Question

我有一个请求，如果操作员输入什么，我将检索所有用户。但是如果运营商多次输入一些用户名，我只会退还用户的名字。名字是输入的，我想使用下面的子查询

我使用子查询（表b）作为IN条件来获取用户＆＃39;来自表a的信息，它运作良好。

SELECT * FROM a WHERE UserName IN (SELECT UserName FROM b)

但是，如果表格b没有，我需要从a中选择所有用户，我不知道该怎么做。在我只能为一个用户执行此操作之前，如果typeInUserName = NULL，则WHERE UserName = UserName将为我重新提供所有用户。

SELECT * FROM a WHERE UserName = NVL(typeInUserName,UserName)

Answer 1

create table a (username varchar2(20));

insert into a values('gaurav');
insert into a values('rahul');
insert into a values('rohan');
insert into a values('niharika');

create table b (username varchar2(20));

 insert into b values('gaurav');
 insert into b values('rahul');

案例1 ：表B中没有记录

Delete from  b;

WITH cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0;

USERNAME

gaurav              
niharika            
rahul               
rohan               

4 rows selected.

案例2 当表B中有两个匹配的记录时

 insert into b values('gaurav');
 insert into b values('rahul');

WITH cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0;


USERNAME            
--------------------
gaurav              
rahul               

2 rows selected.

案例3 当表B中有记录但没有匹配记录时。

delete from  b

insert into b values('rajat');
insert into b values('garima');

with cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0

no rows selected.

干杯GS </ p>

需要pl / sql存储过程的帮助

1 个答案: