MySQL打卡3-表的增删改查@Datawhale

1. mysql表的数据类型

1.1 数值类型

MySQL支持所有标准SQL数值数据类型。

这些类型包括严格数值数据类型(INTEGER、SMALLINT、DECIMAL和NUMERIC),以及近似数值数据类型(FLOAT、REAL和DOUBLE PRECISION)。

关键字INT是INTEGER的同义词,关键字DEC是DECIMAL的同义词。

BIT数据类型保存位字段值,并且支持MyISAM、MEMORY、InnoDB和BDB表。

作为SQL标准的扩展,MySQL也支持整数类型TINYINT、MEDIUMINT和BIGINT。下面的表显示了需要的每个整数类型的存储和范围。
MySQL打卡3-表的增删改查@Datawhale

1.2 日期和时间类型

表示时间值的日期和时间类型为DATETIME、DATE、TIMESTAMP、TIME和YEAR。

每个时间类型有一个有效值范围和一个"零"值,当指定不合法的MySQL不能表示的值时使用"零"值。

TIMESTAMP类型有专有的自动更新特性。
MySQL打卡3-表的增删改查@Datawhale

1.3 字符串类型

字符串类型指CHAR、VARCHAR、BINARY、VARBINARY、BLOB、TEXT、ENUM和SET。该节描述了这些类型如何工作以及如何在查询中使用这些类型。
MySQL打卡3-表的增删改查@Datawhale
CHAR 和 VARCHAR 类型类似,但它们保存和检索的方式不同。它们的最大长度和是否尾部空格被保留等方面也不同。在存储或检索过程中不进行大小写转换。

BINARY 和 VARBINARY 类似于 CHAR 和 VARCHAR,不同的是它们包含二进制字符串而不要非二进制字符串。也就是说,它们包含字节字符串而不是字符字符串。这说明它们没有字符集,并且排序和比较基于列值字节的数值值。

BLOB 是一个二进制大对象,可以容纳可变数量的数据。有 4 种 BLOB 类型:TINYBLOB、BLOB、MEDIUMBLOB 和 LONGBLOB。它们区别在于可容纳存储范围不同。

有 4 种 TEXT 类型:TINYTEXT、TEXT、MEDIUMTEXT 和 LONGTEXT。对应的这 4 种 BLOB 类型,可存储的最大长度不同,可根据实际情况选择。
MySQL 5.0 以上的版本:

1、一个汉字占多少长度与编码有关:
UTF-8:一个汉字=3个字节
GBK:一个汉字=2个字节
2、varchar(n) 表示 n 个字符,无论汉字和英文,Mysql 都能存入 n 个字符,仅是实际字节长度有所区别
3、MySQL 检查长度,可用 SQL 语言来查看:

select LENGTH(fieldname) from tablename

2. 用SQL语句创建表

mysql> CREATE TABLE World3 (
	--设置表名称
    -> name VARCHAR(50) NOT NULL PRIMARY KEY,
    --设置列名、列类型、列大小、约束、主键
    -> continent VARCHAR(50) NOT NULL,
    -> area INT NOT NULL,
    -> population INT NOT NULL,
    -> gdp INT NOT NULL
    -> );
Query OK, 0 rows affected (0.02 sec)

3. 增

--插入数据,不指定列名
mysql> INSERT INTO World3 VALUES( 'Afghanistan', 'Asia',652230,25500100,20343000);
Query OK, 1 row affected (0.01 sec)
--插入数据,指定列名
mysql> INSERT INTO World3 (name, continent, area, population, gdp) VALUES( 'Angola' , 'Africa' ,1246700,20609294,100990000);
Query OK, 1 row affected (0.00 sec)

4. 删

--DELETE
mysql> select * from World3;
+-------------+-----------+---------+------------+-----------+
| name        | continent | area    | population | gdp       |
+-------------+-----------+---------+------------+-----------+
| Afghanistan | Asia      |  652230 |   25500100 |  20343000 |
| Albania     | Europe    |   28748 |    2831741 |  12960000 |
| Angola      | Africa    | 1246700 |   20609294 | 100990000 |
+-------------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)

mysql> delete from World3 where name = 'Angola';
Query OK, 1 row affected (0.01 sec)

mysql> select * from World3;
+-------------+-----------+--------+------------+----------+
| name        | continent | area   | population | gdp      |
+-------------+-----------+--------+------------+----------+
| Afghanistan | Asia      | 652230 |   25500100 | 20343000 |
| Albania     | Europe    |  28748 |    2831741 | 12960000 |
+-------------+-----------+--------+------------+----------+
2 rows in set (0.00 sec)
--TRUNCATE
mysql> TRUNCATE table World3;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from World3;
Empty set (0.00 sec)
--DROP
mysql> Drop table World3;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from World3;
ERROR 1146 (42S02): Table 'yiibaidb.world3' doesn't exist
------------------------------------------------------
--MySql的Delete、Truncate、Drop区别
相同点:
  truncate 和不带 where 子句的 delete,以及 drop 都会删除表内的数据
 不同点:
  1.     truncatedelete 只删除数据不删除表的结构(定义)
  drop 语句将删除表的结构被依赖的约束(constrain)、触发器(trigger)、索引(index);依赖于该表的存储过程/函数将保留,但是变为 invalid 状态。
  2.     delete 语句是数据库操作语言(dml),这操作会放到rollback segement 中,事务提交之后才生效;如果有相应的 trigger,执行的时候将被触发。
  truncatedrop 是数据库定义语言(ddl),操作立即生效,原数据不放到 rollback segment 中,不能回滚,操作不触发 trigger。
  3.    delete 语句不影响表所占用的 extent,高水线(high watermark)保持原位置不动
  显然 drop 语句将表所占用的空间全部释放。
  truncate 语句缺省情况下见空间释放到 minextents个 extent,除非使用reuse storage;truncate 会将高水线复位(回到最开始)。
  4.   速度,一般来说: drop> truncate > delete
  5.  安全性:小心使用 droptruncate,尤其没有备份的时候.否则哭都来不及
  使用上,想删除部分数据行用 delete,注意带上where子句. 回滚段要足够大.
  想删除表,当然用 drop
  想保留表而将所有数据删除,如果和事务无关,用truncate即可。如果和事务有关,或者想触发trigger,还是用delete。
  如果是整理表内部的碎片,可以用truncate跟上reuse stroage,再重新导入/插入数据。

5. 改

--修改列名
mysql> select * from World3;
+-------------+-----------+---------+------------+-----------+
| name        | continent | area    | population | gdp       |
+-------------+-----------+---------+------------+-----------+
| Afghanistan | Asia      |  652230 |   25500100 |  20343000 |
| Albania     | Europe    |   28748 |    2831741 |  12960000 |
| Angola      | Africa    | 1246700 |   20609294 | 100990000 |
+-------------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)

mysql> alter table World3 change  column name name_new varchar(50);
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from World3;
+-------------+-----------+---------+------------+-----------+
| name_new    | continent | area    | population | gdp       |
+-------------+-----------+---------+------------+-----------+
| Afghanistan | Asia      |  652230 |   25500100 |  20343000 |
| Albania     | Europe    |   28748 |    2831741 |  12960000 |
| Angola      | Africa    | 1246700 |   20609294 | 100990000 |
+-------------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)
--修改列数据类型 - V1
mysql> alter table World3 modify name_new char(30);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe World3;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| name_new   | char(30)    | NO   | PRI |         |       |
| continent  | varchar(50) | NO   |     | NULL    |       |
| area       | int(11)     | NO   |     | NULL    |       |
| population | int(11)     | NO   |     | NULL    |       |
| gdp        | int(11)     | NO   |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
--修改列数据类型 - V2
mysql> alter table World3 change name_new address  char(40);
Query OK, 3 rows affected (0.04 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> describe World3;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| address    | char(40)    | NO   | PRI |         |       |
| continent  | varchar(50) | NO   |     | NULL    |       |
| area       | int(11)     | NO   |     | NULL    |       |
| population | int(11)     | NO   |     | NULL    |       |
| gdp        | int(11)     | NO   |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
--修改表中的数据
mysql> select * from World3;
+-------------+-----------+---------+------------+-----------+
| country     | continent | area    | population | gdp       |
+-------------+-----------+---------+------------+-----------+
| Afghanistan | Asia      |  652230 |   25500100 |  20343000 |
| Albania     | Europe    |   28748 |    2831741 |  12960000 |
| Angola      | Africa    | 1246700 |   20609294 | 100990000 |
+-------------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)

mysql> UPDATE World3 SET country ='China',area = 9600000 WHERE country = 'Afghanistan';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from World3;
+---------+-----------+---------+------------+-----------+
| country | continent | area    | population | gdp       |
+---------+-----------+---------+------------+-----------+
| Albania | Europe    |   28748 |    2831741 |  12960000 |
| Angola  | Africa    | 1246700 |   20609294 | 100990000 |
| China   | Asia      | 9600000 |   25500100 |  20343000 |
+---------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)
--删除行
mysql> select * from World3;
+---------+-----------+---------+------------+-----------+
| country | continent | area    | population | gdp       |
+---------+-----------+---------+------------+-----------+
| Albania | Europe    |   28748 |    2831741 |  12960000 |
| Angola  | Africa    | 1246700 |   20609294 | 100990000 |
| China   | Asia      | 9600000 |   25500100 |  20343000 |
+---------+-----------+---------+------------+-----------+
3 rows in set (0.00 sec)

mysql> DELETE FROM World3 WHERE country = 'China';
Query OK, 1 row affected (0.00 sec)

mysql> select * from World3;
+---------+-----------+---------+------------+-----------+
| country | continent | area    | population | gdp       |
+---------+-----------+---------+------------+-----------+
| Albania | Europe    |   28748 |    2831741 |  12960000 |
| Angola  | Africa    | 1246700 |   20609294 | 100990000 |
+---------+-----------+---------+------------+-----------+
2 rows in set (0.00 sec)
--删除列
mysql> select * from World3;
+---------+-----------+---------+------------+-----------+
| country | continent | area    | population | gdp       |
+---------+-----------+---------+------------+-----------+
| Albania | Europe    |   28748 |    2831741 |  12960000 |
| Angola  | Africa    | 1246700 |   20609294 | 100990000 |
+---------+-----------+---------+------------+-----------+
2 rows in set (0.00 sec)

mysql> alter table World3 drop column gdp;
Query OK, 2 rows affected (0.05 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from World3;
+---------+-----------+---------+------------+
| country | continent | area    | population |
+---------+-----------+---------+------------+
| Albania | Europe    |   28748 |    2831741 |
| Angola  | Africa    | 1246700 |   20609294 |
+---------+-----------+---------+------------+
2 rows in set (0.00 sec)
--新建列
mysql> alter table World3 add  column gdp_2 int(11);
Query OK, 2 rows affected (0.03 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from World3;
+---------+-----------+---------+------------+-------+
| country | continent | area    | population | gdp_2 |
+---------+-----------+---------+------------+-------+
| Albania | Europe    |   28748 |    2831741 |  NULL |
| Angola  | Africa    | 1246700 |   20609294 |  NULL |
+---------+-----------+---------+------------+-------+
2 rows in set (0.00 sec)

--新建行
mysql> INSERT INTO World3 VALUES( 'Japan' , 'Asia' ,123456,10000000,98000000);
Query OK, 1 row affected (0.01 sec)

mysql> select * from World3;
+---------+-----------+---------+------------+----------+
| country | continent | area    | population | gdp_2    |
+---------+-----------+---------+------------+----------+
| Albania | Europe    |   28748 |    2831741 |     NULL |
| Angola  | Africa    | 1246700 |   20609294 |     NULL |
| Japan   | Asia      |  123456 |   10000000 | 98000000 |
+---------+-----------+---------+------------+----------+
3 rows in set (0.00 sec)

6. 作业

项目三:超过5名学生的课(难度:简单)
创建如下所示的courses 表 ,有: student (学生) 和 class (课程)。
例如,表:
+---------+------------+
| student | class      |
+---------+------------+
| A       | Math       |
| B       | English    |
| C       | Math       |
| D       | Biology    |
| E       | Math       |
| F       | Computer   |
| G       | Math       |
| H       | Math       |
| I       | Math       |
| A      | Math       |
+---------+------------+

编写一个 SQL 查询,列出所有超过或等于5名学生的课。
应该输出:
+---------+
| class   |
+---------+
| Math    |
+---------+
Note:
学生在每个课中不应被重复计算。
--代码
mysql> CREATE TABLE courses (
    -> student CHAR(1) NOT NULL ,
    -> class VARCHAR(20) NOT NULL
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> describe courses;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| student | char(1)     | NO   |     | NULL    |       |
| class   | varchar(20) | NO   |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> insert into courses VALUES('D', 'Biology');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('E', 'Math');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('F', 'Computer');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('G', 'Math');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('H', 'Math');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('I', 'Math');
Query OK, 1 row affected (0.01 sec)

mysql> insert into courses VALUES('A', 'Math');
Query OK, 1 row affected (0.01 sec)

mysql> select * from courses;
+---------+----------+
| student | class    |
+---------+----------+
| A       | Math     |
| B       | English  |
| C       | Math     |
| D       | Biology  |
| E       | Math     |
| F       | Computer |
| G       | Math     |
| H       | Math     |
| I       | Math     |
| A       | Math     |
+---------+----------+
10 rows in set (0.00 sec)
mysql> select class from courses group by class having count(distinct student) >= 5;
+-------+
| class |
+-------+
| Math  |
+-------+
1 row in set (0.00 sec)

项目四:交换工资(难度:简单)
创建一个 salary表,如下所示,有m=男性 和 f=女性的值 。
例如:
| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | m   | 2500   |
| 2  | B    | f   | 1500   |
| 3  | C    | m   | 5500   |
| 4  | D    | f   | 500    |

交换所有的 f 和 m 值(例如,将所有 f 值更改为 m,反之亦然)。要求使用一个更新查询,并且没有中间临时表。
运行你所编写的查询语句之后,将会得到以下表:
| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | f   | 2500   |
| 2  | B    | m   | 1500   |
| 3  | C    | f   | 5500   |
| 4  | D    | m   | 500    |
#代码
mysql> CREATE TABLE salary (
    -> id int(10) not null ,
    -> name VARCHAR(20) not null,
    -> sex char(1) not null,
    -> salary int(10)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> describe salary;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(10)     | NO   |     | NULL    |       |
| name   | varchar(20) | NO   |     | NULL    |       |
| sex    | char(1)     | NO   |     | NULL    |       |
| salary | int(10)     | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

mysql> insert into salary values(1,'A','m',2500);
Query OK, 1 row affected (0.01 sec)

mysql> insert into salary values(2,'B','f',1500);
Query OK, 1 row affected (0.01 sec)

mysql> insert into salary values(3,'C','m',5500);
Query OK, 1 row affected (0.01 sec)

mysql> insert into salary values(4,'D','f',500);
Query OK, 1 row affected (0.01 sec)
mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | m   |   2500 |
|  2 | B    | f   |   1500 |
|  3 | C    | m   |   5500 |
|  4 | D    | f   |    500 |
+----+------+-----+--------+
4 rows in set (0.00 sec)

mysql> UPDATE `salary` SET `sex` = (
    ->     CASE `sex`
    ->			WHEN 'm' THEN 'f'
    -> 			WHEN 'f' THEN 'm'
    -> 			END);
Query OK, 4 rows affected (0.00 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | f   |   2500 |
|  2 | B    | m   |   1500 |
|  3 | C    | f   |   5500 |
|  4 | D    | m   |    500 |
+----+------+-----+--------+
4 rows in set (0.00 sec)
其他解法学习:
mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | m   |   2500 |
|  2 | B    | f   |   1500 |
|  3 | C    | m   |   5500 |
|  4 | D    | f   |    500 |
+----+------+-----+--------+
4 rows in set (0.00 sec)

mysql> update salary
    -> set sex=IF(sex='f','m','f');
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | f   |   2500 |
|  2 | B    | m   |   1500 |
|  3 | C    | f   |   5500 |
|  4 | D    | m   |    500 |
+----+------+-----+--------+
4 rows in set (0.00 sec)

总结

  • 熟悉增删改查语句
  • 复习having用法
  • update+case when 用法

参考文章:
MySQL 数据类型 | 菜鸟教程
mysql delete/drop/ttruncate区别