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Oracle數(shù)據(jù)庫(kù)表結(jié)構(gòu)基本概念
Oracle數(shù)據(jù)庫(kù)是一種關(guān)系型數(shù)據(jù)庫(kù)管理系統(tǒng)，它采用表結(jié)構(gòu)來(lái)存儲(chǔ)和管理數(shù)據(jù)，在Oracle中，表是由行和列組成的二維結(jié)構(gòu)，每一行表示一條記錄，每一列表示一個(gè)字段，表結(jié)構(gòu)包括字段名、字段類型、字段長(zhǎng)度、約束等屬性，修改表結(jié)構(gòu)是指對(duì)這些屬性進(jìn)行增加、刪除或修改操作。
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Oracle數(shù)據(jù)庫(kù)表結(jié)構(gòu)修改技術(shù)
1、使用SQL語(yǔ)句修改表結(jié)構(gòu)
Oracle提供了ALTER TABLE語(yǔ)句來(lái)修改表結(jié)構(gòu)，可以對(duì)表的字段、索引、約束等進(jìn)行增加、刪除或修改操作，以下是一些常用的ALTER TABLE語(yǔ)句：
（1）增加字段
ALTER TABLE 表名 ADD (字段名 字段類型(字段長(zhǎng)度) [約束條件]);
給employees表增加一個(gè)birth_date字段：
ALTER TABLE employees ADD (birth_date DATE);
（2）刪除字段
ALTER TABLE 表名 DROP COLUMN 字段名；
刪除employees表中的email字段：
ALTER TABLE employees DROP COLUMN email;
（3）修改字段
ALTER TABLE 表名 MODIFY (字段名 新字段類型(新字段長(zhǎng)度));
將employees表中的salary字段類型修改為NUMBER(8,2)：
ALTER TABLE employees MODIFY (salary NUMBER(8,2));
（4）添加主鍵約束
ALTER TABLE 表名 ADD PRIMARY KEY (字段名);
給employees表的emp_id字段添加主鍵約束：
ALTER TABLE employees ADD PRIMARY KEY (emp_id);
（5）添加唯一約束
ALTER TABLE 表名 ADD CONSTRAINT 約束名 UNIQUE (字段名);
給employees表的email字段添加唯一約束：
ALTER TABLE employees ADD CONSTRAINT email_unique UNIQUE (email);
（6）添加外鍵約束
ALTER TABLE 表名 ADD CONSTRAINT 約束名 FOREIGN KEY (本表字段名) REFERENCES 目標(biāo)表名 (目標(biāo)表字段名);
給employees表的dept_id字段添加外鍵約束，關(guān)聯(lián)departments表的dept_id字段：
ALTER TABLE employees ADD CONSTRAINT dept_fk FOREIGN KEY (dept_id) REFERENCES departments (dept_id);
2、使用Oracle SQL*Plus工具修改表結(jié)構(gòu)
除了使用SQL語(yǔ)句外，還可以通過(guò)Oracle SQL*Plus工具圖形化地修改表結(jié)構(gòu)，具體操作步驟如下：
（1）打開(kāi)SQL*Plus工具，連接到目標(biāo)數(shù)據(jù)庫(kù)。
（2）輸入命令“DESCRIBE”，查看目標(biāo)表的結(jié)構(gòu)信息。
（3）輸入命令“EDIT”，進(jìn)入表設(shè)計(jì)器界面。
（4）在表設(shè)計(jì)器界面中，可以直接對(duì)字段進(jìn)行增加、刪除、修改操作，以及對(duì)索引、約束等進(jìn)行管理。
（5）完成表結(jié)構(gòu)修改后，保存并退出表設(shè)計(jì)器。
Oracle數(shù)據(jù)庫(kù)表結(jié)構(gòu)修改注意事項(xiàng)
1、在修改表結(jié)構(gòu)前，建議先備份數(shù)據(jù)，以防數(shù)據(jù)丟失。
2、如果表中有大量數(shù)據(jù)，修改表結(jié)構(gòu)可能會(huì)導(dǎo)致鎖定表，影響數(shù)據(jù)的訪問(wèn)和操作，建議在業(yè)務(wù)低峰期進(jìn)行表結(jié)構(gòu)修改操作。
3、修改表結(jié)構(gòu)可能會(huì)影響到已有的存儲(chǔ)過(guò)程、觸發(fā)器等對(duì)象，在修改表結(jié)構(gòu)前，需要評(píng)估潛在的影響，并進(jìn)行相應(yīng)的調(diào)整。
4、如果需要修改的字段是其他表的外鍵引用，需要先處理相關(guān)聯(lián)的外鍵約束，否則可能導(dǎo)致數(shù)據(jù)不一致。
Oracle數(shù)據(jù)庫(kù)表結(jié)構(gòu)修改實(shí)踐案例
假設(shè)有一個(gè)名為employees的表，其結(jié)構(gòu)如下：
CREATE TABLE employees (
emp_id NUMBER(6) NOT NULL,
first_name VARCHAR2(20),
last_name VARCHAR2(25),
email VARCHAR2(50),
hire_date DATE,
job_id VARCHAR2(10),
salary NUMBER(8,2),
dept_id NUMBER(4),
CONSTRAINT employees_pk PRIMARY KEY (emp_id) USING index PCTFREE 10 INITRANS 20 NOCOMPRESS NOLOGGING,
CONSTRAINT email_unique UNIQUE (email) USING index PCTFREE 10 INITRANS 20 NOCOMPRESS NOLOGGING,
CONSTRAINT dept_fk FOREIGN KEY (dept_id) REFERENCES departments (dept_id) ON DELETE CASCADE ON UPDATE CASCADE NOT DEFERRABLE INITIALLY IMMEDIATE SAVEPOINT NOT NULL,
CONSTRAINT employees_uk_job_id UNIQUE (job_id) USING index PCTFREE 10 INITRANS 20 NOCOMPRESS NOLOGGING,
CONSTRAINT fk_jobs FOREIGN KEY (job_id) REFERENCES jobs (job_id) ON DELETE SET NULL ON UPDATE CASCADE NOT DEFERRABLE INITIALLY IMMEDIATE SAVEPOINT NOT NULL,
CONSTRAINT chk_salary_range FOR salary (salary >= 999999999.99 AND salary <= +999999999.99) check for valid salary range values between 999,999,999.99 and +999,999,999.99 inclusive; invalid values will be rejected by the database trigger before update or insert operation is attempted on this column. SUPPORTS NULL values; SUPPORTS nonnull values; NOT NULL; STRICTLY DOCUMENTED; SCOPE = SPECIFIC; ALLOW FUTURE REFERENCES: enable; ALLOW NULLS: enable; ALLOW ZERO IN FIELD: disable; IDENTITY: none; SEQUENCE: none; CACHE: none; OPTIMISTIC: disable; NOT DETERMINISTIC: disable; NOVALIDATE: disable; NOCACHE: disable; UNDO: enable; DBMS_STATS.GATHER_DATABASE_STATS: disable; DBMS_STATS.AUTOSTATS: disable; AUDIT: disable; IMPLICIT: enable; LOCAL: enable; BINARY: enable; MONOTONIC_INCREMENT: disable; DDL_COMMENT = 'Salary'; SERIALLY REPRODUCIBLE: ensure start of sequence is same as previous restart point of sequence; ALTER SESSION set NLS_COMP=LINGUISTIC order to preserve case sensitivity during comparisons of string data types in a case sensitive collation environment; ALTER SESSION set NLS_SORT=GENERIC order to preserve case sensitivity during comparisons of string data types in a case sensitive collation environment; ALTER SESSION set NLS_NCHAR=CHAR order to preserve case sensitivity during comparisons of string data types in a case sensitive collation environment; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read write false to allow database updates without changing this object's physical storage characteristics; ALTER SESSION set session logical read only true to allow database updates without changing this object's physical storage characteristics
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