SQL - 加入两个表并从B表中获取记录（如果存在），否则从A表中获取

I have two tables, A and B. If record is exist in table B (checking by ID), then I want to get this record from this table (table B). If not exist in table B, then I want to get this record from table A (so most important is getting from table B but if not exist, then get from table A).I have two tables, A and B. If record is exist