How to query a CLOB column in Oracle

Question

I'm trying to run a query that has a few columns that are a CLOB datatype. If i run the query like normal, all of those fields just have (CLOB) as the value.

I tried using DBMS_LOB.substr(column) and i get the error

ORA-06502: PL/SQL: numeric or value error: character string buffer too small

How can i query the CLOB column?

Answer 1

When getting the substring of a CLOB column and using a query tool that has size/buffer restrictions sometimes you would need to set the BUFFER to a larger size. For example while using SQL Plus use the SET BUFFER 10000 to set it to 10000 as the default is 4000.

Running the DBMS_LOB.substr command you can also specify the amount of characters you want to return and the offset from which. So using DBMS_LOB.substr(column, 3000) might restrict it to a small enough amount for the buffer.

See oracle documentation for more info on the substr command

    DBMS_LOB.SUBSTR (
       lob_loc     IN    CLOB   CHARACTER SET ANY_CS,
       amount      IN    INTEGER := 32767,
       offset      IN    INTEGER := 1)
      RETURN VARCHAR2 CHARACTER SET lob_loc%CHARSET;

Answer 2

This works

select DBMS_LOB.substr(myColumn, 3000) from myTable

Answer 3

I did run into another condition with HugeClob in my Oracle database. The dbms_lob.substr only allowed a value of 4000 in the function, ex:

dbms_lob.substr(column,4000,1)

so for my HughClob which was larger, I had to use two calls in select:

select dbms_lob.substr(column,4000,1) part1, 
       dbms_lob.substr(column,4000,4001) part2 from .....

I was calling from a Java app so I simply concatenated part1 and part2 and sent as a email.

Answer 4

For big CLOB selects also can be used:

SELECT dbms_lob.substr( column_name, dbms_lob.getlength(column_name), 1) FROM foo

Answer 5

Another option is to create a function and call that function everytime you need to select clob column.

create or replace function clob_to_char_func
(clob_column in CLOB,
 for_how_many_bytes in NUMBER,
 from_which_byte in NUMBER)
return VARCHAR2
is
begin
Return substrb(dbms_lob.substr(clob_column
                            ,for_how_many_bytes
                            ,from_which_byte)
            ,1
            ,for_how_many_bytes);
end;

and call that function as;

SELECT tocharvalue, clob_to_char_func(tocharvalue, 1, 9999)
FROM (SELECT clob_column AS tocharvalue FROM table_name);

Answer 6

If it's a CLOB why can't we to_char the column and then search normally ?

Create a table

CREATE TABLE MY_TABLE(Id integer PRIMARY KEY, Name varchar2(20), message clob);

Create few records in this table

INSERT INTO MY_TABLE VALUES(1,'Tom','Hi This is Row one');
INSERT INTO MY_TABLE VALUES(2,'Lucy', 'Hi This is Row two');
INSERT INTO MY_TABLE VALUES(3,'Frank', 'Hi This is Row three');
INSERT INTO MY_TABLE VALUES(4,'Jane', 'Hi This is Row four');
INSERT INTO MY_TABLE VALUES(5,'Robert', 'Hi This is Row five');
COMMIT;

Search in the clob column

SELECT * FROM MY_TABLE where to_char(message) like '%e%';

Results

ID   NAME    MESSAGE   
===============================  
1    Tom     Hi This is Row one         
3    Frank   Hi This is Row three
5    Robert  Hi This is Row five

Answer 7

To add to the answer.

declare
v_result clob;
begin
---- some operation on v_result
dbms_lob.substr( v_result, 4000 ,length(v_result) - 3999 );

end;
/

In dbms_lob.substr

first parameter is clob which you want to extract .

Second parameter is how much length of clob you want to extract.

Third parameter is from which word you want to extract .

In above example i know my clob size is more than 50000 , so i want last 4000 character .

Answer 8

If you are using SQL*Plus try the following...

set long 8000

select ...