ORACLE

start with id=1 connect by prior id=parent_id

SELECT * FROM T_MENU T START WITH PARENT_ID = 0 CONNECT BY PRIOR ID = PARENT_ID ORDER BY ID;

PRIOR 后面紧跟的是父id则向上查询，否则向下查询

Cast函数

cast 是进行类型转换的, 可以针对各种Oracle数据类型. 修改的是用户的数据类型.

round只是修改的数据显示格式. 对数据做四舍五入. 类似的函数还有ceil(取此数据的最小整数). trunc( 取整函数.)

SQL> create table t1(a varchar(10));

Table created.

SQL> insert into t1 values ('12.3456');

1 row created.

SQL> select round(a) from t1;

ROUND(A)

----------

12

SQL> select round(a,3) from t1;

ROUND(A,3)

----------

12.346

SQL> select cast(a as int) from t1;

CAST(AASINT)

------------

12

SQL> select cast(a as number(8,4)) from t1;

CAST(AASNUMBER(8,4))

--------------------

12.3456