------------------+
| user_name | order_title                          |
+-----------+--------------------------------------+
| 張三      | 《大佬是怎樣煉成的？》               |
| 馬六      | 《MySQL 從萌新到刪庫跑路》           |
| 楊九      | 《菜雞踩坑記》                       |
+-----------+--------------------------------------+

left join
左連接。返回左表所有行，即使右表中沒有匹配的行，不匹配的用 NULL 填充。
mysql> select `users`.`name` as `user_name`, `orders`.`title` as `order_title` from `users` left join `orders` on `orders`.`user_id`=`users`.`id`;
+-----------+--------------------------------------+
| user_name | order_title                          |
+-----------+--------------------------------------+
| 張三      | 《大佬是怎樣煉成的？》               |
| 馬六      | 《MySQL 從萌新到刪庫跑路》           |
| 楊九      | 《菜雞踩坑記》                       |
| steven    | NULL                                 |
| bill      | NULL                                 |
| 李四      | NULL                                 |
| 王五      | NULL                                 |
| 肖七      | NULL                                 |
| 劉八      | NULL                                 |
| 鄭十      | NULL                                 |
| 張三      | NULL                                 |
| 李四      | NULL                                 |
| 王五      | NULL                                 |
+-----------+--------------------------------------+

right join
右連接。和 left join 正好相反，會返回右表所有行，即使左表中沒有匹配的行，不匹配的用 NULL 填充。
mysql> select `groups`.`title` as `group_title`, `users`.`name` as `user_name` from `groups` right join `users` on `users`.`group_id`=`groups`.`id`;
+-------------+-----------+
| group_title | user_name |
+-------------+-----------+
| 大佬        | steven    |
| 大佬        | bill      |
| 萌新        | 李四      |
| 萌新        | 馬六      |
| 萌新        | 劉八      |
| 萌新        | 鄭十      |
| 萌新        | 李四      |
| 菜雞        | 張三      |
| 菜雞        | 王五      |
| 菜雞        | 王五      |
| NULL        | 肖七      |
| NULL        | 楊九      |
| NULL        | 張三      |
+-------------+-----------+

5.3.3 union

union 用於合併兩個或多個查詢結果，合併的查詢結果必須具有相同數量的列，並且列擁有形似的數據類型，同時列的順序相同。
mysql> (select `id`, `title` from `groups`) union (select `id`, `title` from `orders`);
+----+--------------------------------------+
| id | title                                |
+----+--------------------------------------+
|  1 | 大佬                                 |
|  2 | 萌新                                 |
|  3 | 菜雞                                 |
|  1 | 《大佬是怎樣煉成的？》               |
|  2 | 《MySQL 從萌新到刪庫跑路》           |
|  3 | 《菜雞踩坑記》                       |
+----+--------------------------------------+

6. 函數

6.1 語法

select function(column) from table_name

6.2 合計函數（Aggregate functions）

合計函數的操作面向一系列的值，並返回一個單一的值。通常與 group by 語句一起用。

函數 描述

1. avg(column) 返回某列的平均值
2. count(column) 返回某列的行數（不包括 NULL 值）
3. count(*)返回被選行數
4. first(column) 返回在指定的域中第一個記錄的值
5. last(column) 返回在指定的域中最後一個記錄的值
6. max(column) 返回某列的最高值
7. min(column) 返回某列的最低值
8. sum(column) 返回某列的總和

6.3 標量函數（Scalar functions）

函數 描述

1. ucase© 轉換爲大寫
2. lcase© 轉換爲小寫
3. mid(c, start[, end]) 從文本提取字符
4. len© 返回文本長度
5. instr(c, char) 返回在文本中指定字符的數值位置
6. left(c, number_of_char) 返回文本的左側部分
7. right(c, number_of_char) 返回文本的右側部分
8. round(c, decimals) 對數值指定小數位數四捨五入
9. mod(x, y) 取餘（求模）
10. now() 返回當前的系統日期
11. format(c, format) 格式化顯示
12. datediff(d, date1, date2) 日期計算

注：以上摘錄自：https://juejin.im/post/5da3ba17f265da5b591b6b09

本人的學習公衆號，歡迎大家騷擾討論