SQL语句面试题目：多表查询、SQL约束、DDL、DML



1 SQL语句多表查询

例如：按照 department_id 查询 employees(员工表)和 departments(部门表)的信息。 

方式一(通用型):SELECT ... FROM ... WHERE 

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SELECT e.last_name,e.department_id,d.department_name  
 FROM employees e,departments d   
where e.department_id = d.department_id   

SELECT e.last_name,e.department_id,d.department_name
FROM employees e,departments d
where e.department_id = d.department_id 

方式二：SELECT ... FROM ...NATURAL JOIN ...

有局限性：会自动连接两个表中相同的列 ( 可能有多个 :department_id 和manager_id) 

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SELECT last_name,department_id,department_name   
FROM employees   
NATURAL JOIN departments   

SELECT last_name,department_id,department_name
FROM employees
NATURAL JOIN departments 

方式三：SELECT ... JOIN ... USING...

 有局限性：好于方式二，但若多表的连接列列名不同，此法不合适 

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SELECT last_name,department_id,department_name   
FROM employees   
JOIN departments   
USING(department_id)   

SELECT last_name,department_id,department_n
4000
ame
FROM employees
JOIN departments
USING(department_id) 

方式四：SELECT ... FROM ... JOIN... ON ... 

常用方式，较方式一，更易实现外联接(左、右、满) 

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SELECT last_name,e.department_id,department_name   
FROM employees e   
JOIN departments d   
ON e.department_id = d.department_id  

SELECT last_name,e.department_id,department_name
FROM employees e
JOIN departments d
ON e.department_id = d.department_id

--内连接

1） 

--等值连接 

--不等值连接

2） 

--非自连接 

--自连接

 

--外连接

--左外连接、右外连接、满外连接

2 创建和管理表(DDL) 

CRAETE TABLE/TRUNCATE TABLE /ALTER TABLE /REANME...TO/DROP TABLE ..操作完以后，自动 commit;所以，rollback 对其操作，没有效果 

1.创建表 

1）直接创建 

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create table emp1(   
name varchar2(20),   
salary number(8,2)default 1000,   
id number(4),   
hire_date date   
);   

create table emp1(
name varchar2(20),
salary number(8,2)default 1000,
id number(4),
hire_date date
); 

2）通过子查询的方式创建 

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create table emp2   
as   
select last_name name,employee_id id,hire_date    
from employees;   

create table emp2
as
select last_name name,employee_id id,hire_date
from employees;

或者 

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create table emp2   
as   
select last_name name,employee_id id,hire_date  
from employees   
where department_id = 80;/where 1=2;   

create table emp2
as
select last_name name,employee_id id,hire_date
from employees
where department_id = 80;/where 1=2; 

2.修改表 

1)增加新的列 

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alter table emp1   
add(birthday date)   

alter table emp1
add(birthday date)

2)修改现有的列 

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alter table emp1   
modify(name varchar2(25) default 'abc')   

alter table emp1
modify(name varchar2(25) default 'abc')

3)重命名现有的列 

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alter table emp1   
rename column salary to sal;  

alter table emp1
rename column salary to sal;

 

4)删除现有的列 

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alter table emp1  
drop column birthday;   

alter table emp1
drop column birthday; 

3.清空表中的数据(与 delete from table_name区分开) 

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truncate table emp2;   

truncate table emp2;

4.重命名表 

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rename emp2 to emp3;   

rename emp2 to emp3;

5.删除表 

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drop table emp3;   

drop table emp3;

3 数据处理  DML  

1）增

1.1 增添一条记录

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insert into    [表名](,,,,,)   
values(,,,,,)   

insert into    [表名](,,,,,)
values(,,,,,) 

1.2 从其它表中拷贝数据 

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insert into [表名]  
select .... from [另一个表]   
where ....   

insert into [表名]
select .... from [另一个表]
where .... 

2）改 

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update [表名]   
set .....   
where ....   

update [表名]
set .....
where ....

3）删 

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delete from [表名]   
where ....  

delete from [表名]
where ....

 

4）查(最常用的数据库操作) 

select .... 

from … 

where …. 

group by … 

having … 

order by ….

4 约束 

对创建的表的列属性、字段进行的限制。诸如 :not null/unique/primary key/foreign key/check

1.如何定义约束---在创建表的同时，添加对应属性的约束

 

1.1 表级约束 & 列级约束 

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create table emp1(   
employee_id number(8),   
salary number(8),   
hire_date date not null,     --列级约束    
dept_id number(8),   
email varchar2(8) constraint emp1_email_uk unique,   
name varchar2(8) constaint emp1_name_uu not null,   
first_name varchar2(8),   
--表级约束   
constraint emp1_emp_id_pk primarykey(employee_id),   
constraint emp1_fir_name_uk unique(first_name),   
constraint emp1_dept_id_fk foreignkey(dept_id) references departments(department_id) ON DELETE CASCADE   
)   

create table emp1(
employee_id number(8),
salary number(8),
hire_date date not null,     --列级约束
dept_id number(8),
email varchar2(8) constraint emp1_email_uk unique,
name varchar2(8) constaint emp1_name_uu not null,
first_name varchar2(8),
--表级约束
constraint emp1_emp_id_pk primarykey(employee_id),
constraint emp1_fir_name_uk unique(first_name),
constraint emp1_dept_id_fk foreignkey(dept_id) references departments(department_id) ON DELETE CASCADE
) 

1.2 只有 not null 只能使用列级约束。其他的约束两种方式皆可 

2.添加和删除表的约束--在创建表以后，只能添加和删除，不能修改 

2.1 添加 

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alter table emp1   
add constaint emp1_sal_ck check(salary > 0)   

alter table emp1
add constaint emp1_sal_ck check(salary > 0) 

2.1.1 对于 not null 来讲，不用 add，需要使用modify: 

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alter table emp1   
modify (salary not null)   

alter table emp1
modify (salary not null) 

2.2 删除 

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alter table emp1   
drop constaint emp1_sal_ck   

alter table emp1
drop constaint emp1_sal_ck 

2.3 使某一个约束失效:此约束还存在于表中，只是不起作用 

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alter table emp1   
disable constraint emp1_email_uk;   

alter table emp1
disable constraint emp1_email_uk;

2.4 使某一个约束激活：激活以后，此约束具有约束力 

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alter table emp1   
enable constraint emp1_email_uk;