Converting data in a string format between different data types is usually a straightforward process in Oracle, but some cases are a little more challenging than others. One of these occurs when you use Oracle to convert clob to varchar2, which is difficult because a varchar2 column has a maximum of 4,000 characters, while clob can store huge amounts of character data. You can still achieve your goal, though, using either the dbms_lob substr method or to_char and specifying a maximum size.
CLOB and VARCHAR2 Explained
CLOB (or "clob" colloquially) is an acronym for "character large object," and it can store up to 4 GB of data. It's used by several database systems, including DB2 and Oracle. CLOB is useful when you need to store vast amounts of data, but the size itself can cause problems in some situations. This is why conversion from an Oracle CLOB to a string format of a more manageable size is often a good approach before performing operations.
The VARCHAR2 format stores smaller amounts of data, usually up to 4,000 bytes, unless you use an extended maximum string size, in which case it can go up to 32,767 bytes. For a single-byte character set, you can usually get up to 4,000 characters in a column. The discrepancy between the maximum sizes of these two data formats is where the problems arise during conversion.
The dbms_lob substr Method
To convert a clob string to varchar2, open your SQL *Plus program. The syntax for the conversion you need to perform is:
SELECT dbms_lob.substr( clob_column, for_how_many_bytes, from_which_byte ) FROM table
Here the values in parentheses are ones you have to change, as well as "table" at the end. These correspond to the column ("clob_column") and the table you're working with. The "for_how_many_bytes" gives you the opportunity to specify how many bytes of data you want to convert, and the "from_which_byte" allows you to specify a starting point.
So, if you have a table called "taxes" and a field called "net_pay," and assuming you want 4,000 bytes starting from the first, you'd enter:
SELECT dbms_lob.substr(net_pay, 4000, 1) FROM taxes
To convert the string into varchar2 format.
The TO_CHAR Method
You can also use the TO_CHAR command with a very similar syntax to achieve the same goal:
SELECT TO_CHAR(SUBSTR (clob_column,0,3999)) FROM table
Here, the beginning and ending bytes are specified as 0 and 3999 (for a 4,000 character string starting from the beginning), but the "clob_column" and "table" fields are used in exactly the same way as before. So, in the same example, you could enter:
SELECT TO_CHAR(SUBSTR (net_pay,0,3999)) FROM taxes
Basically, rather than completing the process in a single step, this uses the SUBSTR command to pull a substring from the larger string, and then it uses "to_char" to handle the conversion. If the clob column in question has fewer than 4,000 bytes of information, you can enter the name of the column in the parentheses after to_char instead of specifying a substring.