内容简介：ORACLE 中NUMBER类型默认的精度和Scale问题
在 ORACLE 数据库中， NUMBER(P,S) 是最常见的数字类型，可以存放数据范围为 10^-130~10^126 （不包含此值 ) ，需要 1~22 字节 (BYTE) 不等的存储空间。 P 是 Precison 的英文缩写，即精度缩写，表示有效数字的位数，最多不能超过 38 个有效数字。 S 是 Scale 的英文缩写，表示从小数点到最低有效数字的位数，它为负数时，表示从最大有效数字到小数点的位数。有时候，我们在创建表的时候， NUMBER 往往没有指定 P ， S 的值，那么默认情况下， NUMBER 的 P 、 S 的值分别是多少呢？相信这个问题能问倒一大片 DBA 。 在之前，我遇到了一个问题，总结整理在 “ ORACLE NUMBER类型Scale为0引发的问题 ” 这篇博客当中，当时武断的判断 “ 如果不指定 p 和 s ， NUMBER 类型，它的默认精度值为 38, 默认的 scale 值为 0 ” ，因为当时参考了官方文档 https://docs.oracle.com/cd/B28359_01/server.111/b28318/datatype.htm#CNCPT1832
当然文档没有错误 ，文档应该是指在定义字段数据类型为 NUMBER 时，指定了 NUMBER 类型的 P值 ，但是没有指定 S的值 ，那么 Scale 默认就是 0 ，如下测试所示，当时应该是我自己没有完全理解文档意思，当然文档也有误导的嫌疑。
SQL> drop table test; Table dropped. SQL> create table test(id number(38)); Table created. SQL> insert into test 2 select 123 from dual union all 3 select 123.123 from dual; 2 rows created. SQL> commit; Commit complete. SQL> select * from test; ID ---------- 123 123 SQL>
当在指定字段类型为 NUMBER 时，如果 P 和 S 都不指定，那么 P 和 S 又是什么值呢？今天特意实验验证了一下，具体实验过程如下：
SQL> drop table test; Table dropped. SQL> create table test(id number, id1 number(38,4)); Table created. SQL> insert into test 2 select 12, 12 from dual union all 3 select 12.123456789, 12.123456789 from dual; 2 rows created. SQL> commit; Commit complete. SQL> col id for 999999999999.999999999999999999999999999999999999; SQL> col id1 for 99999999999.9999999999999999999999999999999999999; SQL> select * from test; ID ID1 -------------------------------------------------- -------------------------------------------------- 12.000000000000000000000000000000000000 12.0000000000000000000000000000000000000 12.123456789000000000000000000000000000 12.1235000000000000000000000000000000000 SQL>
如上所示，当我插入上面两条记录后，发现如果不指定 p 和 s ， NUMBER 类型，此时的 Scale 至少是 9 ，我们继续测试，插入下面数据
SQL> insert into test 2 select 12.123456789123456789123456789123456, 3 12.123456789123456789123456789123456 4 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test; ID ID1 -------------------------------------------------- -------------------------------------------------- 12.000000000000000000000000000000000000 12.0000000000000000000000000000000000000 12.123456789000000000000000000000000000 12.1235000000000000000000000000000000000 12.123456789123456789123456789123456000 12.1235000000000000000000000000000000000
如下所示，此时可以看到 Scale 的值 33 了，那么 Scale 的值是否可以继续变大呢？
SQL> insert into test 2 select 12.123456789123456789123456789123456789123, 3 12.123456789123456789123456789123456789123 4 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test; ID ID1 -------------------------------------------------- -------------------------------------------------- 12.000000000000000000000000000000000000 12.0000000000000000000000000000000000000 12.123456789000000000000000000000000000 12.1235000000000000000000000000000000000 12.123456789123456789123456789123456000 12.1235000000000000000000000000000000000 12.123456789123456789123456789123456789 12.1235000000000000000000000000000000000
如下截图所示，插入的记录为 12.123456789123456789123456789123456789123 ，但是显示的值为 12.123456789123456789123456789123456789 ，总共为 38 位，由于格式化列的缘故，可能导致部分小数位没有显示，
我们继续测试，调整格式化列，我们发现值变为了 12.12345678912345678912345678912345678912 ，总共 40 位了， Scale 的值为 38 了。这个是为什么呢？不是数字精度为38，意味着最多是 38 位吗？
SQL> col id for 999999999999.99999999999999999999999999999999999999999 SQL> col id1 for 99999999999.999999999999999999999999999999999999999999 SQL> select * from test; ID ID1 ------------------------------------------------------- ------------------------------------------------------- 12.00000000000000000000000000000000000000000 12.000000000000000000000000000000000000000000 12.12345678900000000000000000000000000000000 12.123500000000000000000000000000000000000000 12.12345678912345678912345678912345600000000 12.123500000000000000000000000000000000000000 12.12345678912345678912345678912345678912000 12.123500000000000000000000000000000000000000
继续其它测试，我们发现 Sacle 的值会随着小数点前面数字的变化而变化，如下所示：
SQL> insert into test 2 select 123456789.123456789123456789123456789123456, 3 123456789.123456789123456789123456789123456 4 from dual; 1 row created. SQL> commit; Commit complete SQL> insert into test 2 select 123456789123.123456789123456789123456789123456, 3 123456789123.123456789123456789123456789123456 4 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test; ID ID1 ------------------------------------------------------- ------------------------------------------------------- 12.00000000000000000000000000000000000000000 12.000000000000000000000000000000000000000000 12.12345678900000000000000000000000000000000 12.123500000000000000000000000000000000000000 12.12345678912345678912345678912345600000000 12.123500000000000000000000000000000000000000 12.12345678912345678912345678912345678912000 12.123500000000000000000000000000000000000000 123456789.12345678912345678912345678912300000000000 123456789.123500000000000000000000000000000000000000 123456789123.12345678912345678912345678910000000000000 ####################################################### 6 rows selected.
从上面测试可以看出， Scale 的值是变化的，跟数据值有关系，目前看来，小数点前的数字位数和小数点后的数字位数相加为 40 （有时候又是 39 ），为了测试是否这个规律，我特意用下面案例测试一下
SQL> create table test2(id number); Table created. SQL> insert into test2 2 select 0.123456789123456789123456789123456789123456789 from dual; 1 row created. SQL> commit; Commit complete. SQL> col id for 9999999999.9999999999999999999999999999999999999999999999; SQL> select * from test2; ID ---------------------------------------------------------- .1234567891234567891234567891234567891235000000 SQL> insert into test2 2 select 123456789.123456789123456789123456789123456789 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test2; ID ---------------------------------------------------------- .1234567891234567891234567891234567891235000000 123456789.1234567891234567891234567891230000000000000000 SQL> insert into test2 2 select 123456789123.123456789123456789123456789123456789 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test2; ID ---------------------------------------------------------- .1234567891234567891234567891234567891235000000 123456789.1234567891234567891234567891230000000000000000 ########################################################## SQL> col id for 9999999999999.9999999999999999999999999999999999999999999999; SQL> select * from test2; ID ------------------------------------------------------------- .1234567891234567891234567891234567891235000000 123456789.1234567891234567891234567891230000000000000000 123456789123.1234567891234567891234567891000000000000000000 SQL> insert into test2 2 select 12345678912345.12345678912345678912345678912345 from dual; 1 row created. SQL> commit; Commit complete. SQL> select * from test2; ID ------------------------------------------------------------- .1234567891234567891234567891234567891235000000 123456789.1234567891234567891234567891230000000000000000 123456789123.1234567891234567891234567891000000000000000000 ############################################################# SQL> col id for 9999999999999999999.99999999999999999999999999999999999999999999; SP2-0246: Illegal FORMAT string "9999999999999999999.99999999999999999999999999999999999999999999" SQL> col id for 9999999999999999999.9999999999999999999999999999999999999999 SQL> select * from test2; ID ------------------------------------------------------------- .1234567891234567891234567891234567891235 123456789.1234567891234567891234567891230000000000 123456789123.1234567891234567891234567891000000000000 12345678912345.1234567891234567891234567900000000000000 SQL>
这个问题纠结了很久，不明白为什么是39或40，后面在 Oracle Database SQL Reference 10g Release 2 终于找到解释了，如下所示：
p is the precision, or the total number of significant decimal digits, where the most
significant digit is the left-most nonzero digit, and the least significant digit is the
right-most known digit. Oracle guarantees the portability of numbers with
precision of up to 20 base-100 digits, which is equivalent to 39 or 40 decimal digits
depending on the position of the decimal point.
p 是精度，或是有效十进制数的总位数。最大的有效数字是最左边的非零数字，而最小有效位是最右边的数字。 Oracle 保证数字的可移植性
精度高达 20 base-100 digits ，相当于 39 位或 40 位十进制数字，取决于小数点的位置。
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