PostgreSQL實現Oracle merge into功能

合並MERGE語句是Oracle9i新增的語法，用來合並UPDATE和INSERT語句。通過MERGE語句，根據一張表或子查詢的連接條件對另外一張表進行查詢，連接條件匹配上的進行UPDATE，無法匹配的執行INSERT。這個語法僅需要一次全表掃描就完成了全部工作，執行效率要高於INSERT＋UPDATE

with upsert as (
    update a
        set col1 = b.col1,col2 = b.col2
        from b t
        where a.id = t.id and a.code = t.code
        returning a.id,a.code
)
insert
into a
select *
from b t
where not exists(
        select 1
        from upsert b
        where t.id = b.id
          and t.code = b.code
    );

---

merge into A a using B b
on (a.id = b.id and a.code = b.code)
when matched then
update col1 = b.col1,col2 = b.col2
    when not matched then
insert
into a
values (b.col1, b.col2);