单行函数（章节摘要）
1，单行函数可以完成许多独立的小功能。 2，字符函数的主要功能是进行字符串数据的处理，例如将字符串变为大写，小写，查找等。 3，使用ROUND()函数可以进行制定位数的四舍五入操作，使用TRUNC()函数可直接截取掉小数。 4，SYSDATE可以取得当前的系统日期，使
1，单行函数可以完成许多独立的小功能。2，字符函数的主要功能是进行字符串数据的处理，例如将字符串变为大写，小写，查找等。
3，使用ROUND()函数可以进行制定位数的四舍五入操作，使用TRUNC()函数可直接截取掉小数。
4，SYSDATE可以取得当前的系统日期，使用日期函数处理日期可以避免闰年的问题。
5，转换函数中TO_CHAR()函数使用较多，使用TO_CHAR()函数可以完成日期或数字变为格式化字符串的操作。
6，在进行数学计算中，如果存在null，则结果就为null，此时可以使用NVL()或NVL2()函数将null转为制定的内容。
7，使用NULLIF()函数可以判断两个表达式的结果是否相等。
8，Decode()函数和CASE表达式可以实现多条件判断输出操作。
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