oracle sql 排序與比較中的技巧與注意事項(xiàng)（一）

在sql排序中，oracle默認(rèn)采用二進(jìn)制的排序方法。大小寫(xiě)有不同的值，大寫(xiě)的值排在前面。有時(shí)候，我們需要處理的情況是，希望忽略大小寫(xiě)來(lái)進(jìn)行排序。有多種方法可以實(shí)現(xiàn)：

1. 設(shè)置NLS環(huán)境變量

   alter session set NLS_SORT = 'BINARY_CI';

2. 使用UPPER和LOWER函數(shù)

   用UPPER函數(shù)和LOWER函數(shù)把要比較的字段名、文字都轉(zhuǎn)換成大寫(xiě)或者小寫(xiě)后再比較。這種方法的不足之處在于，使用函數(shù)后，標(biāo)準(zhǔn)的索引就不能再使用了，優(yōu)化器無(wú)法正常工作，應(yīng)對(duì)的方式是使用基于功能的索引(function-based index)。

注意:NLS_SORT僅僅影響排序的結(jié)果，并不對(duì)其他大小寫(xiě)操作造成影響。若要解決不區(qū)分大小寫(xiě)的比較操作，我們同樣可以采用設(shè)置NLS環(huán)境變量的方式來(lái)完成:

  alter session set NLS_COMP = 'LINGUISTIC';

官方文檔中關(guān)于NLS_SORT和NLS_COMP有這樣一段話:

  NLS_SORT specifies the collating sequence for ORDER BY queries.

• If the value is BINARY, then the collating sequence for ORDER BY queries is based on the numeric value of

  characters (a binary sort that requires less system overhead).

• 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.

• 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.

• You must use the NLS_SORT operator with comparison operations if you want the linguistic sort behavior.

        根據(jù)上文中標(biāo)紅部分的注視，如果NLS_SORT不是設(shè)置為"Binary"，那么就會(huì)引起全表掃描，是不會(huì)使用索引的，在我們的系統(tǒng)中變更單涉及到的數(shù)據(jù)都是數(shù)據(jù)龐大的表，如果不使用到索引，查詢的效率會(huì)受到影響。

NLS_COMP specifies the collation behavior of the database session.

        Values:

• BINARY

                   Normally, comparisons in the WHERE clause and in PL/SQL blocks is binary unless you specify the NLSSORT                        function.

• LINGUISTIC

                   Comparisons for all SQL operations in the WHERE clause and in PL/SQL blocks should use the linguistic sort                        specified in the NLS_SORT parameter. To improve the performance, you can also define a linguistic index on                      the column for which you want linguistic comparisons.

• ANSI

                   A setting of ANSI is for backwards compatibility; in general, you should set NLS_COMP to LINGUISTIC.

        根據(jù)標(biāo)紅的部分，要提高性能可以在需要比較的列上建立一個(gè)linguistic index。若想使NLS_COMP參數(shù)值為L(zhǎng)INGUISTIC生效，需要設(shè)置NLS_SORT為L(zhǎng)INGUISTIC 排序。