查找字符串’10,A,B’ 中逗号’,’出现的次数cnt
    由于 SQLite 中没有直接统计字符串中子串出现次数的函数，因此本题用length()函数与replace()函数的结合灵活地解决了统计子串出现次数的问题，属于技巧题，即先用replace函数将原串中出现的子串用空串替换，再用原串长度减去替换后字符串的长度，最后除以子串的长度（本题中此步可省略，若子串长度大于1则不可省）。详情请参考

    Sqlite 字符串处理函数replace官方说明： replace(X,Y,Z) The replace(X,Y,Z) function returns a string formed by substituting string Z for every occurrence of string Y in string X. The BINARY collating sequence is used for comparisons. If Y is an empty string then return X unchanged. If Z is not initially a string, it is cast to a UTF-8 string prior to processing. x:要处理的字符串

    y:被替换的字符串

    z:替换后的字符串

    有图有真相

    1. SELECT (length("10,A,B")-length(replace("10,A,B",",","")))/length(",") AS cnt