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Don Burleson Blog 







Compare two CLOB columns with dbms_lob

Oracle Tips by Burleson Consulting

December 26, 2009

Question:  I need to compare the contents of two CLOB datatypes in a column.  How do you compare CLOB columns?  How I can compare CLOB column in two table?

Answer: Oracle provides the dbms_lob package, specifically the procedure for comparing two CLOBs. The procedure returns a zero of the strings match and a -1 if they do not match. 

Example using in SQL

In this example, we use in SQL to compare if two CLOB columns are matching in two separate tables:
  For using in SQL, you can compare two LOB columns within a single table, joining the table against itself, like this example comparing the CLOB column col1 with CLOB column col2. 

   tab1   a,
   tab1   b
   a.tab_key = b.tab_key;

In this example, we might use a cross join to display rows where the CLOB column are duplicates.

BEWARE:  This code below is a cross join because there is no table join criteria in the where clause.  This produces a Cartesian product of all rows in the table, so don't; expect sub-second response time:

   customer   a,
   customer   b
(a.clob_comments, b.clob_comments) = 0;

Now let's look at examples of using to compare two CLOB columns in PL/SQL.

Example using in PL/SQL

Here is a simple example of using to compare two CLOB datatypes in PL/SQL:

   c1 clob := 'don';
   c2 clob := 'burleson';
      dbms_output.put_line( c1, c2 ) );

In this example, would return -1 because the two CLOB values do not match.

We can also use with SQL inside PL/SQL, something like this:

   c1 clob := 'don';
   c2 clob := 'burleson';
      dbms_output.put_line( c1, c2 ) );

You can also use to compare substrings within a CLOB.  In this example, we add three arguments after the c1 and c2 arguments:

  • argument 1 - This is the first CLOB

  • argument 2 - This is the second CLOB

  • argument 3- This is the number of bytes to compare

  • argument 4 - this is the offset into string1 (c1)

  • argument 5 - This is the offset into string2 (c1)

Consider this example and note the area of matching text:

    c1 clob := 'don';
   c2 clob := 'mastadon';
      dbms_output.put_line( c1, c2, 3, 1, 6 ) );

Above we return a zero(0) value because we are matching three letters, the first three in string c1 (don) with the three letters in string 3, starting at position 5 (don).

If you like Oracle tuning, you may enjoy my new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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