Oracle语法:
INTERVAL 'integer [- integer]' {YEAR | MONTH} [(precision)][TO {YEAR | MONTH}]

该数据类型常用来表示一段时间差, 注意时间差只精确到年和月. precision为年或月的精确域, 有效范围是0到9, 默认值为2.

eg:
INTERVAL '123-2' YEAR(3) TO MONTH    
表示: 123年2个月, "YEAR(3)" 表示年的精度为3, 可见"123"刚好为3为有效数值, 如果该处YEAR(n), n<3就会出错, 注意默认是2.

INTERVAL '123' YEAR(3)
表示: 123年0个月

INTERVAL '300' MONTH(3)
表示: 300个月, 注意该处MONTH的精度是3啊.

INTERVAL '4' YEAR    
表示: 4年, 同 INTERVAL '4-0' YEAR TO MONTH 是一样的

INTERVAL '50' MONTH    
表示: 50个月, 同 INTERVAL '4-2' YEAR TO MONTH 是一样

INTERVAL '123' YEAR    
表示: 该处表示有错误, 123精度是3了, 但系统默认是2, 所以该处应该写成 INTERVAL '123' YEAR(3) 或"3"改成大于3小于等于9的数值都可以的

INTERVAL '5-3' YEAR TO MONTH + INTERVAL '20' MONTH =
INTERVAL '6-11' YEAR TO MONTH
表示: 5年3个月 + 20个月 = 6年11个月

与该类型相关的函数:
NUMTODSINTERVAL(n, 'interval_unit')
将n转换成interval_unit所指定的值, interval_unit可以为: DAY, HOUR, MINUTE, SECOND
注意该函数不可以转换成YEAR和MONTH的.

NUMTOYMINTERVAL(n, 'interval_unit')
interval_unit可以为: YEAR, MONTH

eg: (Oracle Version 9204, RedHat Linux 9.0)
SQL> select numtodsinterval(100,'DAY') from dual;

NUMTODSINTERVAL(100,'DAY')                                                     
---------------------------------------------------------------------------    
+000000100 00:00:00.000000000

SQL> c/DAY/SECOND
1* select numtodsinterval(100,'SECOND') from dual
SQL> /

NUMTODSINTERVAL(100,'SECOND')                                                  
---------------------------------------------------------------------------    
+000000000 00:01:40.000000000

SQL> c/SECOND/MINUTE
1* select numtodsinterval(100,'MINUTE') from dual
SQL> /

NUMTODSINTERVAL(100,'MINUTE')                                                  
---------------------------------------------------------------------------    
+000000000 01:40:00.000000000

SQL> c/MINUTE/HOUR
1* select numtodsinterval(100,'HOUR') from dual
SQL> /

NUMTODSINTERVAL(100,'HOUR')                                                    
---------------------------------------------------------------------------    
+000000004 04:00:00.000000000

SQL> c/HOUR/YEAR
1* select numtodsinterval(100,'YEAR') from dual
SQL> /
select numtodsinterval(100,'YEAR') from dual
*
ERROR at line 1:
ORA-01760: illegal argument for function

SQL> select numtoyminterval(100,'year') from dual;

NUMTOYMINTERVAL(100,'YEAR')                                                    
---------------------------------------------------------------------------    
+000000100-00

SQL> c/year/month
1* select numtoyminterval(100,'month') from dual
SQL> /

NUMTOYMINTERVAL(100,'MONTH')                                                   
---------------------------------------------------------------------------    
+000000008-04

时间的计算:
SQL> select to_date('1999-12-12','yyyy-mm-dd') - to_date('1999-12-01','yyyy-mm-dd') from dual;

TO_DATE('1999-12-12','YYYY-MM-DD')-TO_DATE('1999-12-01','YYYY-MM-DD')          
---------------------------------------------------------------------          
11          
-- 可以相减的结果为天.

SQL> c/1999-12-12/1999-01-12
1* select to_date('1999-01-12','yyyy-mm-dd') - to_date('1999-12-01','yyyy-mm-dd') from dual
SQL> /

TO_DATE('1999-01-12','YYYY-MM-DD')-TO_DATE('1999-12-01','YYYY-MM-DD')          
---------------------------------------------------------------------          
-323          
-- 也可以为负数的

SQL> c/1999-01-12/2999-10-12
1* select to_date('2999-10-12','yyyy-mm-dd') - to_date('1999-12-01','yyyy-mm-dd') from dual
SQL> /

TO_DATE('2999-10-12','YYYY-MM-DD')-TO_DATE('1999-12-01','YYYY-MM-DD')          
---------------------------------------------------------------------          
365193

下面看看INTERVAL YEAR TO MONTH怎么用.
SQL> create table bb(a date, b date, c interval year(9) to month);

Table created.

SQL> desc bb;
Name                                      Null?    Type
----------------------------------------- -------- ----------------------------
A                                                  DATE
B                                                  DATE
C                                                  INTERVAL YEAR(9) TO MONTH

SQL> insert into bb values(to_date('1985-12-12', 'yyyy-mm-dd'), to_date('1984-12-01','yyyy-mm-dd'), null)

1 row created.

SQL> select * from bb;

A         B                                                                    
--------- ---------                                                            
C                                                                              
---------------------------------------------------------------------------    
12-DEC-85 01-DEC-84

SQL> update bb set c = numtoyminterval(a-b, 'year');

1 row updated.

SQL> select * from bb;

A         B                                                                   
--------- ---------                                                           
C                                                                             
---------------------------------------------------------------------------   
12-DEC-85 01-DEC-84                                                           
+000000376-00

-- 直接将相减的天变成年了, 因为我指定变成年的
SQL> select a-b, c from bb;

A-B                                                                    
----------                                                                    
C                                                                             
---------------------------------------------------------------------------   
376                                                                    
+000000376-00

SQL> insert into bb values(null,null,numtoyminterval(376,'month'));

1 row created.

SQL> select * from bb;

A         B             C                                                      
--------- ---------    --------------------------------------------   
12-DEC-85 01-DEC-84    +000000376-00                                                                 
+000000031-04

SQL> insert into bb values ( null,null, numtoyminterval(999999999,'year'));

1 row created.

SQL> select * from bb;

A           B            C                               
---------   ---------     ---------------------------------------------------------------------   
12-DEC-85   01-DEC-84   +000000376-00                                                                 
+000000031-04
+999999999-00