前言基礎sql語句大全

前言
SQL語句概述
示例所用E-R圖
Oracle經常使用數據類型規定
第一章 DDL：Data Definition Language
基本表的建立、修改、刪除
View視圖
Sequence序列號
Index索引
Synonym同義詞
前言
由於最近要考ocp（全英文考試，因此下文也會中英夾雜，側重oracle標準。），又要把大學學過的數據庫撿起來。特開此文整理基礎sql語句語法和注意事項。
全部示例代碼均來自oracle官方教材。參考資料：數據庫系統教程和oracle提供的官方教材。

一人書寫這麼多內容，不免有錯，歡迎你們留言。

SQL語句概述
1970年，美國IBM研究中心的E.F.Codd提出關係模型，拉開了將來五十年經久不衰的關係數據庫的帷幕。
SQL原來是指「結構化查詢語句Structured Query Language」，通過不斷髮展，已經不表示任何具體的縮寫含義，成爲一個通用標準。
SQL主要包括：

1. DDL（數據定義語言 data definition language ）：定義基本表，視圖，索引等結構。
2. DML（數據操做語言 data manipulation language ）：分爲數據查詢和數據更新（插入、刪除、修改）
3. DCL（數據權限管理語句data control language）：對基本表和視圖的受權、完整性規則的描述。事務控制語句也能夠包括在這裏。
4. 事務控制管理語句（transaction control）
5. 嵌入式SQL的使用

示例所用E-R圖
不看懂也不要緊，也不影響語法學習。
E-R圖：Human Resources (HR) Schema人力資源計劃

Oracle經常使用數據類型規定
CHAR： 固定長度字符串 最大長度2000 bytes
VARCHAR2： 可變長度的字符串 最大長度4000 bytes 可作索引的最大長度749
NCHAR： 根據字符集而定的固定長度字符串 最大長度2000 bytes
NVARCHAR2： 根據字符集而定的可變長度字符串 最大長度4000 bytes
DATE： 日期（日-月-年） DD-MM-YY（HH-MI-SS） 通過嚴格測試，無千蟲問題
LONG ：超長字符串 最大長度2G（231-1） 足夠存儲大部頭著做
NUMBER(P,S) 數字類型 P爲總位數，S爲小數位數

RAW： 固定長度的二進制數據 最大長度2000 bytes 可存放多媒體圖象聲音等
LONG RAW： 可變長度的二進制數據 最大長度2G 同上
CLOB： 字符數據 最大長度4G
BLOB： 二進制數據 最大長度4G
BFILE： 存放在數據庫外的二進制數據 最大長度4G
ROWID： 數據表中記錄的惟一行號 10 bytes ****..格式，*爲0或1

第一章 DDL：Data Definition Language

Naming Rules命名規則
Table names and column names must:
• Begin with a letter字母開始
• Be 1–30 characters long
• Contain only A–Z, a–z, 0–9, _, $, and #【命名容許使用字符】
• Not duplicate the name of another object owned by the same user【同一用戶下不一樣對象不能命相同的名】
• Not be an Oracle server–reserved word不使用oracle保留字【例如table等常見英文單詞】

基本表的建立、修改、刪除
CREATE TABLE Statement
• You must have:
– The CREATE TABLE privilege
– A storage area

CREATE TABLE [schema.指定用戶]table
(column datatype [DEFAULT expr][, ...]) [CONSTRAINT constraint_name] constraint_type,；
//段名+數據類型+約束名+約束類型
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• 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)//字段名和數據類型,
dname VARCHAR2(14),
loc VARCHAR2(13) NOT NULL,//非空
create_date DATE DEFAULT SYSDATE);
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• Confirm table creation:

DESCRIBE 表名;//查看錶的結構（字段和類型）
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Constraints約束
• 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惟一
– PRIMARY KEY主鍵
– FOREIGN KEY外鍵
– CHECK校驗
• View a constraint in the data dictionary.數據字典中查看
• You can name a constraint or the Oracle server generates a name by using the SYS_Cn format.約束命名
• Create a constraint at either of the following times:建立約束的時間
– At the same time as the creation of the table在建立表的同時
– After the creation of the table建立表以後

//Example of a column-level constraint
CREATE TABLE employees(
employee_id NUMBER(6) CONSTRAINT emp_emp_id_pk約束名稱 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));
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NOT NULL Constraint非空約束：Ensures that null values are not permitted for the column:
UNIQUE Constraint惟一值，無重複：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約束
salary NUMBER(8,2),
commission_pct NUMBER(2,2),
hire_date DATE NOT NULL,
CONSTRAINT emp_email_uk UNIQUE(email)字段名;
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PRIMARY KEY Constraint主鍵約束：非空且惟一
FOREIGN KEY Constraint：外鍵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)//外鍵：關係數據庫的參照完整性，定義從表
REFERENCES departments(department_id),引用主表
CONSTRAINT emp_email_uk UNIQUE(email));
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KEY Constraint: Keywords含有外鍵從表
• 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
主表刪除時，主從表都刪除記錄
• ON DELETE SET NULL: Converts dependent foreign key values to null從表改成空，主表刪除

5. CHECK Constraint校驗約束
   • 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);//使用整數校驗
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Violating Constraint違反約束

UPDATE employees//更新從表employess數據
SET department_id = 55//報錯291，55在主表department中未出現
WHERE department_id = 110;
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You cannot delete a row that contains a primary key that is used as a foreign key in another table.
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DELETE FROM departments刪除主表中數據
WHERE department_id = 60；錯誤
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Creating a Table Using a Subquery使用子查詢，用另外表添加數據
• 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
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• 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;
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ALTER TABLE Statement修改表的結構
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增長字段
• You use the ADD clause to add columns:

ALTER TABLE dept80
ADD (job_id VARCHAR2(9));
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• The new column becomes the last column:全部值自動設置爲空
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));
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• A change to the default value affects only subsequent insertions to the table.
Dropping a Column刪除字段
Use the DROP COLUMN clause to drop columns that you no longer need from the table:

ALTER TABLE dept80
MODIFY (last_name VARCHAR2(30));
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SET UNUSED Option某個字段設置不可用
• 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只讀表
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;//設定爲只讀表，不容許修改
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--perform table maintenance and then return table back to read/write mode
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ALTER TABLE employees READ WRITE;
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Dropping a Table刪除表
• 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; //結構和數據所有刪除,存在回收站，還能夠閃回操做
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View視圖
建立在一個表上的視窗，不存放數據，邏輯視窗
Advantage of views：
to restrict data access
to make complex queries easy
to provide data independence
to present different views of the same data
兩種視圖：簡單視圖和複雜視圖

Create view建立視圖
You embed a subquery in the CREATE VIEW statement：

CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view識圖名 [(alias[， alias] ...) ]
AS subquey子查詢
[WITH CHECK OPTION [CONSTRAINT constraint] ]
[WITH READ ONLY [CONSTRAINT constraint]];
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· 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；
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· 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 修改視圖：
create or replace view 識圖名 as 子查詢
· 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 複雜視圖：使用子查詢鏈接
Create a complex view that contains group functions to display values from two tables：

不建議使用DML語句修改視圖，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 視圖名；刪除視圖
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Sequence序列號
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]增加值
[START WITH n]第一個值
[(MAXVALUE n | NOMAXVALUE)]最大
[(MINVALUE n | NOMINVALUE)]是否包含最小
[(CYCLE | NOCYCLE) ]循環使用,不建議
[(CACHE n | NOCACHE) ]內存一次性裝在幾個，系統崩潰後出錯
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引用序列號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:序列號斷層
—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，必須先刪除
[(MAXVALUE n | NOMAXVALUE)]
[(MINVALUE n | NOMINVALUE)]
[(CYCLE | NOCYCLE) ]
[(CACHE n | NOCACHE) ]
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· 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：樹狀結構，單獨存儲，
· 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
· 自動建立：主鍵或unique的字段
Automatically：A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition.
· 手動建立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,…)和表的字段對應
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Drop index 索引名；刪除1Synonym同義詞複雜的對象設置一個簡單的名稱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————————————————