Structured Query Language 结构化查询语言 包含4种类型语句： 数据查询语言DQL-Data Query Language SELECT 数据操纵语
Structured Query Language 结构化查询语言
包含4种类型语句：
数据查询语言DQL-Data Query Language SELECT
数据操纵语言DML-Data Manipulation Language INSERT, UPDATE, DELETE
数据定义语言DDL-Data Definition Language CREATE, ALTER, DROP
数据控制语言DCL-Data Control Language GRANT REVOKE
事物控制语句TCL-Transacation Contrl Language COMMIT , ROLLBACK
DQL
select
1. 数据定义语言DDL
DDL((Data Definition Language)，，用于定义/修改/删除数据对象(如表)的数据结构，或者说，DDL语言操作的对象是数据库中的对象而非对象所包含的数据。
DDL包含以下语句：
CREATE : 在数据库中创建新的数据对象
ALTER : 修改数据库中对象的数据结构
DROP : 删除数据库中的对象
DISABLE/ENABLE TRIGGER : 修改触发器的状态
UPDATE STATISTIC : 更新表/视图统计信息
TRUNCATE TABLE : 清空表中数据
COMMENT : 给数据对象添加注释
RENAME : 更改数据对象名称
2. 数据操作语言DML
DML(Data Manipulation Language)，用于添加/修改/查询数据库中数据。
DML包含以下语句：
INSERT ：将数据插入到表或视图
DELETE ：从表或视图删除数据
SELECT ：从表或视图中获取数据
UPDATE ：更新表或视图中的数据
MERGE ： 对数据进行合并操作(插入/更新/删除)
3. 数据控制语言DCL
DCL(Data Control Language)用来向用户赋予/取消对数据对象的控制权限。
DCL包含以下语句：
GRANT : 赋予用户某种控制权限
REVOKE ：取消用户某种控制权限
4. 事务控制语言(TCL)
TCL(Transaction Control Language)用来对事务进行管理。
TCL包含以下语句：
COMMIT : 保存已完成事务动作结果
SAVEPOINT : 保存事务相关数据和状态用以可能的回滚操作
ROLLBACK : 恢复事务相关数据至上一次COMMIT操作之后
SET TRANSACTION : 设置事务选项
而其中最复杂的就是DQL数据查询语言的select
如何书写DQL_SQL
大小写不敏感,但单引和双引内的大小写是敏感的.
关键字不能缩写select不能写成sel
字符串用单引 比如 ename='SEKER'
列的别名含特殊字符用双引
可跨行,但不要将关键字和单引的内容跨行.
跨行是为了可读性,一般我们都把select子句和from子句分行写.
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