详解DBMS中delete命令的使用
删除命令是一种数据操作命令，用于从表中删除记录。可以一次性删除所有记录，也可以根据条件删除一组记录。
从表中删除特定行
要删除表中的特定行，我们需要提及where条件。根据条件，从表中删除该特定行。
语法如下-
delete from tablename where condition
例如，
Delete from employee where name=’sneha’;
示例
下面给出的是从表中删除特定行的示例-
create table employee(ename varchar(30),department varchar(20), age number(30), salary number(20));
insert into employee values('ram','projectmanager',40,50000);
insert into employee values('priya','assistant director',45,45000);
insert into employee values('hari','developer',46,30000);
select * from employee;输出结果您将获得以下输出-
ram|projectmanager|40|50000 priya|assistant director|45|45000 hari|developer|46|30000
现在，使用下面给出的程序-
示例
delete from employee where ename='priya'; select * from employee;输出结果
您将获得以下输出-
ram|projectmanager|40|50000 hari|developer|46|30000
根据条件删除数据
例如，
delete from employee where age>45;
示例
下面给出的是删除年龄大于45岁的员工行的示例，其余的将保留在数据库中-
create table employee(ename varchar(30),department varchar(20), age number(30), salary number(20));
insert into employee values('ram','projectmanager',40,50000);
insert into employee values('priya','assistant director',45,45000);
insert into employee values('hari','developer',46,30000);
select * from employee;输出结果您将获得以下输出-
ram|projectmanager|40|50000 priya|assistant director|45|45000 hari|developer|46|30000
现在，使用下面给出的程序-
delete from employee where age>45; select * from employee;输出结果
您将获得以下输出-
ram|projectmanager|40|50000 priya|assistant director|45|45000
根据子查询的结果删除数据
也可以根据子查询的结果进行删除。
例如，
delete from employee where age in (select age from employee where age>45)
从表中删除所有行
如果我们想删除整个表，我们可以使用以下内容-
delete from employee; (or) delete * from employee;
要从表中删除所有行，我们还可以使用TRUNCATE命令。
语法如下-
TRUNCATE table tablename;
例如，
TRUNCATE table employee
TRUNCATE从没有备份的表中删除所有行。
示例
下面给出的是从表中删除所有行的示例-
create table employee(ename varchar(30),department varchar(20), age number(30), salary number(20));
insert into employee values('ram','projectmanager',40,50000);
insert into employee values('priya','assistant director',45,45000);
insert into employee values('hari','developer',46,30000);
delete from employee;
select * from employee;输出结果您将获得以下输出-
no rows displayed