单引号转义
-- 单引号（就近原则）
-- 作用1 ： 引用字符串
-- 作用2： 转义
-- 引用字符串和转义结合时理解
-- 字符串
SELECT '字符串' AS III FROM DUAL; --字符串
SELECT '' AS III FROM DUAL; -- NULL
-- 单引号转义
-- 1. 单引号必须成对出现，最外层一对为引用字符串作用
-- 2. 成对的字符串一个为转义符，另一个被转义
SELECT '''' AS III FROM DUAL; -- '
SELECT '''''' AS III FROM DUAL; -- ''
-- 字符串与字符结合
-- 1. 字符串必须成对(抛开最外层，里面一对引号中间不能有字符串)
-- 2. 在字符串内输出单引号必须使用转义
-- 3. yang'、yang''、 'yang'
SELECT 'yang''' AS III FROM DUAL; -- yang'
SELECT 'yang''''' AS III FROM DUAL; -- yang''
SELECT '''''yang' AS III FROM DUAL; -- ''yang
SELECT '''yang''' AS III FROM DUAL; -- yang''
SELECT '''yang''' AS III FROM DUAL; -- 'yang'
-- 转义与连接符
-- 1. 连接符将对前后面引号进行新一轮转义
-- 2. 连接符两遍字符串必须成对(抛开最外层，里面一对引号中间不能有字符串)
-- 3. 连接符两遍各自计算转义并然后将其连接
SELECT '''yang''''''' AS III FROM DUAL; -- 'yang'''
SELECT '''yang'''||'''' AS III FROM DUAL; -- 'yang''
SELECT '''||'yang'''''''' AS III FROM DUAL; -- 'yang''
SELECT ''''||'yang''''''' AS III FROM DUAL; -- 'yang''
SELECT '''zhi'||'yang''''''' AS III FROM DUAL; -- 'yang''
SELECT 'yang'''||'''''zhi''' AS III FROM DUAL; -- yang'''zhi'
\
