ORA-06502: PL/SQL: 數字或值錯誤（oracle顯示中文的首字母）

我的功能是oracle顯示中文字段的首字母
首先是oracle的function

CREATE OR REPLACE FUNCTION F_TRANS_PINYIN_CAPITAL(P_NAME IN VARCHAR2) RETURN VARCHAR2 AS
V_COMPARE VARCHAR2(100);
V_RETURN VARCHAR2(4000);

FUNCTION F_NLSSORT(P_WORD IN VARCHAR2) RETURN VARCHAR2 AS
BEGIN
RETURN NLSSORT(P_WORD, 'NLS_SORT=SCHINESE_PINYIN_M');
END;
BEGIN
FOR I IN 1..LENGTH(P_NAME) LOOP
  V_COMPARE := F_NLSSORT(SUBSTR(P_NAME, I, 1));
  IF V_COMPARE >= F_NLSSORT(' 吖 ') AND V_COMPARE <= F_NLSSORT('驁 ') THEN
  V_RETURN := V_RETURN || 'a';
  ELSIF V_COMPARE >= F_NLSSORT('八 ') AND V_COMPARE <= F_NLSSORT('簿 ') THEN
  V_RETURN := V_RETURN || 'b';
  ELSIF V_COMPARE >= F_NLSSORT('嚓 ') AND V_COMPARE <= F_NLSSORT('錯 ') THEN
  V_RETURN := V_RETURN || 'c';
  ELSIF V_COMPARE >= F_NLSSORT('咑 ') AND V_COMPARE <= F_NLSSORT('鵽 ') THEN
  V_RETURN := V_RETURN || 'd';
  ELSIF V_COMPARE >= F_NLSSORT('妸 ') AND V_COMPARE <= F_NLSSORT('樲 ') THEN
  V_RETURN := V_RETURN || 'e';
  ELSIF V_COMPARE >= F_NLSSORT('發 ') AND V_COMPARE <= F_NLSSORT('猤 ') THEN
  V_RETURN := V_RETURN || 'f';
  ELSIF V_COMPARE >= F_NLSSORT('旮 ') AND V_COMPARE <= F_NLSSORT('腂 ') THEN
  V_RETURN := V_RETURN || 'g';
  ELSIF V_COMPARE >= F_NLSSORT('妎 ') AND V_COMPARE <= F_NLSSORT('夻 ') THEN
  V_RETURN := V_RETURN || 'h';
  ELSIF V_COMPARE >= F_NLSSORT('丌 ') AND V_COMPARE <= F_NLSSORT('攈 ') THEN
  V_RETURN := V_RETURN || 'j';
  ELSIF V_COMPARE >= F_NLSSORT('咔 ') AND V_COMPARE <= F_NLSSORT('穒 ') THEN
  V_RETURN := V_RETURN || 'k';
  ELSIF V_COMPARE >= F_NLSSORT('垃 ') AND V_COMPARE <= F_NLSSORT('擽 ') THEN
  V_RETURN := V_RETURN || 'l';
  ELSIF V_COMPARE >= F_NLSSORT('嘸 ') AND V_COMPARE <= F_NLSSORT('椧 ') THEN
  V_RETURN := V_RETURN || 'm';
  ELSIF V_COMPARE >= F_NLSSORT('拏 ') AND V_COMPARE <= F_NLSSORT('瘧 ') THEN
  V_RETURN := V_RETURN || 'n';
  ELSIF V_COMPARE >= F_NLSSORT('筽 ') AND V_COMPARE <= F_NLSSORT('漚 ') THEN
  V_RETURN := V_RETURN || 'o';
  ELSIF V_COMPARE >= F_NLSSORT('妑 ') AND V_COMPARE <= F_NLSSORT('曝 ') THEN
  V_RETURN := V_RETURN || 'p';
  ELSIF V_COMPARE >= F_NLSSORT('七 ') AND V_COMPARE <= F_NLSSORT('裠 ') THEN
  V_RETURN := V_RETURN || 'q';
  ELSIF V_COMPARE >= F_NLSSORT('亽 ') AND V_COMPARE <= F_NLSSORT('鶸 ') THEN
  V_RETURN := V_RETURN || 'r';
  ELSIF V_COMPARE >= F_NLSSORT('仨 ') AND V_COMPARE <= F_NLSSORT('蜶 ') THEN
  V_RETURN := V_RETURN || 's';
  ELSIF V_COMPARE >= F_NLSSORT('侤 ') AND V_COMPARE <= F_NLSSORT('籜 ') THEN
  V_RETURN := V_RETURN || 't';
  ELSIF V_COMPARE >= F_NLSSORT('屲 ') AND V_COMPARE <= F_NLSSORT('鶩 ') THEN
  V_RETURN := V_RETURN || 'w';
  ELSIF V_COMPARE >= F_NLSSORT('夕 ') AND V_COMPARE <= F_NLSSORT('鑂 ') THEN
  V_RETURN := V_RETURN || 'x';
  ELSIF V_COMPARE >= F_NLSSORT('丫 ') AND V_COMPARE <= F_NLSSORT('韻 ') THEN
  V_RETURN := V_RETURN || 'y';
  ELSIF V_COMPARE >= F_NLSSORT('帀 ') AND V_COMPARE <= F_NLSSORT('咗 ') THEN
  V_RETURN := V_RETURN || 'z';
  ELSE
    V_RETURN := V_RETURN ||SUBSTR(P_NAME, I, 1);
END IF;
END LOOP;
RETURN V_RETURN;
END;

然後是執行的sql

select (substr(F_TRANS_PINYIN_CAPITAL(NAME), 0,1)) as title from xxxx

報錯的原因也很簡單，有一個空值，是的，我找了一下午的bug就因爲這，共勉