在ORACLE數據庫中，NUMBER(P,S)是最常見的數字類型，可以存放數據范圍為10^-130~10^126（不包含此值)，需要1~22字節(BYTE)不等的存儲空間。P 是Precison的英文縮寫，即精度縮寫，表示有效數字的位數，最多不能超過38個有效數字。S是Scale的英文縮寫，表示從小數點到最低有效數字的位數，它為負數時，表示從最大有效數字到小數點的位數。有時候，我們在創建表的時候，NUMBER往往沒有指定P，S的值，那么默認情況下，NUMBER的P、S的值分別是多少呢？相信這個問題能問倒一大片DBA。 在之前，我遇到了一個問題，總結整理在“ORACLE NUMBER類型Scale為0引發的問題”這篇博客當中，當時武斷的判斷“如果不指定p和s，NUMBER類型，它的默認精度值為38, 默認的scale值為0”，因為當時參考了官方文檔https://docs.oracle.com/cd/B28359_01/server.111/b28318/datatype.htm#CNCPT1832
當然文檔沒有錯誤，文檔應該是指在定義字段數據類型為NUMBER時，指定了NUMBER類型的P值，但是沒有指定S的值，那么Scale默認就是0，如下測試所示，當時應該是我自己沒有完全理解文檔意思，當然文檔也有誤導的嫌疑。
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>
當在指定字段類型為NUMBER時，如果P和S都不指定，那么P和S又是什么值呢？今天特意實驗驗證了一下，具體實驗過程如下：
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，當我插入上面兩條記錄后，發現如果不指定p和s，NUMBER類型，此時的Scale至少是9，我們繼續測試，插入下面數據
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，此時可以看到Scale的值33了，那么Scale的值是否可以繼續變大呢？
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
如下截圖所示，插入的記錄為12.123456789123456789123456789123456789123，但是顯示的值為12.123456789123456789123456789123456789，總共為38位，由於格式化列的緣故，可能導致部分小數位沒有顯示，
我們繼續測試，調整格式化列，我們發現值變為了12.12345678912345678912345678912345678912，總共40位了，Scale的值為38了。這個是為什么呢？不是數字精度為38，意味着最多是38位嗎？
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
繼續其它測試，我們發現Sacle的值會隨着小數點前面數字的變化而變化，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.
從上面測試可以看出，Scale的值是變化的，跟數據值有關系，目前看來，小數點前的數字位數和小數點后的數字位數相加為40（有時候又是39），為了測試是否這個規律，我特意用下面案例測試一下
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>
這個問題糾結了很久，不明白為什么是39或40，后面在Oracle Database SQL Reference 10g Release 2終於找到解釋了，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，或是有效十進制數的總位數。最大的有效數字是最左邊的非零數字，而最小有效位是最右邊的數字。 Oracle保證數字的可移植性
精度高達20 base-100 digits，相當於39位或40位十進制數字，取決於小數點的位置。