ORACLE預設所存的值是取分大小寫的，但有些需求想忽略大小寫，今天就淺談一下nls_sort ,nls_comp實現查詢忽略大小寫查詢

1. [email protected]> select * from testci ;   
2.         ID NAME                 REMARK   
3. ---------- -------------------- --------------------
4.          1 a                    lower code   
5.          1 A                    upper code   
6. [email protected]> select * from testci wherename='a';   
7.         ID NAME                 REMARK   
8. ---------- -------------------- --------------------
9.          1 a                    lower code   
10. [email protected]> select * from  v$nls_parameters where parameter ='NLS_SORT';   
11. PARAMETER                         VALUE   
12. -------------------------------- ----------------
13. NLS_SORT                          BINARY
14. [email protected]> ALTER SESSION SET NLS_SORT='BINARY_CI';   
15. Session altered.   
16. [email protected]> select * from testci wherename='a';   
17.         ID NAME                 REMARK   
18. ---------- -------------------- --------------------
19.          1 a                    lower code   
20. [email protected]> select * from  v$nls_parameters where parameter ='NLS_COMP';   
21. PARAMETER                         VALUE   
22. ---------------------------------- ------------------
23. NLS_COMP                          BINARY
24. [email protected]> ALTER SESSION SET NLS_COMP=LINGUISTIC;   
25. Session altered.   
26. [email protected]> select * from testci wherename='a';   
27.         ID NAME                 REMARK   
28. ---------- -------------------- --------------------
29.          1 a                    lower code   
30.          1 A                    upper code  

[email protected]> select * from testci ;

        ID NAME                 REMARK
---------- -------------------- --------------------
         1 a                    lower code
         1 A                    upper code

[email protected]> select * from testci where name='a';

        ID NAME                 REMARK
---------- -------------------- --------------------
         1 a                    lower code

[email protected]> select * from  v$nls_parameters where parameter ='NLS_SORT';

PARAMETER                         VALUE
-------------------------------- ----------------
NLS_SORT                          BINARY

[email protected]> ALTER SESSION SET NLS_SORT='BINARY_CI';

Session altered.

[email protected]> select * from testci where name='a';

        ID NAME                 REMARK
---------- -------------------- --------------------
         1 a                    lower code

[email protected]> select * from  v$nls_parameters where parameter ='NLS_COMP';

PARAMETER                         VALUE
---------------------------------- ------------------
NLS_COMP                          BINARY

[email protected]> ALTER SESSION SET NLS_COMP=LINGUISTIC;

Session altered.

[email protected]> select * from testci where name='a';

        ID NAME                 REMARK
---------- -------------------- --------------------
         1 a                    lower code
         1 A                    upper code

NLS_COMP:
確定資料庫中值的比較，10G中增加了LINGUISTIC，根據nls_sort引數指定進行比較，用來取代ansi,但ANSI為了向後相容保留，還有一個值是BINARY根據字元的二進位制值比較也是預設值，總共三個值

note:To improve the performance, you can also define a linguistic index on the column for which you want linguistic comparisons.

nls_sort:
指定order by 的查詢順序，If the value is BINARY, then the collating sequence for ORDER BY queries is based on the numeric value of characters.

note: If the value is a named linguistic sort, sorting is based on the order of the defined linguistic sort. Most (but not all) languages supported by the NLS_LANGUAGE parameter also support a linguistic sort with the same name.

值非常多，可以通過V$NLS_VALID_VALUES where parameter=’SORT’ 查詢

不過修改任何引數都要考慮效能問題

Setting NLS_SORT to anything other than BINARY causes a sort to use a full table scan, regardless of the path chosen by the optimizer. BINARY is the exception because indexes are built according to a binary order of keys. Thus the optimizer can use an index to satisfy the ORDER BY clause when NLS_SORT is set to BINARY. If NLS_SORT is set to any linguistic sort, the optimizer must include a full table scan and a full sort in the execution plan.
因篇幅原因 就說這些，其實與這兩個引數相關的還有很多…