Oracle Text Literals (Q quote_delimiter )

在SQL或PL/SQL中，是表示一个字符串时，通常要用单引号('')括起来。
比如：insert或是select的时候。
insert into test values('test');
select 'test' from dual;

但如果字符串本身包含'或者"这样的字符，那么实现起来有点麻烦。
比如: Smith's bag

--直接用单引号括起来，会报错，因为不配对
SQL> select 'Smith's bag' from dual;
ERROR:
ORA-01756: quoted string not properly terminated

--可以用一个'符号来转义
SQL> select 'Smith''s bag' sb from dual;

SB
----------------------
Smith's bag

--双引号，则不用转义，它还是代表他本来的意义
SQL> select '"Smith"s bag' sb from dual;

SB
------------------------
"Smith"s bag

--要显示'Smith'，需要写成'''Smith'''
SQL> select '''Smith''' sb from dual;

SB
-----------
'Smith'

SQL> select '''Smith''s bag''' sb from dual;

SB
--------------------------
'Smith's bag'

在Oracle中，single-quote(')是一个表示字符串的关键字。所以在字符串中用两个''表示一个实际的单引号字符。
其实Oracle提供了一个Q-quote的表达式，用来简化SQL或PLSQL中字符串的表示。

SQL> select q'[Smith's bag]' sb from dual;

SB
----------------------
Smith's bag

SQL> select q'/Smith's bag/' sb from dual;

SB
----------------------
Smith's bag

SQL> select q'#Smith's bag#' sb from dual;

SB
----------------------
Smith's bag

SQL> select q'<Smith's bag>' sb from dual;

SB
----------------------
Smith's bag

SQL> select q'\Smith's bag\' sb from dual;

SB
----------------------
Smith's bag

SQL> select q'|Smith's bag|' sb from dual;

SB
----------------------
Smith's bag

语法还是比较简单的，必须将要表示的字符串用一对特殊字符括起来，这对字符必须一致。
字符对不一致，就会报错，比如下面的例子：

SQL> select q'[Smith's bag|' sb from dual;
ERROR:
ORA-01756: quoted string not properly terminated

联机文档描述原文：
      http://docs.oracle.com/cd/E11882_01/server.112/e41084/sql_elements003.htm#SQLRF00218

         
     1) Q or q indicates that the alternative quoting mechanism will be used. This mechanism allows a wide range of delimiters for the text string.

    2) The outermost ' ' are two single quotation marks that precede and follow, respectively, the opening and closing quote_delimiter.

    3) c is any member of the user's character set. You can include quotation marks (") in the text literal made up of c characters. You can also include the quote_delimiter, as long as it is not immediately followed by a single quotation mark.

    4) quote_delimiter is any single- or multibyte character except space, tab, and return. The quote_delimiter can be a single quotation mark. However, if the quote_delimiter appears in the text literal itself, ensure that it is not immediately followed by a single quotation mark.
    If the opening quote_delimiter is one of [, {, <, or (, then the closing quote_delimiter must be the corresponding ], }, >, or ). In all other cases, the opening and closing quote_delimiter must be the same character.