insert一条目标表中不存在的记录

目标表的创建语句：

create table test_T(
field_a integer,
field_b integer
)

【Oracle】说明：test_T 是目标表，插入记录 1，1

insert into test_T (field_a,field_b) 
select t.field_a,t.field_b
from (select 1 as field_a,1 as field_b from dual) t
left join test_T tt on tt.field_a=t.field_a and tt.field_b=t.field_b
where tt.field_a is null

【Sybase】说明：test_T 是目标表，插入记录 1，1

insert into test_T (field_a,field_b) 
select t.field_a,t.field_b
from (select 1 as field_a,1 as field_b ) t
left join test_T tt on tt.field_a=t.field_a and tt.field_b=t.field_b
where tt.field_a is null

注意：Oracle 与 Sybase 的区别：
select 1 as field_a,1 as field_b                【Sybase】
与
select 1 as field_a,1 as field_b from dual      【Oracle】