如果 id 存在于另一个表中，则有条件地插入另一个表，否则插入到 oracle 中的两个表中

Question

如果A表中存在客户ID，则在B表中插入订单。 如果表 A 中不存在客户 ID，则将客户 ID 插入表 A 中，然后在表 B 中订购。 我一直在尝试使用 if/else 和 merge 来实现这一点，但一直遇到 无效的 sql 语句。

IF EXISTS (SELECT CustomerID FROM Customer_T WHERE CustomerID = 18)
    Insert into Order_T
    values(79,18,to_date('09/28/2021','mm/dd/yyyy'),to_date('10/01/2021','mm/dd/yyyy'),1,3)
ELSE
  insert INTO Customer_T VALUES (18,'Capitol Industries Ltd', '999 Fifth Avenue', 'New York', 'NY','10015')
  insert into Order_T values (79,18,to_date('09/28/2021','mm/dd/yyyy'),to_date('10/01/2021','mm/dd/yyyy'),1,3)
END IF;

Answer 1

假设您有 2 个非常简单的表格。

表格

create table T1( num_ )
as
select 1 from dual ;

create table T2( num_ )
as
select 200 from dual ;

与您问题中的代码类似，包含 IF .. ELSE .. END IF 和 EXISTS() 的匿名块会导致错误：

函数或伪列 'EXISTS' 可以在 SQL 语句中使用 只有

    begin
      if exists( select num_ from T1 where num_  = 2 ) then
        insert into T2( num_ ) values( 2 ) ;
        dbms_output.put_line( 'if' ) ;
      else
        insert into T1( num_ ) values( 2 ) ;
        insert into T2( num_ ) values( 2 ) ;
        dbms_output.put_line( 'else' ) ;
      end if ;
    end ;
    /
-- error:
... function or pseudo-column 'EXISTS' may be used inside a SQL statement only

一种解决方案可能是执行以下操作（请参阅 asktom.oracle.com - Equivalent for EXISTS() in an IF statement）

begin
  for x in ( select count(*) cnt
             from dual
             where exists ( select num_ from T1 where num_  = 2  ) 
  ) loop
    if ( x.cnt = 1 ) then                  -- found
      insert into T2( num_ ) values( 2 ) ;
      dbms_output.put_line( 'if' ) ;
    else                                   -- not found
      insert into T1( num_ ) values( 2 ) ;
      insert into T2( num_ ) values( 2 ) ;
      dbms_output.put_line( 'else' ) ; 
    end if;
  end loop;
end;
/

-- output:
1 rows affected

dbms_output:
else  

第一次执行匿名块后，表包含以下行：

select num_, '<- T1' as table_ from T1
union all
select num_, '<- T2' from T2 ;

-- result
NUM_    TABLE_
1       <- T1
2       <- T1
200     <- T2
2       <- T2

再次执行匿名块，你得到...

1 rows affected

dbms_output:
if

-- tables
NUM_    TABLE_
1       <- T1
2       <- T1
200     <- T2
2       <- T2
2       <- T2

DBfiddle here.

Answer 2

这种情况不需要IF THEN ELSE 逻辑。而是使用数据库内置功能。 customerid 应该是您的主键，因此如果您尝试插入并且它已经存在，则会引发 DUP_VAL_ON_INDEX 异常。

检查以下示例：

-- create tables
create table customers (
    id                             number generated by default on null as identity 
                                   constraint customers_id_pk primary key,
    name                           varchar2(255 char)
)
;

create table orders (
    id                             number generated by default on null as identity 
                                   constraint orders_id_pk primary key,
    customer_id                    number
                                   constraint orders_customer_id_fk
                                   references customers on delete cascade,
    product                        varchar2(100 char)
)
;

BEGIN
  BEGIN
    insert INTO customers VALUES (2,'Capitol Industries Ltd');
  EXCEPTION WHEN DUP_VAL_ON_INDEX THEN
    NULL; 
  END;
  insert into orders (customer_id,product) values (2,'a book');
END;  
/

运行上述代码块几次。只有第一次才会插入客户。

Answer 3

您可以使用multitable insert 并使用这样一个事实，即没有group by 的聚合函数在不存在（= 不满足where 条件）行的情况下总是返回带有null 的行。

代码如下：

insert into customers(id, name, company, state)
values (1, 'Some name', 'Some company', 'NY')

1 行受影响

insert all
  when cust_exists = 0
  then
    into customers (id, name, company, state)
    values (cust_id, cust_name, company, state)

  when 1 = 1
  then
    into orders (id, customer_id, order_date, due_date, some_id)
    values(order_id, cust_id, order_date, due_date, some_id)
select
  1 as order_id,
  1 as cust_id,
  'Some other name' as cust_name,
  'Company' as company,
  'NY' as state,
  date '2021-09-28' as order_date,
  date '2021-10-03' as due_date,
  100 as some_id,
  nvl(max(1), 0) as cust_exists
from customers
where id = 1

1 行受影响

insert all
  when cust_exists = 0
  then
    into customers (id, name, company, state)
    values (cust_id, cust_name, company, state)

  when 1 = 1
  then
    into orders (id, customer_id, order_date, due_date, some_id)
    values(order_id, cust_id, order_date, due_date, some_id)
select
  2 as order_id,
  2 as cust_id,
  'Some other name' as cust_name,
  'Company' as company,
  'NY' as state,
  date '2021-09-28' as order_date,
  date '2021-10-03' as due_date,
  100 as some_id,
  nvl(max(1), 0) as cust_exists
from customers
where id = 2

2 行受影响

您还可以使用两个带有 documented ignore_row_on_dupkey_index 提示的插入，正如其名称所暗示的那样。

insert /*+
  ignore_row_on_dupkey_index(customers(id))
*/
into customers (id, name, company, state)
values (2, 'Name', 'Comp', 'NY')

✓

insert into orders (id, customer_id, order_date, due_date, some_id)
values (3, 2, date '2021-09-30', date '2021-10-07', 5)

1 行受影响

select *
from customers

ID	NAME	COMPANY	STATE
1	Some name	Some company	NY
2	Some other name	Company	NY

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