一、 数据库介绍
1、什么是数据库?
数据库(Database)是按照数据结构来组织、存储和管理数据的仓库。每个数据库都有一个或多个不同的API用于创建,访问,管理,搜索和复制所保存的数据。我们也可以将数据存储在文件中,但是在文件中读写数据速度相对较慢。所以,现在我们使用关系型数据库管理系统(RDBMS)来存储和管理的大数据量。所谓的关系型数据库,是建立在关系模型基础上的数据库,借助于集合代数等数学概念和方法来处理数据库中的数据。
RDBMS即关系数据库管理系统(Relational Database Management System)的特点: ①数据以表格的形式出现 ②每行为各种记录名称 ③每列为记录名称所对应的数据域 ④许多的行和列组成一张表单 ⑤若干的表单组成database2、RDBMS 术语
①数据库: 数据库是一些关联表的集合。. ②数据表: 表是数据的矩阵。在一个数据库中的表看起来像一个简单的电子表格。 ③列: 一列(数据元素) 包含了相同的数据, 例如邮政编码的数据。 ④行:一行(=元组,或记录)是一组相关的数据,例如一条用户订阅的数据。 ⑤冗余:存储两倍数据,冗余可以使系统速度更快。(表的规范化程度越高,表与表之间的关系就越多;查询时可能经常需要在多个表之间进行连接查询;而进行连接操作会降低查询速度。例如,学生的信息存储在student表中,院系信息存储在department表中。通过student表中的dept_id字段与department表建立关联关系。如果要查询一个学生所在系的名称,必须从student表中查找学生所在院系的编号(dept_id),然后根据这个编号去department查找系的名称。如果经常需要进行这个操作时,连接查询会浪费很多的时间。因此可以在student表中增加一个冗余字段dept_name,该字段用来存储学生所在院系的名称。这样就不用每次都进行连接操作了。) ⑥主键:主键是唯一的。一个数据表中只能包含一个主键。你可以使用主键来查询数据。 ⑦外键:外键用于关联两个表。 ⑧复合键:复合键(组合键)将多个列作为一个索引键,一般用于复合索引。 ⑨索引:使用索引可快速访问数据库表中的特定信息。索引是对数据库表中一列或多列的值进行排序的一种结构。类似于书籍的目录。 ⑩参照完整性: 参照的完整性要求关系中不允许引用不存在的实体。与实体完整性是关系模型必须满足的完整性约束条件,目的是保证数据的一致性。二、MySQL管理
MySQL的安装、配置、启动、该密码等操作自己动手吧。。。
1、数据类型MySQL中定义数据字段的类型对你数据库的优化是非常重要的。MySQL支持多种类型,大致可以分为三类:数值、日期/时间和字符串(字符)类型。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,将在后面描述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。它们只是可容纳值的最大长度不同。 *有4种TEXT类型:TINYTEXT、TEXT、MEDIUMTEXT和LONGTEXT。这些对应4种BLOB类型,有相同的最大长度和存储需求
三、MySQL常用命令
以下列出了使用Mysql数据库过程中常用的命令:
USE 数据库名 :选择要操作的Mysql数据库,使用该命令后所有Mysql命令都只针对该数据库。 SHOW DATABASES: 列出 MySQL 数据库管理系统的数据库列表。 SHOW TABLES: 显示指定数据库的所有表,使用该命令前需要使用 use命令来选择要操作的数据库。 SHOW COLUMNS FROM 数据表: 显示数据表的属性,属性类型,主键信息 ,是否为 NULL,默认值等其他信息。 create database testdb charset "utf8"; 创建一个叫testdb的数据库,且让其支持中文 drop database testdb; 删除数据库 SHOW INDEX FROM 数据表:显示数据表的详细索引信息,包括PRIMARY KEY(主键)<1> 创建数据表:
1 mysql> create table student( 2 -> stu_id INT NOT NULL AUTO_INCREMENT, 3 -> name CHAR(32) NOT NULL, 4 -> age INT NOT NULL, 5 -> register_date DATE, 6 -> PRIMARY KEY ( stu_id ) 7 -> ); 8 Query OK, 0 rows affected (0.41 sec) 9 10 mysql> desc student;11 +---------------+----------+------+-----+---------+----------------+12 | Field | Type | Null | Key | Default | Extra |13 +---------------+----------+------+-----+---------+----------------+14 | stu_id | int(11) | NO | PRI | NULL | auto_increment |15 | name | char(32) | NO | | NULL | |16 | age | int(11) | NO | | NULL | |17 | register_date | date | YES | | NULL | |18 +---------------+----------+------+-----+---------+----------------+19 4 rows in set (0.01 sec)
注:AUTO_INCREMENT定义列为自增的属性,一般用于主键,数值会自动加1;如果你不想字段为 NULL 可以设置字段的属性为 NOT NULL, 在操作数据库时如果输入该字段的数据为NULL ,就会报错;PRIMARY KEY关键字用于定义列为主键。 您可以使用多列来定义主键,列间以逗号分隔
<2>插入数据表:
1 mysql> insert into student (name,age,register_date) values ("liwei",22,"2017-11-29"); 2 Query OK, 1 row affected (0.01 sec) 3 4 mysql> select * from student; 5 +--------+------------+-----+---------------+ 6 | stu_id | name | age | register_date | 7 +--------+------------+-----+---------------+ 8 | 1 | liwei | 22 | 2017-11-29 | 9 +--------+------------+-----+---------------+10 1 row in set (0.00 sec)
<3> 查询数据表:
语法:
SELECT column_name,column_name FROM table_name [WHERE Clause] [OFFSET M ][LIMIT N]
#查询语句中你可以使用一个或者多个表,表之间使用逗号(,)分割,并使用WHERE语句来设定查询条件。
#SELECT 命令可以读取一条或者多条记录。 #你可以使用星号(*)来代替其他字段,SELECT语句会返回表的所有字段数据 #你可以使用 WHERE 语句来包含任何条件。 #你可以通过OFFSET指定SELECT语句开始查询的数据偏移量。默认情况下偏移量为0。 #你可以使用 LIMIT 属性来设定返回的记录数。①一般查询
1 mysql> select * from student limit 3 offset 2; 2 +--------+------------+-----+---------------+ 3 | stu_id | name | age | register_date | 4 +--------+------------+-----+---------------+ 5 | 3 | XinZhiYu | 12 | 2015-02-08 | 6 | 4 | Alex li | 32 | 2016-08-08 | 7 | 5 | liwei | 22 | 2017-11-29 | 8 +--------+------------+-----+---------------+ 9 3 rows in set (0.00 sec)10 比如这个SQL ,limit后面跟的是3条数据,offset后面是从第3条开始读取11 12 mysql> select * from student limit 3 ,1;13 +--------+---------+-----+---------------+14 | stu_id | name | age | register_date |15 +--------+---------+-----+---------------+16 | 4 | Alex li | 32 | 2016-08-08 |17 +--------+---------+-----+---------------+18 1 row in set (0.00 sec)19 而这个SQL,limit后面是从第3条开始读,读取1条信息
②where子句:
语法:SELECT field1, field2,...fieldN FROM table_name1, table_name2...[WHERE condition1 [AND [OR]] condition2.....
以下为操作符列表,可用于 WHERE 子句中
1 mysql> select * from student where register_date > '2017-11-11' 2 -> ; 3 +--------+------------+-----+---------------+ 4 | stu_id | name | age | register_date | 5 +--------+------------+-----+---------------+ 6 | 1 | liwei | 22 | 2017-11-29 | 7 | 2 | GaoCheng | 22 | 2017-11-29 | 8 | 4 | Alex li | 32 | 2017-11-29 | 9 | 5 | liwei | 22 | 2017-11-29 |10 | 6 | liwei | 22 | 2017-11-29 |11 | 7 | liwei | 22 | 2017-11-29 |12 | 8 | liwei | 22 | 2017-11-29 |13 +--------+------------+-----+---------------+14 7 rows in set (0.00 sec)15 16 mysql> select * from student where stu_id = 3 ;17 +--------+----------+-----+---------------+18 | stu_id | name | age | register_date |19 +--------+----------+-----+---------------+20 | 3 | XinZhiYu | 12 | 2015-02-08 |21 +--------+----------+-----+---------------+22 1 row in set (0.01 sec)
③模糊查询(like):
1 mysql> select * from student where register_date like "2017-12-%"; 2 +--------+------------+-----+---------------+ 3 | stu_id | name | age | register_date | 4 +--------+------------+-----+---------------+ 5 | 1 | liwei | 22 | 2017-12-08 | 6 | 2 | GaoCheng | 22 | 2017-12-08 | 7 | 4 | Alex li | 32 | 2017-12-08 | 8 | 5 | liwei | 22 | 2017-12-08 | 9 | 6 | liwei | 22 | 2017-12-08 |10 | 7 | liwei | 22 | 2017-12-08 |11 | 8 | liwei | 22 | 2017-12-08 |12 +--------+------------+-----+---------------+13 7 rows in set, 1 warning (0.00 sec)
④查询排序:
1 SELECT field1, field2,...fieldN table_name1, table_name2... 2 ORDER BY field1, [field2...] [ASC [DESC]] 3 使用 ASC 或 DESC 关键字来设置查询结果是按升序或降序排列。 默认情况下,它是按升序排列 4 5 mysql> select * from student order by age desc; 6 +--------+------------+-----+---------------+ 7 | stu_id | name | age | register_date | 8 +--------+------------+-----+---------------+ 9 | 6 | wupeiqi | 33 | 2017-12-08 |10 | 4 | Alex li | 32 | 2017-12-08 |11 | 1 | liwei | 18 | 2017-12-08 |12 | 3 | XinZhiYu | 12 | 2017-12-08 |13 +--------+------------+-----+---------------+14 4 rows in set (0.00 sec)
⑤分组统计:
1 #语法 2 SELECT column_name, function(column_name) 3 FROM table_name 4 WHERE column_name operator value 5 GROUP BY column_name; 6 7 mysql> select * from student; 8 +--------+------------+-----+---------------+ 9 | stu_id | name | age | register_date |10 +--------+------------+-----+---------------+11 | 1 | liwei | 18 | 2017-12-29 |12 | 3 | XinZhiYu | 12 | 2015-02-08 |13 | 4 | Alex li | 32 | 2017-12-29 |14 | 6 | wupeiqi | 33 | 2017-12-29 |15 | 9 | liwei | 18 | 2017-12-29 |16 | 10 | liwei | 18 | 2016-02-08 |17 +--------+------------+-----+---------------+18 6 rows in set (0.00 sec)19 20 接下来我们使用 GROUP BY 语句 将数据表按名字进行分组,并统计每个人有多少条记录:21 mysql> select name,count(*) from student group by name;22 +------------+----------+23 | name | count(*) |24 +------------+----------+25 | Alex li | 1 |26 | liwei | 3 |27 | wupeiqi | 1 |28 | XinZhiYu | 1 |29 +------------+----------+30 4 rows in set (0.00 sec)31 32 按名字进行分组,并统计按组划分显示所有年龄的汇总33 mysql> select name,sum(age) from student group by name with rollup;34 +------------+----------+35 | name | sum(age) |36 +------------+----------+37 | Alex li | 32 |38 | liwei | 54 |39 | wupeiqi | 33 |40 | XinZhiYu | 12 |41 | NULL | 131 |42 +------------+----------+43 5 rows in set (0.00 sec)44 45 其中记录 NULL 表示所有人的年龄和46 我们可以使用 coalesce 来设置一个可以取代 NUll 的名称,coalesce 语法:47 mysql> select coalesce(name,"Total"),sum(age) from student group by name with rollup;48 +------------------------+----------+49 | coalesce(name,"Total") | sum(age) |50 +------------------------+----------+51 | Alex li | 32 |52 | liwei | 54 |53 | wupeiqi | 33 |54 | XinZhiYu | 12 |55 | Total | 131 |56 +------------------------+----------+57 5 rows in set (0.00 sec)
<4>修改数据表
1 #语法 2 UPDATE table_name SET field1=new-value1, field2=new-value2 3 [WHERE Clause] 4 5 mysql> update student set name = "wupeiqi",age = 33 where stu_id =6; 6 Query OK, 1 row affected (0.06 sec) 7 Rows matched: 1 Changed: 1 Warnings: 0 8 9 mysql>10 mysql> select * from student;11 +--------+------------+-----+---------------+12 | stu_id | name | age | register_date |13 +--------+------------+-----+---------------+14 | 1 | liwei | 22 | 2017-12-29 |15 | 2 | GaoCheng | 22 | 2017-12-29 |16 | 3 | XinZhiYu | 12 | 2015-02-08 |17 | 4 | Alex li | 32 | 2017-12-29 |18 | 5 | liwei | 22 | 2017-12-29 |19 | 6 | wupeiqi | 33 | 2017-12-29 |20 | 7 | liwei | 22 | 2017-12-29 |21 | 8 | liwei | 22 | 2017-12-29 |22 +--------+------------+-----+---------------+23 8 rows in set (0.00 sec)
<5> 删除数据表:
1 mysql> delete from student where age = 22; 2 Query OK, 4 rows affected (0.01 sec) 3 4 mysql> select * from student; 5 +--------+------------+-----+---------------+ 6 | stu_id | name | age | register_date | 7 +--------+------------+-----+---------------+ 8 | 1 | liwei | 18 | 2017-11-29 | 9 | 3 | XinZhiYu | 12 | 2015-02-08 |10 | 4 | Alex li | 32 | 2017-11-29 |11 | 6 | wupeiqi | 33 | 2017-11-29 |12 +--------+------------+-----+---------------+13 4 rows in set (0.00 sec)
<6>操作表字段(alter):
我们需要修改数据表名或者修改数据表字段时,就需要使用到MySQL ALTER命令
①插入表字段:
1 mysql> alter table student add sex enum("F","M") not null ; 2 Query OK, 6 rows affected (0.08 sec) 3 Records: 6 Duplicates: 0 Warnings: 0 4 5 mysql> select * from student; 6 +--------+------------+-----+---------------+-----+ 7 | stu_id | name | age | register_date | sex | 8 +--------+------------+-----+---------------+-----+ 9 | 1 | liwei | 18 | 2017-11-29 | F |10 | 3 | XinZhiYu | 12 | 2015-02-08 | F |11 | 4 | Alex li | 32 | 2017-11-29 | F |12 | 6 | wupeiqi | 33 | 2017-11-29 | F |13 | 9 | liwei | 18 | 2017-11-29 | F |14 | 10 | liwei | 18 | 2016-02-08 | F |15 +--------+------------+-----+---------------+-----+16 6 rows in set (0.00 sec)17 18 mysql> desc student;19 +---------------+---------------+------+-----+---------+----------------+20 | Field | Type | Null | Key | Default | Extra |21 +---------------+---------------+------+-----+---------+----------------+22 | stu_id | int(11) | NO | PRI | NULL | auto_increment |23 | name | char(32) | NO | | NULL | |24 | age | int(11) | NO | | NULL | |25 | register_date | date | YES | | NULL | |26 | sex | enum('F','M') | NO | | NULL | |27 +---------------+---------------+------+-----+---------+----------------+28 5 rows in set (0.00 sec)
②修改表字段:
1 #change修改字段名 类型 2 3 mysql> alter table student change sex gender char(32) not null default "X"; 4 Query OK, 6 rows affected (0.08 sec) 5 Records: 6 Duplicates: 0 Warnings: 0 6 7 mysql> select * from student; 8 +--------+------------+-----+---------------+--------+ 9 | stu_id | name | age | register_date | gender |10 +--------+------------+-----+---------------+--------+11 | 1 | liwei | 18 | 2017-11-29 | F |12 | 3 | XinZhiYu | 12 | 2015-02-08 | F |13 | 4 | Alex li | 32 | 2017-11-29 | F |14 | 6 | wupeiqi | 33 | 2017-11-29 | F |15 | 9 | liwei | 18 | 2017-11-29 | F |16 | 10 | liwei | 18 | 2016-02-08 | F |17 +--------+------------+-----+---------------+--------+18 6 rows in set (0.00 sec)19 20 mysql> desc student;21 +---------------+----------+------+-----+---------+----------------+22 | Field | Type | Null | Key | Default | Extra |23 +---------------+----------+------+-----+---------+----------------+24 | stu_id | int(11) | NO | PRI | NULL | auto_increment |25 | name | char(32) | NO | | NULL | |26 | age | int(11) | NO | | NULL | |27 | register_date | date | YES | | NULL | |28 | gender | char(32) | NO | | X | |29 +---------------+----------+------+-----+---------+----------------+30 5 rows in set (0.00 sec)
③删除表字段:
1 mysql> alter table student drop sex; 2 ERROR 1091 (42000): Can't DROP 'sex'; check that column/key exists 3 mysql> alter table student drop gender; 4 Query OK, 6 rows affected (0.04 sec) 5 Records: 6 Duplicates: 0 Warnings: 0 6 7 mysql> select * from student; 8 +--------+------------+-----+---------------+ 9 | stu_id | name | age | register_date |10 +--------+------------+-----+---------------+11 | 1 | liwei | 18 | 2017-11-29 |12 | 3 | XinZhiYu | 12 | 2015-02-08 |13 | 4 | Alex li | 32 | 2017-11-29 |14 | 6 | wupeiqi | 33 | 2017-11-29 |15 | 9 | liwei | 18 | 2017-11-29 |16 | 10 | liwei | 18 | 2016-02-08 |17 +--------+------------+-----+---------------+18 6 rows in set (0.00 sec)
<7>外键
一个特殊的索引,用于关联2个表,只能是指定内容1 #先创建两张表 2 mysql> create table class( 3 -> id int not null primary key, 4 -> name char(16)); 5 Query OK, 0 rows affected (0.02 sec) 6 7 CREATE TABLE `student2` ( 8 `id` int(11) NOT NULL, 9 `name` char(16) NOT NULL,10 `class_id` int(11) NOT NULL,11 PRIMARY KEY (`id`),12 KEY `fk_class_key` (`class_id`),13 CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`)14 )15 16 mysql> desc class;17 +-------+----------+------+-----+---------+-------+18 | Field | Type | Null | Key | Default | Extra |19 +-------+----------+------+-----+---------+-------+20 | id | int(11) | NO | PRI | NULL | |21 | name | char(16) | YES | | NULL | |22 +-------+----------+------+-----+---------+-------+23 2 rows in set (0.00 sec)24 25 mysql> desc student226 -> ;27 +----------+----------+------+-----+---------+-------+28 | Field | Type | Null | Key | Default | Extra |29 +----------+----------+------+-----+---------+-------+30 | id | int(11) | NO | PRI | NULL | |31 | name | char(16) | NO | | NULL | |32 | class_id | int(11) | NO | MUL | NULL | |33 +----------+----------+------+-----+---------+-------+34 3 rows in set (0.00 sec)35 36 #操作37 此时如果class 表中不存在id 1,student表也插入不了,这就叫外键约束38 mysql> insert into student2(id,name,class_id) values(1,'alex', 1);39 ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`testdb`.`student2`,40 CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`))41 42 43 mysql> insert into class(id,name) values(1,"linux");44 Query OK, 1 row affected (0.01 sec)45 46 mysql> insert into student2(id,name,class_id) values(1,'alex', 1);47 Query OK, 1 row affected (0.00 sec)48 49 50 #如果有student表中跟这个class表有关联的数据,你是不能删除class表中与其关联的纪录的51 mysql> delete from class where id =1;52 ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`testdb`.`student2`,53 CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`))
<8>NULL 值处理
我们已经知道MySQL使用 SQL SELECT 命令及 WHERE 子句来读取数据表中的数据,但是当提供的查询条件字段为 NULL 时,该命令可能就无法正常工作。
为了处理这种情况,MySQL提供了三大运算符: ①IS NULL: 当列的值是NULL,此运算符返回true。 ②IS NOT NULL: 当列的值不为NULL, 运算符返回true。 ③<=>: 比较操作符(不同于=运算符),当比较的的两个值为NULL时返回true。关于 NULL 的条件比较运算是比较特殊的。你不能使用 = NULL 或 != NULL 在列中查找 NULL 值 。在MySQL中,NULL值与任何其它值的比较(即使是NULL)永远返回false,即 NULL = NULL 返回false 。MySQL中处理NULL使用IS NULL和IS NOT NULL运算符。<9>连接(left join, right join, inner join ,full join)
我们已经学会了如果在一张表中读取数据,这是相对简单的,但是在真正的应用中经常需要从多个数据表中读取数据。
本章节我们将向大家介绍如何使用 MySQL 的 JOIN 在两个或多个表中查询数据。你可以在SELECT, UPDATE 和 DELETE 语句中使用 Mysql 的 JOIN 来联合多表查询。JOIN 按照功能大致分为如下三类: ①INNER JOIN(内连接,或等值连接):获取两个表中字段匹配关系的记录。 ②LEFT JOIN(左连接):获取左表所有记录,即使右表没有对应匹配的记录。 ③RIGHT JOIN(右连接): 与 LEFT JOIN 相反,用于获取右表所有记录,即使左表没有对应匹配的记录。 ④Full join(全连接):存在匹配,匹配显示;同时,将各个表中不匹配的数据与空数据行匹配进行显示。可以看成是左外连接与右外连接的并集。1 #给出两张表作为以下操作的的初始数据 2 A B 3 - - 4 1 3 5 2 4 6 3 5 7 4 6 8 9 ①INNER JOIN(内连接,或等值连接)10 select * from a INNER JOIN b on a.a = b.b;11 select a.*,b.* from a,b where a.a = b.b;12 13 a | b14 --+--15 3 | 316 4 | 417 18 #其实就是只显示2个表的交集。19 ②LEFT JOIN(左连接)20 select * from a LEFT JOIN b on a.a = b.b;21 22 a | b23 --+-----24 1 | null25 2 | null26 3 | 327 4 | 428 29 ③RIGHT JOIN(右连接)30 select * from a RIGHT JOIN b on a.a = b.b;31 32 a | b33 -----+----34 3 | 335 4 | 436 null | 537 null | 638 39 ④Full join(全连接)40 select * from a FULL JOIN b on a.a = b.b;41 42 a | b43 -----+-----44 1 | null45 2 | null46 3 | 347 4 | 448 null | 649 null | 550 51 #mysql 并不直接支持full join,but 总是难不到我们52 select * from a left join b on a.a = b.b UNION select * from a right join b on a.a = b.b;53 +------+------+54 | a | b |55 +------+------+56 | 3 | 3 |57 | 4 | 4 |58 | 1 | NULL |59 | 2 | NULL |60 | NULL | 5 |61 | NULL | 6 |62 +------+------+63 6 rows in set (0.00 sec)
<10>事务
MySQL事务主要用于处理操作量大,复杂度高的数据。比如说,在人员管理系统中,你删除一个人员,你即需要删除人员的基本资料,也要删除和该人员相关的信息,如信箱,文章等等,这样,这些数据库操作语句就构成一个事务!
在MySQL中只有使用了Innodb数据库引擎的数据库或表才支持事务
事务处理可以用来维护数据库的完整性,保证成批的SQL语句要么全部执行,要么全部不执行事务用来管理insert,update,delete语句一般来说,事务是必须满足4个条件(ACID): 1、原子性(Atomicity):一组事务,要么成功;要么撤回。 2、稳定性(Consistency): 有非法数据(外键约束之类),事务撤回。 3、隔离性(Isolation):事务独立运行。一个事务处理后的结果,影响了其他事务,那么其他事务会撤回。事务的100%隔离,需要牺牲速度。 4、可靠性(Durability):软、硬件崩溃后,InnoDB数据表驱动会利用日志文件重构修改。可靠性和高速度不可兼得, innodb_flush_log_at_trx_commit选项 决定什么时候吧事务保存到日志里。在Mysql控制台使用事务来操作:
1 mysql> begin; #开始一个事务2 mysql> insert into a (a) values(555);3 mysql>rollback; 回滚 , 这样数据是不会写入的4 5 #如果上面的数据没问题,就输入commit提交命令就行;
<11>索引
①普通索引
1 1.创建索引: 2 这是最基本的索引,它没有任何限制 3 CREATE INDEX indexName ON mytable(username(length)); 4 #如果是CHAR,VARCHAR类型,length可以小于字段实际长度;如果是BLOB和TEXT类型,必须指定 length 5 6 2.修改表结构: 7 ALTER mytable ADD INDEX [indexName] ON (username(length)) 8 9 3.创建表的时候直接指定:10 CREATE TABLE mytable( 11 ID INT NOT NULL, 12 username VARCHAR(16) NOT NULL,13 INDEX [indexName] (username(length))14 );15 16 4.删除索引的语法:17 DROP INDEX [indexName] ON mytable;
②唯一索引
它与前面的普通索引类似,不同的就是:索引列的值必须唯一,但允许有空值。如果是组合索引,则列值的组合必须唯一。1 1.创建索引: 2 CREATE UNIQUE INDEX indexName ON mytable(username(length)) 3 4 2.修改表结构 5 ALTER mytable ADD UNIQUE [indexName] ON (username(length)) 6 7 3.创建表的时候直接指定 8 CREATE TABLE mytable( 9 ID INT NOT NULL, 10 username VARCHAR(16) NOT NULL, 11 UNIQUE [indexName] (username(length)) 12 );13 14 4.使用ALTER 命令添加和删除索引:15 有四种方式来添加数据表的索引:16 ALTER TABLE tbl_name ADD PRIMARY KEY (column_list): 该语句添加一个主键,这意味着索引值必须是唯一的,且不能为NULL。17 ALTER TABLE tbl_name ADD UNIQUE index_name (column_list): 这条语句创建索引的值必须是唯一的(除了NULL外,NULL可能会出现多次)。18 ALTER TABLE tbl_name ADD INDEX index_name (column_list): 添加普通索引,索引值可出现多次。19 ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list):该语句指定了索引为 FULLTEXT ,用于全文索引。20 21 以下实例为在表中添加索引。22 mysql> ALTER TABLE testalter_tbl ADD INDEX (c);23 你还可以在 ALTER 命令中使用 DROP 子句来删除索引。尝试以下实例删除索引:24 mysql> ALTER TABLE testalter_tbl DROP INDEX (c);25 26 5.使用 ALTER 命令添加和删除主键27 主键只能作用于一个列上,添加主键索引时,你需要确保该主键默认不为空(NOT NULL)。实例如下:28 mysql> ALTER TABLE testalter_tbl MODIFY i INT NOT NULL;29 mysql> ALTER TABLE testalter_tbl ADD PRIMARY KEY (i);30 31 你也可以使用 ALTER 命令删除主键:32 mysql> ALTER TABLE testalter_tbl DROP PRIMARY KEY;33 删除指定时只需指定PRIMARY KEY,但在删除索引时,你必须知道索引名。34 35 6.显示索引信息:36 mysql> SHOW INDEX FROM table_name\G
select * from score where num>60;
select teacher_id,count(cname) from score group by teacher_id;
select * from courseleft join teacher on course.teacher_id=teacher_id;
select gender,count(sid) from student group by gender;
#思路:根据学生分组,使用avg获取平均值,通过having对avg进行筛选select student_id,avg(num) from score group by student_id having avg(num) > 60;
select B.student_id,student.sname,b.cc from (select student_id,avg(num) as cc from score group by student_id having avg(num) > 60) as Bleft join student on B.student_id = student.sid;
select score.student_id,student.sname,count(score.student_id),sum(score.num)from score left join student on score.student_id = student.sid group by score.student_id;
select count(tid) from teacher where tname like '李%';#//select count(1) from (select tid from teacher where tname like '李%') as B
#思路:先查到“李平老师”教的所有课ID、获取选过“李平老师”课的所有学生ID、学生表中筛选select student.sid,student.sname from student where sid not in ( select DISTINCT student_id from score where score.course_id in ( select course.cid from course left join teacher on course.teacher_id = teacher.tid where tname = '李平老师' ) group by student_id);
#思路:1、获取所有有生物课程的人(学号,成绩) - 临时表# 2、获取所有有物理课程的人(学号,成绩) - 临时表# 3、根据【学号】连接两个临时表:学号 - 物理成绩 - 生物成绩 # 4、然后再进行筛选select A.student_id,A.num,B.num from(select student_id,num as sw from score left join course on score.course_id = course.cid where course.cname = '生物') as Ainner join //为空的时候不显示(select student_id,num as wl from score left join course on score.course_id = course.cid where course.cname = '物理') as Bon A.student_id = B.student_id where A.num > B.num;
#思路:1、先查到选择001或者选择002课程的所有同学# 2、根据学生进行分组,如果学生数量等于2表示,两门均已选择select score.student_id,student.sname from scoreleft join student on score.student_id = student.sid where course_id = 1 or course_id = 2 group by student_id HAVING count(course_id) > 1;
select student_id from score where course_id in ( select cid from course left join teacher on course.teacher_id = teacher.tid where teacher.tname = "李平") group by student_id having count(course_id) = (select count(cid) from course left join teacher on course.teacher_id = teacher.tid where teacher.tname = "李平");
select sid,sname from student where sid in ( select distinct student_id from score where num < 60);
#思路:1、在分数表中根据学生进行分组,获取每一个学生选课数量# 2、如果数量 == 总课程数量,表示已经选择了所有课程select student_id,snamefrom score left join student on score.student_id = student.sidgroup by student_id HAVING count(1) < (select count(1) from course);
#思路:获取 001 同学选择的所有课程# 获取课程在其中的所有人以及所有课程# 根据学生筛选,获取所有学生信息# 再与学生表连接,获取姓名select student_id,sname, count(course_id)from score left join student on score.student_id = student.sidwhere student_id != 1 and course_id in (select course_id from score where student_id = 1) group by student_id
#思路:先找到和001的学过的所有人# 然后个数 = 001所有学科 ==》 其他人可能选择的更多select student_id,sname, count(course_id)from score left join student on score.student_id = student.sidwhere student_id != 1 and course_id in (select course_id from score where student_id = 1) group by student_id having count(course_id) = (select count(course_id) from score where student_id = 1)
#思路:1、个数相同# 2、002学过的也学过select student_id,sname from score left join student on score.student_id = student.sid where student_id in (select student_id from score where student_id != 1 group by student_id HAVING count(course_id) = (select count(1) from score where student_id = 1)) and course_id in (select course_id from score where student_id = 1) group by student_id HAVING count(course_id) = (select count(1) from score where student_id = 1)
delete from score where course_id in ( select cid from course left join teacher on course.teacher_id = teacher.tid where teacher.name = '李平')
#思路:1、由于insert 支持 :inset into tb1(xx,xx) select x1,x2 from tb2; # 2、获取所有没上过002课的所有人,获取002的平均成绩insert into score(student_id, course_id, num) select student_id,2,(select avg(num) from score where course_id = 2)from student where sid where course_id != 2
select B.student_id,(select num from score left join course on score.course_id = course.cid where course.cname = "语文" and score.student_id=B.student_id) as 语文,(select num from score left join course on score.course_id = course.cid where course.cname = "数学" and score.student_id=B.student_id) as 数学,(select num from score left join course on score.course_id = course.cid where course.cname = "英语" and score.student_id=B.student_id) as 英语,count(B.course_id),avg(B.num)from score as Bgroup by student_id desc
select course_id, max(num) as max_num, min(num) as min_num from score group by course_id
#思路:case when .. thenselect course_id, avg(num) as avgnum,sum(case when score.num > 60 then 1 else 0 END)/sum(1) as percent from score group by course_id order by avgnum asc,percent desc;
select score.course_id,course.cname,avg(if(isnull(score.num),0,score.num)),teacher.tname from scoreleft join score on score.course_id = course.cidleft join teacher on teacher.tid = course.teacher_idgroup by score.course_id
select score.sid,score.course_id,score.num,B.first_num,B.second_num from score left join(select sid, (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num, (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 2,1) as second_numfrom score as s1) as Bon score.sid =B.sidwhere score.num <= B.first_num and score.num >= B.second_num
select course.cname,count(1) from scoreleft join course on score.course_id = course.cidgroup by course_id
select course_id, count(1) from score group by course_id having count(1) > 5
select student.sid, student.sname, count(1) from scoreleft join student on score.student_id = student.sidgroup by course_id having count(1) = 1
select gender,count(1) from student group by gender
select sname from student where sname like '张%'
select sname,count(1) as count from student group by sname;
select course_id,avg(if(isnull(num), 0 ,num)) as avg from score group by course_id order by avg asc,course_id desc
select student_id,sname, avg(if(isnull(num), 0 ,num)) from score left join student on score.student_id = student.sid group by student_id
select student.sname,score.num from scoreleft join course on score.course_id = course.cidleft join student on score.student_id = student.sidwhere score.num < 60 and course.cname = '数学'
select * from score where score.student_id = 3 and score.num > 80
select count(distinct student_id) from score
select sname,num from scoreleft join student on score.student_id = student.sidwhere score.course_id in (select course.cid from course left join teacher on course.teacher_id = teacher.tid where tname='杨艳') order by num desc limit 1
select DISTINCT s1.course_id,s2.course_id,s1.num,s2.num from score as s1, score as s2 where s1.num = s2.num and s1.course_id != s2.course_id
select student_id from score group by student_id having count(student_id) > 1
select course_id,count(1) from score group by course_id having count(1) = (select count(1) from student)
select sid,student.sname from student where sid not in (select student_id from score where course_id in(select cid from course left join teacher on course.teacher_id = teacher.tid where tname = '李平')) group by sid
select student_id,count(1) from score where num < 60 group by student_id having count(1) > 2
select student_id from score where num< 60 and course_id = 4 order by num desc;
delete from score where course_id = 1 and student_id = 2