MySQL学习总结之路（第二章：表）
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目录
mysql学习总结之路（第三章：数据类型）
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1、创建表
1.1、创建表基本语法：
CREATE TABLE tablename (column_name_1 column_type_1 constraints， column_name_2 column_type_2 constraints ， ……）
column_name 是列的名字
column_type 是列的数据类型
contraints 是这个列的约束条件
1.1.1、创建一张简单的表
mysql> create table orders (ordername varchar(10),createtime date,ordermoney decimal(10,2),ordernumber int(2)); Query OK, 0 rows affected (0.23 sec)
1.1.2、查看创建表定义
1.1.2.1、结构化定义
mysql> desc orders; +-------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------+---------------+------+-----+---------+-------+ | ordername | varchar(10) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumber | int(2) | YES | | NULL | | +-------------+---------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
1.1.2.2、表详细定义
查看详细的表定义
mysql> show create table orders \\G; *************************** 1. row *************************** Table: orders Create Table: CREATE TABLE `orders` ( `ordername` varchar(10) DEFAULT NULL, `createtime` date DEFAULT NULL, `ordermoney` decimal(10,2) DEFAULT NULL, `ordernumber` int(2) DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=latin1 1 row in set (0.00 sec) ERROR: No query specified
由此可以看到表的 ENGINE（存储引擎）是InnoDB
CHARSET（字符集）是Latin1
“\\G”选项的含义是使得记录能够按照字段竖着排列，对于内容比较长的记录更易于显示。
2、删除表
命令：
DROP TABLE tablename
删除orders：
mysql> drop table orders -> ; Query OK, 0 rows affected (0.14 sec)
3、修改表
3.1、修改表类型命令：
ALTER TABLE tablename MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
例：修改表 orders 的 name 字段定义，将 varchar(10)改为 varchar(20)：
mysql> alter table orders modify ordername varchar(20); Query OK, 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc orders; +-------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------+---------------+------+-----+---------+-------+ | ordername | varchar(20) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumber | int(2) | YES | | NULL | | +-------------+---------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
3.2、字段改名命令：
ALTER TABLE tablename CHANGE [COLUMN] old_col_name column_definition [FIRST|AFTER col_name]
例：表 orders 上将ordernumber修改为ordernumbers
mysql> alter table orders change column ordernumber ordernumbers int(4); Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc orders; +--------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+---------------+------+-----+---------+-------+ | ordername | varchar(20) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumbers | int(4) | YES | | NULL | | +--------------+---------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
特别说明：change 和 modify 都可以修改表的定义，不同的是 change 后面需要写两次列名，不方便。但是 change 的优点是可以修改列名称，modify 则不能。
3.3、增加表字段命令：
ALTER TABLE tablename ADD [COLUMN] column_definition [FIRST | AFTER col_name]
例：表 orders 上新增加字段 username，类型为 varchar(3)：
mysql> alter table orders add column username varchar(30); Query OK, 0 rows affected (0.39 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc orders; +-------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------+---------------+------+-----+---------+-------+ | ordername | varchar(20) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumber | int(2) | YES | | NULL | | | username | varchar(30) | YES | | NULL | | +-------------+---------------+------+-----+---------+-------+ 5 rows in set (0.00 sec)
3.4、删除表列字段命令
ALTER TABLE tablename DROP [COLUMN] col_name
例：表 orders 上删除字段 username：
mysql> alter table orders drop column username; Query OK, 0 rows affected (0.53 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc orders; +-------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------+---------------+------+-----+---------+-------+ | ordername | varchar(20) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumber | int(2) | YES | | NULL | | +-------------+---------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
3.5、表改名命令
ALTER TABLE tablename RENAME [TO] new_tablename
例：表 orders 名字改为goodsorders
mysql> alter table orders rename goodsorders; Query OK, 0 rows affected (0.16 sec) mysql> desc orders; ERROR 1146 (42S02): Table \'ordermanage.orders\' doesn\'t exist mysql> desc goodsorders; +--------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+---------------+------+-----+---------+-------+ | ordername | varchar(20) | YES | | NULL | | | createtime | date | YES | | NULL | | | ordermoney | decimal(10,2) | YES | | NULL | | | ordernumbers | int(4) | YES | | NULL | | +--------------+---------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
4、DML 语句
插入（insert）、查询（select）、更新（update）、删除（delete）
4.1、插入记录 命令
INSERT INTO tablename (field1,field2,……fieldn) VALUES(value1,value2,……valuesn);
例：表 goodsorders 中插入一条记录，ordername 为zhang，createtime为2021-05-12，ordermoney为100.00,ordernumbers为：1
mysql> insert into goodsorders (ordername,createtime,ordermoney,ordernumbers) values(\'zhang\',\'2021-05-12\',100.00,1); Query OK, 1 row affected (0.03 sec)
也可以省略(field1,field2,……fieldn)这一部分
mysql> insert into goodsorders values(\'zhang1\',\'2021-05-12\',1001.00,11); Query OK, 1 row affected (0.05 sec)
4.2、查看插入数据命名
4.2.1、查询全部
SELECT * FROM tablename [WHERE CONDITION]
例：查看goodsorders中所有插入数据
mysql> select * from goodsorders; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | zhang | 2021-05-12 | 100.00 | 1 | | zhang1 | 2021-05-12 | 1001.00 | 11 | +-----------+------------+------------+--------------+ 2 rows in set (0.00 sec)
其中“*”表示要将所有的记录都选出来
4.2.2、查询不重复记录命令关键字
distinct
例：查询非goodsorders中非重复创建时间（createtime）的数据
mysql> select * from goodsorders; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | zhang | 2021-03-11 | 50.00 | 1 | | li | 2020-05-12 | 70.00 | 15 | | li | 2020-03-12 | 70.00 | 15 | | li | 2020-03-11 | 70.00 | 15 | | li | 2021-03-11 | 70.00 | 15 | +-----------+------------+------------+--------------+ 5 rows in set (0.00 sec) mysql> select distinct createtime from goodsorders; +------------+ | createtime | +------------+ | 2021-03-11 | | 2020-05-12 | | 2020-03-12 | | 2020-03-11 | +------------+ 4 rows in set (0.00 sec)
由此可以看到，将重复的一条时间数据2021-03-11去掉了
4.2.3、多条件查询关键字
where 后面的条件是一个字段的‘=’比较，还可以使用>、<、>=、<=、!=等比较运算符； 多个条件之间还可以使用 or、and 等逻辑运算符进行多条件联合查询，
例：查询非goodsorders中 ordername=\'li\'并且createtime为2020-03-11
mysql> select * from goodsorders where ordername=\'li\'and createtime =\'2020-03-11\'; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | li | 2020-03-11 | 70.00 | 15 | +-----------+------------+------------+--------------+ 1 row in set (0.00 sec)
4.2.4、排序查询命名
SELECT * FROM tablename [WHERE CONDITION] [ORDER BY field1 [DESC|ASC] ， field2 [DESC|ASC]，……fieldn [DESC|ASC]]
例：把 goodsorders表中的记录按照创建时间高低进行排序显示
mysql> select * from goodsorders order by createtime; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | li | 2020-03-11 | 70.00 | 15 | | li | 2020-03-12 | 70.00 | 15 | | li | 2020-05-12 | 70.00 | 15 | | zhang | 2021-03-11 | 50.00 | 1 | | li | 2021-03-11 | 70.00 | 15 | +-----------+------------+------------+--------------+ 5 rows in set (0.01 sec)
4.2.5、显示一部分，而不是全部，指令
SELECT ……[LIMIT offset_start,row_count]
offset_start 表示记录的起始偏移量
row_count 表示显示的行数
例如1：显示 goodsorders表中按照 createtiem 排序后的前 3 条记录：
mysql> select * from goodsorders order by createtime limit 3; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | li | 2020-03-11 | 70.00 | 15 | | li | 2020-03-12 | 70.00 | 15 | | li | 2020-05-12 | 70.00 | 15 | +-----------+------------+------------+--------------+ 3 rows in set (0.00 sec)
例如2：如果要显示 goodsorders表中按照 createtiem 排序后 从第二条记录开始，显示3条数据：
mysql> select * from goodsorders order by createtime limit 2,3; +-----------+------------+------------+--------------+ | ordername | createtime | ordermoney | ordernumbers | +-----------+------------+------------+--------------+ | li | 2020-05-12 | 70.00 MySQL学习总结之路（第三章：数据类型）[原创]java WEB学习笔记61：Struts2学习之路--通用标签 property，uri，param,set,push,if-else,itertor,sort,date,a标签等(代码片段