*单行函数
*length(获取字节个数）
select length("join") 返回4
*concat(拼接）
select concat("firstname","_","last_name"); firstname_last_name
*upper(将所有字母变大写）
select upper(zqc); ZQC
*lower(将所有字母变小写）
select lower(ZQC); zqc
*substr(截取字符）
select substr("abcdef",3) cdef
select substr("abcdef",2,3) bcd
*instr(返回子串第一次出现的开始索引，如果没有返回0)
select instr("abcdefg","cd") 3
select instr("abcdefg","cfd") 0
*trim(去掉前后的空格）
select trim(" abc "); abc
select trim("a" from "aaaaabbbbbaaaaaa") bbbbb
*lpad (左填充）
select lpad("abc",10,"a") aaaaaaaabc
select lpad("abc",2,"a") ab
*rpad(右填充）
select rpad ("abc",10,"a") abcaaaaaaa
select rpad("abc",2,"a") ab
*replace (代替）
select replace("abccdefg","abc","haha") hahacdefg
select replace("abcdefgabcabc","abc","ha") hadefghaha