7天快速掌握MySQL-DAY3

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发布于 2019-12-26 10:47:24

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发布于 2019-12-26 10:47:24

文章被收录于专栏：datartisan

首先公布一下DAY2的作业答案。

项目一：

mysql> select distinct Email from email
    -> group by Email
    -> having count(Email) > 1;
+---------+
| Email   |
+---------+
| a@b.com |
+---------+
1 row in set (0.01 sec)

项目二：

mysql> select name, population, area
    -> from World
    -> where area > 3000000 or (population > 25000000 and gdp > 20000000);
+-------------+------------+---------+
| name        | population | area    |
+-------------+------------+---------+
| Afghanistan |   25500100 |  652230 |
| Algeria     |   37100000 | 2381741 |
+-------------+------------+---------+
2 rows in set (0.00 sec)

你正确查询出结果了吗？咱们继续DAY3。

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。

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。

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。

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 删除指定条目，保留表结构
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  截断表，清空数据，保留表结构，速度快
mysql> TRUNCATE table World3;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from World3;
Empty set (0.00 sec)
--DROP 删除数据内容及数据表，慎用！
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. truncate 和 delete 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。

　　truncate、drop 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 drop 和 truncate，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。

delete延伸

# 单表删除内容
mysql> select * from students2;
+-----+-------+--------+---------+
| sid | sname | gender | dept_id |
+-----+-------+--------+---------+
|   1 | 555   | 1      |       2 |
|   2 | 555   | 2      |       2 |
|   3 | 555   | 3      |       2 |
|   4 | 555   | 3      |       1 |
|   5 | 555   | 1      |       1 |
+-----+-------+--------+---------+
5 rows in set (0.00 sec)

mysql> delete from students2 where sid=4;
Query OK, 1 row affected (0.05 sec)

mysql> select * from students2;
+-----+-------+--------+---------+
| sid | sname | gender | dept_id |
+-----+-------+--------+---------+
|   1 | 555   | 1      |       2 |
|   2 | 555   | 2      |       2 |
|   3 | 555   | 3      |       2 |
|   5 | 555   | 1      |       1 |
+-----+-------+--------+---------+
4 rows in set (0.00 sec)

# 多表删除内容
mysql> select * from person;
+----------+-----------+----------+
| personid | firstname | lastname |
+----------+-----------+----------+
|      101 | ming      | Yao      |
|      102 | lei       | Wu       |
|      103 | zhi       | Zheng    |
+----------+-----------+----------+
3 rows in set (0.00 sec)

mysql> select * from address;
+-----------+----------+----------+-------+
| addressid | personid | city     | state |
+-----------+----------+----------+-------+
|      8000 |     9999 | xian     | UA    |
|      8008 |      102 | beijing  | AB    |
|      9009 |      103 | hangzhou | AC    |
+-----------+----------+----------+-------+
3 rows in set (0.00 sec)

mysql> delete p1, a1
    -> from person p1 inner join address a1
    -> where p1.personid = a1.personid;
Query OK, 4 rows affected (0.01 sec)

mysql> select * from person;
+----------+-----------+----------+
| personid | firstname | lastname |
+----------+-----------+----------+
|      101 | ming      | Yao      |
+----------+-----------+----------+
1 row in set (0.00 sec)

mysql> select * from address;
+-----------+----------+------+-------+
| addressid | personid | city | state |
+-----------+----------+------+-------+
|      8000 |     9999 | xian | UA    |
+-----------+----------+------+-------+
1 row in set (0.00 sec)

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

6.1 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。
--建表
mysql> CREATE TABLE courses (
    -> student CHAR(1) NOT NULL ,
    -> class VARCHAR(20) NOT NULL
    -> );
Query OK, 0 rows affected (0.01 sec)
--查看数据结构
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)
--插入数据
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)
--核查插入结果
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)

6.2 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    |
--建表sql
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)
--查询数据结构
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)
--插入数据
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)
--核查插入结果
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)

答案DAY4揭晓。

喜欢就给个好看吧~~

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原始发表：2019-03-09，如有侵权请联系 cloudcommunity@tencent.com 删除

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