想要查询连接第一个表的第一行和第二个表的前两列

Question

我想连接查询来连接第一张表的第一行和第二张表的前两列，

表A

ID     Date        Username  Password              
1    19/2/2016       XYZ       ******               
2    19/2/2016       ABC       ******    

表B，

ID     Date        Username    City                     
1    19/2/2016       XYZ       NYC                       
2    19/2/2016       ABC       LA                      

当我在表A的第一行中插入一些数据时，我想检查表B的ID，DATE是否有可用数据

Answer 1

您是说要在这两个表之间强制引用完整性吗？ 在这种情况下，您需要一个外键约束

ALTER TABLE table_a
ADD CONSTRAINT reference_table_b_fk
   FOREIGN KEY (id, date)
   REFERENCES table_b (id, date);

Answer 2

如果您只想在执行“插入”选项之前进行检查，请尝试以下操作：

IF EXISTS (SELECT ID FROM TableB WHERE ID=1 AND Date='19/2/2016')

// Your either insert or not query

ELSE 

// Your else logic will be here 

Answer 3

因此，如果仅在table2中存在等效条目时才将条目插入到table1中，请执行以下脚本：

create table table1 (id number(2), date_t1 date, username varchar2(5), password varchar2(8));
create table table2 (id number(2), date_t1 date, username varchar2(5), city varchar2(8));

insert into table1 values (1, to_date('16.02.2016', 'dd.mm.yyyy'), 'XYZ', 'ABC123');
insert into table1 values (2, to_date('16.02.2016', 'dd.mm.yyyy'), 'ABC', 'XYZ123');

insert into table2 values (1, to_date('16.02.2016', 'dd.mm.yyyy'), 'XYZ', 'NYC');
insert into table2 values (2, to_date('16.02.2016', 'dd.mm.yyyy'), 'ABC', 'LA');


declare
  n_id number(2);
  d_date date;
  v_username varchar2(5);
  v_password varchar2(8);
  n_check number(1);
begin
  n_check := 0;
  -- fill the variables with data which you want to insert:
  n_id          := 2;
  d_date        := to_date('16.02.2016', 'dd.mm.yyyy');
  v_username    := 'ABC';
  v_password    := 'CCCCC';

  -- check whether an entry exists in table2:
  begin
    select count(1)
    into n_check
    from table2 t2
    where t2.id = n_id
    and trunc(t2.date_t1) = trunc(d_date);
  exception when no_data_found then n_check := 0;
  end;

  -- if an entry exists in table2, then insert into table1:
  if n_check <> 0 then
    insert into table1 (id, date_t1, username, password) 
    values (n_id, d_date, v_username, v_password);
  end if;
end;  
/

select * from table1;
select * from table2;

delete from table1;
delete from table2;