复杂SQL

1. 根据不同条件去 为表字段设置不同值

UPDATE A as a,
    (SELECT * from b where bb=1) p,
    (SELECT * from c where cc=2) m
    set  a.aa=
      CASE
      WHEN  a.a1= p.p1 THEN 3
     WHEN  a.a2=m.m1 THEN 2
      ELSE a.aa
      END
  where a.id=p.pId and a.id=m.mId

else 必须要有，没有的话会导致a.aa的值为空