sql number转varchar_MySQL 指南之 SQL 语句基础

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零、结构化查询语言：SQL(Structured Query Language)

DDL 数据定义语言管理库，表

DML 数据操作语言增删改查

DCL 数据控制语言数据控制，权限访问等

准备活动：创建库和表

CREATE DATABASE datatype;

USE datatype;

CREATE TABLE type_number(

type CHAR(12),

byte TINYINT UNSIGNED,

range_singed VARCHAR(20),

range_unsinged VARCHAR(20),

info VARCHAR(40)

);

目前状态：

mysql> SHOW DATABASES;

+--------------------+

| Database |

+--------------------+

| datatype |

| information_schema |

| mycode |

| mysql |

| performance_schema |

| seckill |

+--------------------+

mysql> USE datatype;

Database changed

mysql> SHOW TABLES;

+--------------------+

| Tables_in_datatype |

+--------------------+

| type_number |

+--------------------+

mysql> DESC type_number;

+----------------+---------------------+------+-----+---------+-------+

+----------------+---------------------+------+-----+---------+-------+

| type | char(12) | YES | | NULL | |

| byte | tinyint(3) unsigned | YES | | NULL | |

| range_singed | varchar(20) | YES | | NULL | |

| range_unsinged | varchar(20) | YES | | NULL | |

| info | varchar(40) | YES | | NULL | |

+----------------+---------------------+------+-----+---------+-------+

一、DML 数据库记录操作 `LEVEL1`

LEVEL 1 先简单掌握一下下面的用法

1、记录的插入操作

INSERT INTO(属性,...)VALUES(值,...),...;

|-- 插入一条数据 INSERT INTO (属性,...) VALUES (值,...);

INSERT INTO

type_number(type,byte,range_singed,range_unsinged,info)

VALUES

('TINYINT',1,'-2⁷ ~ 2⁷-1','0 ~ 2⁸-1','很小整数');

|-- 查询所有 SELECT * FROM ;

mysql> SELECT * FROM type_number;

+---------+------+----------------+----------------+--------------+

+---------+------+----------------+----------------+--------------+

| TINYINT | 1 | -2⁷ ~ 2⁷-1 | 0 ~ 2⁸-1 | 很小整数 |

+---------+------+----------------+----------------+--------------+

|-- 你也可以一次，插入多条数据

INSERT INTO

type_number(type,byte,range_singed,range_unsinged,info)

VALUES

('TINYINT',1,'-2⁷ ~ 2⁷-1','0 ~ 2⁸-1','很小整数'),

('SMALLINT',2,'-2¹⁶ ~ 2¹⁶-1','0 ~ 2¹⁶-1','小整数'),

('MEDIUMINT',3,'-2²⁴ ~ 2²⁴-1','0 ~ 2²⁴-1','中等整数'),

('INT',4,'-2³² ~ 2³²-1','0 ~ 2³²-1','标准整数'),

('BIGINT',8,'-2⁶⁴ ~ 2⁶⁴-1','0 ~ 2⁶⁴-1','大整数');

mysql> SELECT * FROM type_number;

+-----------+------+----------------------+----------------+--------------+

+-----------+------+----------------------+----------------+--------------+

| TINYINT | 1 | -2⁷ ~ 2⁷-1 | 0 ~ 2⁸-1 | 很小整数 |

| SMALLINT | 2 | -2¹⁶ ~ 2¹⁶-1 | 0 ~ 2¹⁶-1 | 小整数 |

| MEDIUMINT | 3 | -2²⁴ ~ 2²⁴-1 | 0 ~ 2²⁴-1 | 中等整数 |

| INT | 4 | -2³² ~ 2³²-1 | 0 ~ 2³²-1 | 标准整数 |

| BIGINT | 8 | -2⁶⁴ ~ 2⁶⁴-1 | 0 ~ 2⁶⁴-1 | 大整数 |

+-----------+------+----------------------+----------------+--------------+

2、记录的更新操作

UPDATE SET 属性=值,...WHERE 条件;

UPDATE type_number

SET

info='微型整数'

WHERE type = 'TINYINT';

mysql> SELECT * FROM type_number;

+-----------+------+----------------------+----------------+--------------+

+-----------+------+----------------------+----------------+--------------+

| TINYINT | 1 | -2⁷ ~ 2⁷-1 | 0 ~ 2⁸-1 | 微型整数 |

| SMALLINT | 2 | -2¹⁶ ~ 2¹⁶-1 | 0 ~ 2¹⁶-1 | 小整数 |

| MEDIUMINT | 3 | -2²⁴ ~ 2²⁴-1 | 0 ~ 2²⁴-1 | 中等整数 |

| INT | 4 | -2³² ~ 2³²-1 | 0 ~ 2³²-1 | 标准整数 |

| BIGINT | 8 | -2⁶⁴ ~ 2⁶⁴-1 | 0 ~ 2⁶⁴-1 | 大整数 |

+-----------+------+----------------------+----------------+--------------+

3.记录的删除操作

DELETE FROM WHERE 条件;

|--- 删除操作

DELETE FROM type_number

WHERE type = 'TINYINT';

mysql> SELECT * FROM type_number;

+-----------+------+----------------------+----------------+--------------+

+-----------+------+----------------------+----------------+--------------+

| SMALLINT | 2 | -2¹⁶ ~ 2¹⁶-1 | 0 ~ 2¹⁶-1 | 小整数 |

| MEDIUMINT | 3 | -2²⁴ ~ 2²⁴-1 | 0 ~ 2²⁴-1 | 中等整数 |

| INT | 4 | -2³² ~ 2³²-1 | 0 ~ 2³²-1 | 标准整数 |

| BIGINT | 8 | -2⁶⁴ ~ 2⁶⁴-1 | 0 ~ 2⁶⁴-1 | 大整数 |

+-----------+------+----------------------+----------------+--------------+

4.记录的查询操作

SELECT 属性,...FROM WHERE 条件;

mysql>

SELECT

type,range_unsinged

FROM type_number

WHERE byte>=4;

+--------+----------------+

| type | range_unsinged |

+--------+----------------+

| INT | 0 ~ 2³²-1 |

| BIGINT | 0 ~ 2⁶⁴-1 |

+--------+----------------+

二、图片表 pic `(LEVER2)`

这个是用来记录图片信息的表，数据准备过程详见番外篇：[番外]-练习MySQL没素材？来一波字符串操作

1.建表语句

CREATE TABLE pic(

id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,

pic_path VARCHAR(120) NOT NULL,

pic_length INT UNSIGNED DEFAULT 0,

pic_mime TINYINT UNSIGNED,

pic_width SMALLINT UNSIGNED,

pic_height SMALLINT UNSIGNED

);

|--- id 为主键自增长

|--- pic_path表示名字，不定长度，给个VARCHAR 120 吧，差不多够用吧

|--- 图片文件大小不会非常大，给个INT足够了，给个默认值 0

|--- pic_mime 0 表示 image/png 1表示 image/jpeg 给个最小的

|--- pic_width和pic_height也不会非常大，无符号SMALLINT足够

2.查询操作 `AS` 的作用

|-- 查询高大于1200像素的记录，使用AS 来临时更改查询输出的属性名(不会改变实际记录)

mysql>

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height>1200;

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| 30000X20000.jpg | 30000 | 20000 |

| 3000X2000.jpg | 3000 | 2000 |

| ecNKedygCmSjTWWF.jpg | 700 | 1352 |

| gtQiXnRfkvvTLinw.jpg | 2880 | 2025 |

| HXqqASHJETSlvpnc.jpg | 3600 | 2400 |

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

| screen.png | 1080 | 1920 |

| XQWGrglfjGVuJfzJ.jpg | 1200 | 1696 |

+----------------------+--------+--------+

3.查询是属性可参与运算

|-- CONCAT函数用于连接字符串注意：\需要转义

mysql>

SELECT

CONCAT('E:\\SpringBootFiles\\imgs\\',pic_path) AS 绝对路径,

pic_width * pic_height AS '像素点个数'

FROM pic

WHERE pic_height>1200;

+----------------------------------------------+-----------------+

| 绝对路径 | 像素点个数 |

+----------------------------------------------+-----------------+

| E:\SpringBootFiles\imgs\30000X20000.jpg | 600000000 |

| E:\SpringBootFiles\imgs\3000X2000.jpg | 6000000 |

| E:\SpringBootFiles\imgs\ecNKedygCmSjTWWF.jpg | 946400 |

| E:\SpringBootFiles\imgs\gtQiXnRfkvvTLinw.jpg | 5832000 |

| E:\SpringBootFiles\imgs\HXqqASHJETSlvpnc.jpg | 8640000 |

| E:\SpringBootFiles\imgs\ndbMXlwKuCpiiVqC.jpg | 3857868 |

| E:\SpringBootFiles\imgs\screen.png | 2073600 |

| E:\SpringBootFiles\imgs\XQWGrglfjGVuJfzJ.jpg | 2035200 |

+----------------------------------------------+-----------------+

4. `WHERE`条件的千变万化

4.1: 条件 `与` -- `AND` 和 `&&`

条件必须全部满足

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height>1200 AND

pic_width > 1500;

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| 30000X20000.jpg | 30000 | 20000 |

| 3000X2000.jpg | 3000 | 2000 |

| gtQiXnRfkvvTLinw.jpg | 2880 | 2025 |

| HXqqASHJETSlvpnc.jpg | 3600 | 2400 |

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

+----------------------+--------+--------+

|--- AND 效果等于 &&

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height>1200 &&

pic_width > 1500;

4.2: 条件 `或` -- `OR` 和 `||`

条件满足一个即可

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height>1200 OR

pic_width > 1500;

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| 30000X20000.jpg | 30000 | 20000 |

| 3000X2000.jpg | 3000 | 2000 |

| ecNKedygCmSjTWWF.jpg | 700 | 1352 |

| gtQiXnRfkvvTLinw.jpg | 2880 | 2025 |

| HXqqASHJETSlvpnc.jpg | 3600 | 2400 |

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

| screen.png | 1080 | 1920 |

| XQWGrglfjGVuJfzJ.jpg | 1200 | 1696 |

+----------------------+--------+--------+

|--- OR 效果等于 ||

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height>1200 ||

pic_width > 1500;

4.3: 条件 `非` -- `NOT` 和 `!`

对条件取反

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE NOT pic_height < 1200;

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| 30000X20000.jpg | 30000 | 20000 |

| 3000X2000.jpg | 3000 | 2000 |

| ecNKedygCmSjTWWF.jpg | 700 | 1352 |

| gtQiXnRfkvvTLinw.jpg | 2880 | 2025 |

| HXqqASHJETSlvpnc.jpg | 3600 | 2400 |

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

| screen.png | 1080 | 1920 |

| XQWGrglfjGVuJfzJ.jpg | 1200 | 1696 |

+----------------------+--------+--------+

4.4: 散点匹配 `IN(v1,v2,v3,...)`

符合 v1,v2,v3,...之一可匹配

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height IN (1696,2268);

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

| XQWGrglfjGVuJfzJ.jpg | 1200 | 1696 |

+----------------------+--------+--------+

4.5: 区间匹配 `BETWEEN v1 AND v2`

v1,v2 之间可匹配

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_height BETWEEN 1696 AND 2268;

+----------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------+--------+--------+

| 3000X2000.jpg | 3000 | 2000 |

| gtQiXnRfkvvTLinw.jpg | 2880 | 2025 |

| ndbMXlwKuCpiiVqC.jpg | 1701 | 2268 |

| screen.png | 1080 | 1920 |

| XQWGrglfjGVuJfzJ.jpg | 1200 | 1696 |

+----------------------+--------+--------+

4.6：模糊查询： `LIKE`

'%' 匹配任意多个字符,'_' 匹配任意单个字符

mysql>

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_path LIKE 'androi%';

+----------------------------------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+----------------------------------------------+--------+--------+

| android\008525ebc2b7d434070e74c00841a30f.png | 544 | 544 |

| android\054d98e2d96dc42d9b2b036126fccf49.png | 544 | 544 |

| android\05baf2d03651d1110d7a403f14aee877.png | 544 | 544 |

| android\0655e07d6717847489cd222c9c9e0b1d.png | 500 | 500 |

| android\079c4cb46c95b2365b5bc5150e7d5213.png | 544 | 544 |

| android\07a4dc9b4b207cb420a71cbf941ad45a.png | 544 | 544 |

| android\07abb7972a5638b53afa3b5eb98b19c1.png | 500 | 500 |

......

mysql>

SELECT

pic_path AS 路径 ,

pic_width AS '宽/px',

pic_height AS '高/px'

FROM pic

WHERE pic_path LIKE 'p_em%';

+--------------------------------------------+--------+--------+

| 路径 | 宽/px | 高/px |

+--------------------------------------------+--------+--------+

| poem\世界·绽放.jpg | 1148 | 712 |

| poem\我爱你，是火山岩的缄默.jpg | 690 | 397 |

| poem\枝·你是树的狂舞.jpg | 500 | 333 |

| poem\海与鹿王.jpg | 799 | 499 |

| poem\游梦人·诗的诞生.jpg | 800 | 444 |

| poem\珊瑚墓地.jpg | 1104 | 719 |

+--------------------------------------------+--------+--------+

4.7:比较符号 `=!=<><=>=`

小学生都知道的，就不废话了，查看一下小于 10Kb 的图片

mysql>

SELECT

pic_path AS 路径 ,

pic_length AS '大小/byte'

FROM pic

WHERE pic_length < 10*1024;

+----------------------------------------------+-------------+

| 路径 | 大小/byte |

+----------------------------------------------+-------------+

| 30X20.jpg | 10158 |

| android\613f2b8f0eaa8f63bedce9781527c9ab.png | 4001 |

| android\94b5c41232f9761403890c09c2b1aae3.png | 4001 |

| android\d3fd676f224f0734beb48d0c0d2f4e66.png | 4001 |

| udp发送与接收消息_控制台.png | 9184 |

+----------------------------------------------+-------------+

5. `GROUP BY`分组查询

会先排序，再列出

|--- GROUP BY

SELECT

pic_mime AS "类型",

avg(pic_length) AS '平均大小/byte' ,

count(pic_length) AS '总数量/个' ,

min(pic_length) AS '最小值/byte' ,

max(pic_length) AS '最大值/byte' ,

sum(pic_length) AS '总和/byte'

FROM pic

GROUP BY pic_mime;

+--------+-------------------+---------------+----------------+----------------+-------------+

+--------+-------------------+---------------+----------------+----------------+-------------+

| 0 | 141518.8734 | 229 | 4001 | 829338 | 32407822 |

| 1 | 2133272.8000 | 60 | 10158 | 116342886 | 127996368 |

+--------+-------------------+---------------+----------------+----------------+-------------+

6.结果集筛选： `HAVING`

现在查询宽高比在 1.1 和 1.3 之间的图片

|-- 如果用WHERE 来查询感觉有点不优雅

SELECT

pic_path AS 路径 ,

pic_width/pic_height AS '宽高比'

FROM pic

WHERE pic_width/pic_height > 1.1 && pic_width/pic_height<1.3;

+------------------------------------------------------------------+-----------+

| 路径 | 宽高比 |

+------------------------------------------------------------------+-----------+

| dQXbnTRjUdNxhiyl.jpg | 1.2308 |

| JsXHWmKqOlziKmeA.jpg | 1.2600 |

| logo\android\Android原生绘图之让你了解View的运动.png | 1.2884 |

| 洛天依.jpg | 1.1990 |

+------------------------------------------------------------------+-----------+

|-- AS 相当于将列取了变量，对结果集再进行筛选用HAVING,用WHERE则报错，找不到列

SELECT

pic_path AS 路径 ,

pic_width/pic_height AS ratio

FROM pic

HAVING ratio > 1.1 && ratio <1.3;

+------------------------------------------------------------------+--------+

| 路径 | ratio |

+------------------------------------------------------------------+--------+

| dQXbnTRjUdNxhiyl.jpg | 1.2308 |

| JsXHWmKqOlziKmeA.jpg | 1.2600 |

| logo\android\Android原生绘图之让你了解View的运动.png | 1.2884 |

| 洛天依.jpg | 1.1990 |

+------------------------------------------------------------------+--------+

7.结果排序： `ORDER BY`

按照 ratio 将序排列

SELECT

pic_path AS 路径 ,

pic_width/pic_height AS ratio

FROM pic

HAVING ratio > 1.1 && ratio <1.3;

ORDER BY ratio DESC

+------------------------------------------------------------------+--------+

| 路径 | ratio |

+------------------------------------------------------------------+--------+

| dQXbnTRjUdNxhiyl.jpg | 1.2308 |

| JsXHWmKqOlziKmeA.jpg | 1.2600 |

| logo\android\Android原生绘图之让你了解View的运动.png | 1.2884 |

| 洛天依.jpg | 1.1990 |

+------------------------------------------------------------------+--------+

8.控制条目数： `LIMIT`

|-- 偏移一条，取两条

SELECT

pic_path AS 路径 ,

pic_width/pic_height AS ratio

FROM pic

HAVING ratio > 1.1 && ratio <1.3

ORDER BY ratio DESC LIMIT 1,2;

+----------------------+--------+

| 路径 | ratio |

+----------------------+--------+

| JsXHWmKqOlziKmeA.jpg | 1.2600 |

| dQXbnTRjUdNxhiyl.jpg | 1.2308 |

+----------------------+--------+

三、子查询 `(LEVER3)`

1.查询大于平均尺寸的图片 -- `WHERE`

|--- 出现在其他SQL语句内的SELECT语句

|--- 子查询必须在()内

|--- 增删改查都可以进行子查询,返回：标量，行，列或子查询

|-- 1-1：查出图片平均大小

SELECT

ROUND(AVG(pic_length),2) AS '平均大小'

FROM pic;

+--------------+

| 平均大小 |

+--------------+

| 555031.80 |

+--------------+

1 row in set (0.00 sec)

|-- 1-2：在用WHERE 筛选

SELECT

pic_path AS 路径 ,

pic_length AS '大小/byte'

FROM pic

WHERE pic_length > 555031.80;

+----------------------------------------------+-------------+

| 路径 | 大小/byte |

+----------------------------------------------+-------------+

| 30000X20000.jpg | 116342886 |

| 3000X2000.jpg | 3404969 |

| android\12284e5f7197d8be737fa967c8b00fbe.png | 829338 |

| android\594665add495ac9da8b6bbee1c63f1b8.png | 598974 |

| android\7cc97458727e23f7d161b8a1a7c6b453.png | 559420 |

| android\cbb1524f5ab4266698f3a6fc2992ccae.png | 829338 |

| android\d52539b1b508a594d1f2865037ff50c5.png | 598974 |

| android\f07ddfe5a103e4a024e14e2569f1d70e.png | 829338 |

| android\f0d1e7713d5557a8f9c74c9904843e09.png | 559420 |

| bg.png | 688207 |

| gtQiXnRfkvvTLinw.jpg | 771187 |

| poem\珊瑚墓地.jpg | 984472 |

| XoazFNMQROveEPQn.jpg | 795364 |

+----------------------------------------------+-------------+

|--- 也就是将一个语句包在WHERE 条件里

SELECT

pic_path AS 路径 ,

pic_length AS '大小/byte'

FROM pic

WHERE pic_length > (

SELECT

ROUND(AVG(pic_length),2)

FROM pic

);

2.查出每种类型的最新插入的图片 -- `WHERE`

SELECT

pic_path AS 路径 ,

pic_mime AS 类型

FROM pic

WHERE id IN (

SELECT

max(id)

FROM pic

GROUP BY pic_mime

);

+------------------+--------+

| 路径 | 类型 |

+------------------+--------+

| 洛天依.jpg | 1 |

| 虚拟机栈.png | 0 |

+------------------+--------+

3.FROM子查询 -- `FROM`

SELECT

id,

pic_path AS 路径 ,

pic_length AS '大小/byte'

FROM pic

WHERE id>=10&&id<=15

ORDER BY pic_length DESC;

+----+----------------------------------------------+-------------+

| id | 路径 | 大小/byte |

+----+----------------------------------------------+-------------+

| 15 | android\0f3bf63796ac370a08ee97b056b0587b.png | 178849 |

| 14 | android\0951ef0be68f0c498ca34ffcd7fc7faa.png | 175842 |

| 11 | android\079c4cb46c95b2365b5bc5150e7d5213.png | 86996 |

| 10 | android\0655e07d6717847489cd222c9c9e0b1d.png | 53764 |

| 12 | android\07a4dc9b4b207cb420a71cbf941ad45a.png | 46270 |

| 13 | android\07abb7972a5638b53afa3b5eb98b19c1.png | 43360 |

+----+----------------------------------------------+-------------+

|--- 将查询结果当做一张表，再查询操作

SELECT

id,路径 FROM (

SELECT

id,

pic_path AS 路径 ,

pic_length AS '大小/byte'

FROM pic

WHERE id>=10&&id<=15

ORDER BY pic_length DESC

) AS result

WHERE `大小/byte` < 59999;

+----+----------------------------------------------+

| id | 路径 |

+----+----------------------------------------------+

| 10 | android\0655e07d6717847489cd222c9c9e0b1d.png |

| 12 | android\07a4dc9b4b207cb420a71cbf941ad45a.png |

| 13 | android\07abb7972a5638b53afa3b5eb98b19c1.png |

+----+----------------------------------------------+

四、连接查询

0.创建关联表

首先连接查询要多张表，现在建一个 mime_type 的表

|--- 建表

CREATE TABLE mime_type(

mime_id SMALLINT UNSIGNED PRIMARY KEY,

mime_info CHAR(24)

);

|--- 插入数据

INSERT INTO mime_type(mime_id,mime_info) VALUES

(0,'image/png'),

(1,'image/jpeg'),

(2,'image/svg+xml'),

(3,'video/mp4'),

(4,'text/plain');

|--- 效果

mysql> select * from mime_type;

+---------+---------------+

| mime_id | mime_info |

+---------+---------------+

| 0 | image/png |

| 1 | image/jpeg |

| 2 | image/svg+xml |

| 3 | video/mp4 |

| 4 | text/plain |

+---------+---------------+

|-- 为了说明问题，pic表添加一条测试数据：pic_mime = 8 也就是 mime_type表找不到时

INSERT INTO pic(pic_path,pic_length,pic_mime,pic_width,pic_height) VALUES('test.jpg',100,8,300,200);

1.内连接查询 `INNER JOIN`

SELECT 待查属性 FROM 表1I NNER JOIN 表2O N条件 WHERE 条件

SELECT id, pic_path AS 路径 , mime_type.mime_info AS 类型 , pic_length

FROM pic INNER JOIN mime_type

ON pic.pic_mime = mime_type.mime_id

ORDER BY id DESC LIMIT 4;

+-----+------------------+------------+------------+

| id | 路径 | 类型 | pic_length |

+-----+------------------+------------+------------+

| 289 | 虚拟机栈.png | image/png | 63723 |

| 288 | 统一返回.png | image/png | 29485 |

| 287 | 洛天依.jpg | image/jpeg | 42117 |

| 286 | 标记整理.png | image/png | 29288 |

+-----+------------------+------------+------------+

2.左连接查询 : `LEFT JOIN`

保持左表的记录完整性，右表查不到就摆 NULL

SELECT id, pic_path AS 路径 , mime_type.mime_info AS 类型 , pic_length

FROM pic LEFT JOIN mime_type

ON pic.pic_mime = mime_type.mime_id

ORDER BY id DESC LIMIT 4;

+-----+------------------+------------+------------+

| id | 路径 | 类型 | pic_length |

+-----+------------------+------------+------------+

| 290 | test.jpg | NULL | 100 |

| 289 | 虚拟机栈.png | image/png | 63723 |

| 288 | 统一返回.png | image/png | 29485 |

| 287 | 洛天依.jpg | image/jpeg | 42117 |

+-----+------------------+------------+------------+

3. 右(外)连接查询 : `RIGHT JOIN`

保持右表的记录完整性，左表查不到就摆 NULL

SELECT id, pic_path AS 路径 , mime_type.mime_info AS 类型 , pic_length

FROM pic RIGHT JOIN mime_type

ON pic.pic_mime = mime_type.mime_id

ORDER BY id LIMIT 8;

+------+--------------------------------------+---------------+------------+

| id | 路径 | 类型 | pic_length |

+------+--------------------------------------+---------------+------------+

| 1 | 30000X20000.jpg | image/jpeg | 116342886 |

| 2 | 3000X2000.jpg | image/jpeg | 3404969 |

| 3 | 300X200.jpg | image/jpeg | 99097 |

| 4 | 30X20.jpg | image/jpeg | 10158 |

| 5 | 6dc9e8455c47d964e1a8a4ef04cf9477.jpg | image/jpeg | 236254 |

+------+--------------------------------------+---------------+------------+

4. 全(外)连接 (伪): `使用UNION`

MySQL 不支持全外连接，所以只能采取关键字 UNION 来联合左、右连接的方法 UNION : 将若干条 sql 的查询结果集合并成一个。 UNION ALL不会覆盖相同结果

SELECT id, pic_path AS 路径 , mime_type.mime_info AS 类型 , pic_length

FROM pic LEFT JOIN mime_type ON pic.pic_mime = mime_type.mime_id

UNION(

SELECT id, pic_path AS 路径 , mime_type.mime_info AS 类型 , pic_length

FROM pic RIGHT JOIN mime_type ON pic.pic_mime = mime_type.mime_id

)

ORDER BY id DESC;

+------+------------------------------------------------------------------------------------+---------------+------------+

| id | 路径 | 类型 | pic_length |

+------+------------------------------------------------------------------------------------+---------------+------------+

| 290 | test.jpg | NULL | 100 |

| 289 | 虚拟机栈.png | image/png | 63723 |

| 288 | 统一返回.png | image/png | 29485 |

| 287 | 洛天依.jpg | image/jpeg | 42117 |

...

| 3 | 300X200.jpg | image/jpeg | 99097 |

| 2 | 3000X2000.jpg | image/jpeg | 3404969 |

| 1 | 30000X20000.jpg | image/jpeg | 116342886 |

+------+------------------------------------------------------------------------------------+---------------+------------+

5. UNION 小测试

CREATE TABLE a(

id CHAR(4),

num INT

);

INSERT INTO a(id,num) VALUES

('a',4),('b',6),('c',2),('d',8);

CREATE TABLE b(

id CHAR(4),

num INT

);

INSERT INTO b(id,num) VALUES

('b',8),('c',7),('d',3),('e',18);

mysql> SELECT * FROM a; mysql> SELECT * FROM b;

+------+------+ +------+------+

| id | num | | id | num |

+------+------+ +------+------+

| a | 4 | | b | 8 |

| b | 6 | | c | 7 |

| c | 2 | | d | 3 |

| d | 8 | | e | 18 |

+------+------+ +------+------+

SELECT id,sum(num) FROM

(SELECT * FROM a

UNION ALL

SELECT * FROM b) as temp

GROUP BY id;

+------+----------+

| id | sum(num) |

+------+----------+

| a | 4 |

| b | 14 |

| c | 9 |

| d | 11 |

| e | 18 |

+------+----------+

六、DDL 建库/表

1、关于操作数据库

SHOW DATABASES; # 显示所有的数据库

SHOW CREATE DATABASE # 查看数据库创建信息

USE ; # 使用数据库

CREATE DATABASE [CHARACTER SET ]; # 创建一个将的数据库指定字符集

ALTER DATABASE CHARACTER SET ; # 修改数据库字符集

DROP DATABASE ; # 传说中的删库跑路

SELECT DATABASE(); # 查看当前选中的数据库

2.显示数据库信息

SHOW TABLES; # 展示当前数据库中的表

SHOW TABLES FROM mysql # 展示指定数据库中的表

DESC ; # 查看表结构

SHOW COLUMNS FROM ; # 查看表结构

3.创建表

|-- UNSIGNED 无符号 AUTO_INCREMENT 自增长

|-- ZEROFILL 前面自动填 0 , 默认 UNSIGNED

CREATE TABLE create_test(

id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,

code TINYINT(5) ZEROFILL DEFAULT 0

);

INSERT INTO create_test(code) VALUES (5);

INSERT INTO create_test VALUES (); #默认值测试

mysql> SELECT * FROM create_test;

+----+-------+

| id | code |

+----+-------+

| 1 | 00005 |

| 2 | 00000 |

+----+-------+

4.为表增加属性

ALTER TABLEADD属性信息[AFTER属性];

|-- 看一下当前表结构

mysql> DESC create_test;

+-------+------------------------------+------+-----+---------+----------------+

+-------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

+-------+------------------------------+------+-----+---------+----------------+

mysql> ALTER TABLE create_test ADD age SMALLINT UNSIGNED NOT NULL;

mysql> DESC create_test;

+-------+------------------------------+------+-----+---------+----------------+

+-------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+-------+------------------------------+------+-----+---------+----------------+

|-- AFTER可将属性排在指定属性之后(强迫症专用)

|-- ALTER TABLE create_test ADD password VARCHAR(32) AFTER id;

mysql> ALTER TABLE create_test ADD password VARCHAR(32) AFTER id;

mysql> DESC create_test;

+----------+------------------------------+------+-----+---------+----------------+

+----------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| password | varchar(32) | YES | | NULL | |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+----------+------------------------------+------+-----+---------+----------------+

|-- 一次添加多个属性

ALTER TABLE create_test ADD (aaa VARCHAR(32), bbb VARCHAR(32),ccc VARCHAR(32));

mysql> DESC create_test;

+----------+------------------------------+------+-----+---------+----------------+

+----------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| password | varchar(32) | YES | | NULL | |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

| aaa | varchar(32) | YES | | NULL | |

| bbb | varchar(32) | YES | | NULL | |

| ccc | varchar(32) | YES | | NULL | |

+----------+------------------------------+------+-----+---------+----------------+

5.为表删除属性

ALTER TABLE DROP 属性

ALTER TABLE create_test DROP aaa,DROP bbb,DROP ccc;

mysql> DESC create_test;

+----------+------------------------------+------+-----+---------+----------------+

+----------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| password | varchar(32) | YES | | NULL | |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+----------+------------------------------+------+-----+---------+----------------+

6.修改属性的类型

ALTER TABLE MODIFY 属性属性类型[FIRST];

|-- 把password改成VARCHAR(40)

ALTER TABLE create_test MODIFY password VARCHAR(40);

mysql> DESC create_test;

+----------+------------------------------+------+-----+---------+----------------+

+----------+------------------------------+------+-----+---------+----------------+

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| password | varchar(40) | YES | | NULL | |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+----------+------------------------------+------+-----+---------+----------------+

|-- 将某个属性移到最顶

ALTER TABLE create_test MODIFY password VARCHAR(40) FIRST;

+----------+------------------------------+------+-----+---------+----------------+

+----------+------------------------------+------+-----+---------+----------------+

| password | varchar(40) | YES | | NULL | |

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+----------+------------------------------+------+-----+---------+----------------+

7.修改表的属性名

ALTER TABLE CHANGE 原属性新属性新属性类型;

mysql> ALTER TABLE create_test CHANGE password pw varchar(40);

mysql> DESC create_test;

+-------+------------------------------+------+-----+---------+----------------+

+-------+------------------------------+------+-----+---------+----------------+

| pw | varchar(40) | YES | | NULL | |

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| code | tinyint(5) unsigned zerofill | YES | | 00000 | |

| age | smallint(5) unsigned | NO | | NULL | |

+-------+------------------------------+------+-----+---------+----------------+

8.修改表名

方式一： ALTER TABLE 旧表名 RENAME 新表名;方式二： RENAME TABLE 旧表名 TO 新表名;

ALTER TABLE create_test RENAME 阿姆斯特朗回旋加速喷气式阿姆斯特朗炮;

mysql> SHOW TABLES;

+--------------------------------------------------------+

| Tables_in_datatype |

+--------------------------------------------------------+

| 阿姆斯特朗回旋加速喷气式阿姆斯特朗炮 |

| a |

| b |

| mime_type |

| pic |

| type_number |

+--------------------------------------------------------+

RENAME TABLE 阿姆斯特朗回旋加速喷气式阿姆斯特朗炮 TO toly;

mysql> SHOW TABLES;

+--------------------+

| Tables_in_datatype |

+--------------------+

| a |

| b |

| mime_type |

| pic |

| toly |

| type_number |

+--------------------+

SQL 的基础就这样 , 下篇见