求某個列第2大的值的多種方法

 

    create table t (x int);  --創建測試表
    insert into t values(1);
    insert into t values(3);
    insert into t values(5);
    insert into t values(7);
    insert into t values(9);
    法一：
    select x
        from ( select x, rownum r
                 from ( select x from t order by x desc )
                        where rownum <= 2 )
       where r = 2;
    註釋：子查詢不進行擴展(unest),那麼是先求出最裏層的數據
    法二：
    select *
     from t
     where (select count(*)
     from t t2
     where t2.x > t.x ) = 1
     /
    註釋：子查詢中的條件跟其他高輩分的SELECT有關聯（關聯子查詢）
法三：
select max(x)
 from t
 where x < (select max(x) from t)
 /
法四：
select x
 from ( select x, row_number() over ( order by x desc ) r
          from t
        )
 where r = 2
/
註釋：row_number() 是分析函數