oracle中的instr

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用途
    返回父串中子串的下标，如果父串中不存在该子串就返回0
语法
    instr(string1, string2 [, start_position [, nth_appearance ] ] )
        string1，父串；
        string2，子串；
        start_position，查找的起始位置；
        nth_appearance，第几次出现；
示例
     instr('Tech on the net', 'e') would return 2; the first occurrence of 'e'
      instr('Tech on the net', 'e', 1, 1) would return 2; the first occurrence of 'e'
     instr('Tech on the net', 'e', 1, 2) would return 11; the second occurrence of 'e'
     instr('Tech on the net', 'e', 1, 3) would return 14; the third occurrence of 'e'
     instr('Tech on the net', 'e', -3, 2) would return 2.