oracle笔记--DML语句优化

update table A a
set a.c1 = (select b.c1 from B b where a.id = b.id)
where exists(select 1 from B b where a.id = b.id )

可以考虑使用以下方式来更新表中的数据避免对B的两次查询

update (select a.c1 a_c1, b.c1 b_c1 from A a, B where a.id = b.id)
set a_c1 = b_c1