substr('This is a test', 6, 2)   would return 'is'
   substr('This is a test', 6)   would return 'is a test'
   substr('TechOnTheNet', -3, 3)   would return 'Net'
   substr('TechOnTheNet', -6, 3)   would return 'The'
 select substr('Thisisatest', -4, 2) value from dual

SELECT INSTR('CORPORATE FLOOR', 'OR', -1, 1) "Instring" FROM DUAL
--INSTR(源字符串, 目标字符串, 起始位置, 匹配序号)
SELECT INSTR('CORPORATE FLOOR','OR', 3, 2) "Instring" FROM DUAL
SELECT INSTR('32.8,63.5',',', 1, 1) "Instring" FROM DUAL
SELECT SUBSTR('32.8,63.5',INSTR('32.8,63.5',',', 1, 1)+1) "INSTRING" FROM DUAL
SELECT SUBSTR('32.8,63.5',1,INSTR('32.8,63.5',',', 1, 1)-1) "INSTRING" FROM DUAL
-- CREATED ON 2008-9-26 BY ADMINISTRATOR
DECLARE
 -- LOCAL VARIABLES HERE
 T  VARCHAR2(2000);
 S  VARCHAR2(2000);
 NUM INTEGER;
 I  INTEGER;
 POS INTEGER;
BEGIN
 -- TEST STATEMENTS HERE
 T := '12.3,23.0;45.6,54.2;32.8,63.5;';
 SELECT LENGTH(T) - LENGTH(REPLACE(T, ';', '')) INTO NUM FROM DUAL;
 DBMS_OUTPUT.PUT_LINE('NUM:' || NUM);
 POS := 0;
 FOR I IN 1 .. NUM LOOP
  DBMS_OUTPUT.PUT_LINE('I:' || I);
  DBMS_OUTPUT.PUT_LINE('POS:' || POS);
  DBMS_OUTPUT.PUT_LINE('==:' || INSTR(T, ';', 1, I));
  DBMS_OUTPUT.PUT_LINE('INSTR:' || SUBSTR(T, POS + 1, INSTR(T, ';', 1, I) - 1));
  POS := INSTR(T, ';', 1, I);
 END LOOP;
END;
 -- Created on 2008-9-26 by ADMINISTRATOR
declare
 -- Local variables here
 i integer;
 T   VARCHAR2(2000);
 S   VARCHAR2(2000);
begin
 -- Test statements here
  --历史状态
 T := '12.3,23.0;45.6,54.2;32.8,63.5;';
 IF (T IS NOT NULL) AND (LENGTH(T) > 0) THEN
  --T := T || ',';
  WHILE LENGTH(T) > 0 LOOP
   --ISTATUSID := 0;
   S     := TRIM(SUBSTR(T, 1, INSTR(T, ';') - 1));
   IF LENGTH(S) > 0 THEN
     DBMS_OUTPUT.PUT_LINE('LAT:'||SUBSTR('32.8,63.5',1,INSTR('32.8,63.5',',', 1, 1)-1));
     DBMS_OUTPUT.PUT_LINE('LON:'||SUBSTR('32.8,63.5',INSTR('32.8,63.5',',', 1, 1)+1));
    -- COMMIT;
   END IF;
   T := SUBSTR(T, INSTR(T, ';') + 1);
  END LOOP;
 END IF; 
end;

SELECT INSTR('CORPORATE FLOOR', 'OR', 0, 1) FROM DUAL; 返回值为0
 SELECT INSTR('CORPORATE FLOOR', 'OR', 2, 1) FROM DUAL; 返回值为2
 SELECT INSTR('CORPORATE FLOOR', 'OR', 2, 2) FROM DUAL; 返回值为5
 SELECT INSTR('CORPORATE FLOOR', 'OR', -1, 1) FROM DUAL; 返回值为14
 SELECT INSTR('CORPORATE FLOOR', 'OR', -5, 1) FROM DUAL; 返回值为5

SELECT SUBSTR('This is a test', 0, 2) value from dual; 返回值Th
 SELECT SUBSTR('This is a test', 1, 2) value from dual; 返回值Hi
 SELECT SUBSTR('This is a test', -1, 2) value from dual; 返回值t
 SELECT SUBSTR('This is a test', -2, 2) value from dual; 返回值st

SELECT nvl('pos1',null) from dual; 返回值为pos1
 SELECT nvl(null,'pos2') from dual; 返回值为pos1
 SELECT nvl(null,null) from dual;  返回值为null