Oracle可以处理LOB字段的常用字符函数
以前处理LOB字段都是使用DBMS_LOB包，最近看了文档才发现，原来很多常用的字符函数都是支持LOB字段的。
建立一个测试表：
SQL> CREATE TABLE T_LOB (ID NUMBER, CONTENTS CLOB);
Table created.
SQL> DECLARE
2 V_LOB CLOB;
3 BEGIN
4 INSERT INTO T_LOB
5 VALUES (1, EMPTY_CLOB())
6 RETURN CONTENTS INTO V_LOB;
7 FOR I IN 1..100 LOOP
8 DBMS_LOB.WRITEAPPEND(V_LOB, 32767, LPAD('A', 32767, 'A'));
9 END LOOP;
10 END;
11 /
PL/SQL procedure successfully completed.
SQL> SELECT ID, DBMS_LOB.GETLENGTH(CONTENTS) FROM T_LOB;
ID DBMS_LOB.GETLENGTH(CONTENTS)
---------- ----------------------------
1 3276700
SQL> SELECT ID, LENGTH(CONTENTS) FROM T_LOB;
ID LENGTH(CONTENTS)
---------- ----------------
1 3276700
这里的LENGTH函数显然不是对CLOB进行了隐式的TO_CHAR转换，因为那样的话，要不然因为CLOB长度太大而报错，要不然CLOB自动截取前4000个字符而LENGTH返回的长度则是4000。这里的LENGTH函数明显接收的是CLOB类型的参数。
除了LENGTH外，常用的INSTR、SUBSTR等都是支持LOB字段的：
SQL> DECLARE
2 V_LOB CLOB;
3 BEGIN
4 SELECT CONTENTS
5 INTO V_LOB
6 FROM T_LOB
7 WHERE ID = 1
8 FOR UPDATE;
9 DBMS_LOB.WRITEAPPEND(V_LOB, 1000, RPAD('BCDEFG', 1000, 'A'));
10 COMMIT;
11 END;
12 /
PL/SQL procedure successfully completed.
SQL> SELECT DBMS_LOB.INSTR(CONTENTS, 'B')
2 FROM T_LOB
3 WHERE ID = 1;
DBMS_LOB.INSTR(CONTENTS,'B')
----------------------------
3276701
SQL> SELECT INSTR(CONTENTS, 'B')
2 FROM T_LOB
3 WHERE ID = 1;
INSTR(CONTENTS,'B')
-------------------
3276701
SQL> SELECT DBMS_LOB.SUBSTR(CONTENTS, 10, 3276701)
2 FROM T_LOB
3 WHERE ID = 1;
DBMS_LOB.SUBSTR(CONTENTS,10,3276701)
-------------------------------------------------------------------------------------------
BCDEFGAAAA
SQL> SELECT SUBSTR(CONTENTS, 3276701, 10)
2 FROM T_LOB
3 WHERE ID = 1;
SUBSTR(CONTENTS,3276701,10)
--------------------------------------------------------------------------------
BCDEFGAAAA
另外，LPAD、RPAD以及LTRIM和RTRIM也是支持CLOB类型的：
SQL> SELECT LTRIM(RTRIM(CONTENTS, 'A'), 'A')
2 FROM T_LOB
3 WHERE ID = 1;
LTRIM(RTRIM(CONTENTS,'A'),'A')
--------------------------------------------------------------------------------
BCDEFG
SQL> SELECT LENGTH(LPAD(CONTENTS, 10000000, 'B'))
2 FROM T_LOB
3 WHERE ID = 1;
LENGTH(LPAD(CONTENTS,10000000,'B'))
-----------------------------------
10000000
字符连接操作||也支持LOB类型：
SQL> SELECT LENGTH(CONTENTS || CONTENTS)
2 FROM T_LOB
3 WHERE ID = 1;
LENGTH(CONTENTS||CONTENTS)
--------------------------
6555400
如果想找更加详细的支持LOB的函数说明可以参考LOB相关的文档。
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