前言

因为最近要考ocp（全英文考试，所以下文也会中英夹杂，侧重oracle标准。），又要把大学学过的数据库捡起来。特开此文整理基础sql语句语法和注意事项。
所有示例代码均来自oracle官方教材。参考资料：数据库系统教程和oracle提供的官方教材。

一人书写这么多内容，难免有错，欢迎大家留言。

SQL语句概述

1970年，美国IBM研究中心的E.F.Codd提出关系模型，拉开了未来五十年经久不衰的关系数据库的帷幕。
SQL原来是指“结构化查询语句Structured Query Language”，经过不断发展，已经不表示任何具体的缩写含义，成为一个通用标准。
SQL主要包括：
1. DDL（数据定义语言 data definition language ）：定义基本表，视图，索引等结构。
2. DML（数据操作语言 data manipulation language ）：分为数据查询和数据更新（插入、删除、修改）
3. DCL（数据权限管理语句data control language）：对基本表和视图的授权、完整性规则的描述。事务控制语句也可以包括在这里。
4. 事务控制管理语句（transaction control）
5. 嵌入式SQL的使用

示例所用E-R图

不看懂也没关系，也不影响语法学习。
E-R图：Human Resources (HR) Schema人力资源计划

Oracle常用数据类型规定

CHAR： 固定长度字符串 最大长度2000 bytes
VARCHAR2： 可变长度的字符串 最大长度4000 bytes 可做索引的最大长度749
NCHAR： 根据字符集而定的固定长度字符串 最大长度2000 bytes
NVARCHAR2： 根据字符集而定的可变长度字符串 最大长度4000 bytes
DATE： 日期（日-月-年） DD-MM-YY（HH-MI-SS） 经过严格测试，无千虫问题
LONG ：超长字符串 最大长度2G（231-1） 足够存储大部头著作
NUMBER(P,S) 数字类型 P为总位数，S为小数位数

RAW： 固定长度的二进制数据 最大长度2000 bytes 可存放多媒体图象声音等
LONG RAW： 可变长度的二进制数据 最大长度2G 同上
CLOB： 字符数据 最大长度4G
BLOB： 二进制数据 最大长度4G
BFILE： 存放在数据库外的二进制数据 最大长度4G
ROWID： 数据表中记录的唯一行号 10 bytes ********.****.****格式，*为0或1

第一章 DDL：Data Definition Language

Naming Rules命名规则
Table names and column names must:
• Begin with a letter字母开始
• Be 1–30 characters long
• Contain only A–Z, a–z, 0–9, _, $, and #【命名允许使用字符】
• Not duplicate the name of another object owned by the same user【同一用户下不同对象不能命相同的名】
• Not be an Oracle server–reserved word不使用oracle保留字【例如table等常见英文单词】

基本表的创建、修改、删除

CREATE TABLE Statement
• You must have:
– The CREATE TABLE privilege
– A storage area

CREATE TABLE [schema.指定用户]table (column datatype [DEFAULT expr][, ...]) [CONSTRAINT constraint_name] constraint_type,；    //段名+数据类型+约束名+约束类型 

• You specify:
– The table name
– The column name, column data type, and column size
• Create the table:例子

CREATE TABLE dept//表名 (deptno      NUMBER(2)//字段名和数据类型,   dname     VARCHAR2(14),   loc      VARCHAR2(13) NOT NULL,//非空   create_date   DATE DEFAULT SYSDATE); 

• Confirm table creation:

DESCRIBE 表名;//查看表的结构（字段和类型） 

Constraints约束
• Constraints enforce rules at the table level.
• Constraints ensure the consistency and integrity of the database.
• The following constraint types are valid:
– NOT NULL非空
– UNIQUE唯一
– PRIMARY KEY主键
– FOREIGN KEY外键
– CHECK校验
• View a constraint in the data dictionary.数据字典中查看
• You can name a constraint or the Oracle server generates a name by using the SYS_Cn format.约束命名
• Create a constraint at either of the following times:创建约束的时间
– At the same time as the creation of the table在创建表的同时
– After the creation of the table创建表之后

//Example of a column-level constraint CREATE TABLE employees(    employee_id  NUMBER(6) CONSTRAINT emp_emp_id_pk约束名称 PRIMARY KEY,     first_name    VARCHAR2(20), ...); //Example of a table-level constraint: CREATE TABLE employees(   employee_id   NUMBER(6),    first_name    VARCHAR2(20),   job_id      VARCHAR2(10) NOT NULL, CONSTRAINT emp_emp_id_pk PRIMARY KEY (EMPLOYEE_ID指定)); 

1. NOT NULL Constraint非空约束：Ensures that null values are not permitted for the column:
2. UNIQUE Constraint唯一值，无重复：Defined at either the table level or the column level:

CREATE TABLE employees(   employee_id NUMBER(6),   last_name    VARCHAR2(25) NOT NULL,   email      VARCHAR2(25),//unique约束   salary      NUMBER(8,2),   commission_pct  NUMBER(2,2),   hire_date     DATE NOT NULL, CONSTRAINT emp_email_uk UNIQUE(email)字段名; 

3. PRIMARY KEY Constraint主键约束：非空且唯一
4. FOREIGN KEY Constraint：外键Defined at either the table level or the column level:

CREATE TABLE employees(    employee_id    NUMBER(6),    last_name     VARCHAR2(25) NOT NULL,    email       VARCHAR2(25),    salary       NUMBER(8,2),    commission_pct   NUMBER(2,2),    hire_date      DATE NOT NULL,    department_id    NUMBER(4), CONSTRAINT emp_dept_fk FOREIGN KEY (department_id)//外键：关系数据库的参照完整性，定义从表 REFERENCES departments(department_id),引用主表    CONSTRAINT emp_email_uk UNIQUE(email)); 

KEY Constraint: Keywords含有外键从表
• FOREIGN KEY: Defines the column in the child table at the table-constraint level
• REFERENCES: Identifies the table and column in the parent table
• ON DELETE CASCADE: Deletes the dependent rows in the child table when a row in the parent table is deleted
主表删除时，主从表都删除记录
• ON DELETE SET NULL: Converts dependent foreign key values to null从表改为空，主表删除
5. CHECK Constraint校验约束
• It defines a condition that each row must satisfy.
• The following expressions are not allowed:
– References to CURRVAL, NEXTVAL, LEVEL, and ROWNUM pseudocolumns
– Calls to SYSDATE, UID, USER, and USERENV functions
– Queries that refer to other values in other rows

salary NUMBER(2) CONSTRAINT emp_salary_min CHECK (salary > 0);//使用整数校验 

Violating Constraint违反约束

UPDATE  employees//更新从表employess数据 SET    department_id = 55//报错291，55在主表department中未出现 WHERE  department_id = 110; 

You cannot delete a row that contains a primary key that is used as a foreign key in another table. 

DELETE FROM departments删除主表中数据 WHERE department_id = 60；错误 

Creating a Table Using a Subquery使用子查询，用另外表添加数据
• Create a table and insert rows by combining the CREATE TABLE statement and the AS subquery option.

CREATE TABLE table   [(column, column...)] AS subquery;匹配字段 

• Match the number of specified columns to the number of subquery columns.
• Define columns with column names and default values.

Creating a Table Using a Subquery  CREATE TABLE   dept80   AS SELECT  employee_id, last_name,salary*12 ANNSAL, hire_date   FROM  employees    WHERE  department_id = 80; 

ALTER TABLE Statement修改表的结构
Use the ALTER TABLE statement to:
• Add a new column
• Modify an existing column definition
• Define a default value for the new column
• Drop a column
• Rename a column
• Change table to read-only status
Use the ALTER TABLE statement to add, modify, or drop columns:

Adding a Column增加字段
• You use the ADD clause to add columns:

ALTER TABLE dept80 ADD (job_id VARCHAR2(9)); 

• The new column becomes the last column:所有值自动设置为空
Modifying a Column修改字段
• You can change a column’s data type, size, and default value.

ALTER TABLE  dept80 MODIFY (last_name VARCHAR2(30)); 

• A change to the default value affects only subsequent insertions to the table.
Dropping a Column删除字段
Use the DROP COLUMN clause to drop columns that you no longer need from the table:

ALTER TABLE  dept80 MODIFY (last_name VARCHAR2(30)); 

SET UNUSED Option某个字段设置不可用
• You use the SET UNUSED option to mark one or more columns as unused.
• You use the DROP UNUSED COLUMNS option to remove the columns that are marked as unused.
• You can specify the ONLINE keyword to indicate that DML operations on the table will be allowed while marking the column or columns UNUSED.

Read-Only Tables只读表
You can use the ALTER TABLE syntax to:
• Put a table in read-only mode, which prevents DDL or DML changes during table maintenance
• Put the table back into read/write mode

ALTER TABLE employees READ ONLY;//设定为只读表，不允许修改 

--perform table maintenance and then return table back to read/write mode 

ALTER TABLE employees READ WRITE; 

Dropping a Table删除表
• Moves a table to the recycle bin
• Removes the table and all its data entirely if the PURGE clause is specified
• Invalidates dependent objects and removes object privileges on the table

DROP TABLE 表名; //结构和数据全部删除,存在回收站，还可以闪回操作 

View视图

创建在一个表上的视窗，不存放数据，逻辑视窗
Advantage of views：
to restrict data access
to make complex queries easy
to provide data independence
to present different views of the same data
两种视图：简单视图和复杂视图

Create view创建视图
You embed a subquery in the CREATE VIEW statement：

CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view识图名 [(alias[， alias] ...) ] AS  subquey子查询 [WITH CHECK OPTION [CONSTRAINT constraint] ] [WITH READ  ONLY [CONSTRAINT constraint]]; 

· Create the EMPVU80 view，which contains details of the employees in department 80：

CREATE VIEW  empvu80  AS SELECT    employee_id， last_name，salary FROM employees WHERE department id=80； 

· Describe the structure of the view by using the SQL*Plus DESCRIBE command：

DESCRIBE emp vu 80

The subquery can contain complex SELECT syntax
Modify view 修改视图：
create or replace view 识图名 as 子查询
· Modify the EMPVU80 view by using a CREATE OR REPLACE VIEW clause.Add an alias for each columnname：

· Column aliases in the CREATE OR REPLACE VIEW clause are listed in the same order as the columns in the subquery.
Complex view 复杂视图：使用子查询连接
Create a complex view that contains group functions to display values from two tables：

不建议使用DML语句修改视图，WITH READ ONLY；

· You can usually perform DML operations on simple views.
· You can not remove a row if the view contains the following：

• Group functions
• A GROUP BY clause
• The DISTINCT keyword
• The pseudo column ROWNUM keyword
  You can not modify data in a view if it contains：
  · Group functions
  · A GROUP BY clause
  · The DISTINCT keyword
  · The pseudo column ROWNUM keyword
  · Columns defined by expressions
  You can not add data through a view if the view includes：
  · The pseudo column ROWNUM keyword
  · Group functions
  · A GROUP BY clause
  · The DISTINCT keyword
  · Columns defined by expressions
  · NOT NULL columns in the base tables that are not selected by the view

Drop view 视图名；删除视图 

Sequence序列号

A sequence：
Can automatically generate unique numbers
is a shareable object
Can be used to create a primary key value
Replaces application code
Speeds up the efficiency of accessing sequence values when cached in memory

CREATE SEQUENCE 序列名 [INCREMENT BY n]增长值 [START WITH n]第一个值 [(MAXVALUE n  |  NOMAXVALUE)]最大 [(MINVALUE n  |  NOMINVALUE)]是否包含最小 [(CYCLE |  NOCYCLE) ]循环使用,不建议 [(CACHE n  |  NOCACHE) ]内存一次性装在几个，系统崩溃后出错 

引用序列号NEXTVAL and CURRVAL Pseudocolun
· NEXTVAL returns the next available sequence value.It returns a unique value everytime it is referenced， even for different users.
· CURRVAL obtains the current sequence value.
· NEXTVAL must be issued for that sequence before CURRVAL contains a value.
Insert a new department named“Support”in location ID 2500：

View the current value for the DEPT_DEPTID_SEQ sequence：

Caching Sequence Values
· Caching sequence values in memory gives faster access to those values.
· Gaps in sequence values can occur when:序列号断层
—A rollback occurs
—The system crashes
—A sequence is used in another table
Guidelines for Modifying a Sequence

ALTER SEQUENCE 序列名   [INCREMENT BY n] [START WITH n]不能修改初始值，必须先删除   [(MAXVALUE n  |  NOMAXVALUE)] [(MINVALUE n  |  NOMINVALUE)] [(CYCLE |  NOCYCLE) ] [(CACHE n  |  NOCACHE) ] 

· You must be the owner or have the ALTER privilege for the sequence.
· Only future sequence numbers are affected.
· The sequence must be dropped and re-created to restart the sequence at a different number.
· Some validation is performed.
· To remove a sequence， use the DROP statement：

DROP SEQUENCE dept_dept id_seq；

Index索引

An index：树状结构，单独存储，
· is a schema object
· Can be used by the Oracle server to speed up the retrieval of rows by using a pointer
· Can reduce disk input/output(I/O) by using a rapid path access method to locate data quickly
· is independent of the table that it indexes
· is used and maintained automatically by the oracle server
· 自动创建：主键或unique的字段
Automatically：A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition.
· 手动创建Manually：Users can create nonunique indexes on columns to speed up access to the rows.

create [unique][bitmap]index 索引名   on table表名(column字段,…)和表的字段对应 

Drop index 索引名；删除 

Synonym同义词

复杂的对象设置一个简单的名称
Simplify access to objects by creating a synonym(another name for an object) .With synonyms， you can：
· Create an easier reference to a table that is owned by another user
· Shorten lengthy object names

CREATE [PUBLIC]  SYNONYM   synonym FOR object；  DROP SYNONYM 同义词；删除