9
SQL Semantics and LOBs

This chapter describes SQL semantics that are supported for LOBs. These techniques allow you to use LOBs directly in SQL code and provide an alternative to using LOB-specific APIs for some operations.

This chapter covers the following topics:

Using LOBs in SQL

You can access CLOB and NCLOB datatypes using SQL VARCHAR2 semantics, such as SQL string operators and functions. (LENGTH functions can be used with BLOB datatypes as well as CLOB and NCLOBs.) These techniques are beneficial in the following situations:

When performing operations on LOBs that are relatively small in size (up to about 100K bytes).
After migrating your database from LONG columns to LOB datatypes, any SQL string functions, contained in your existing PL/SQL application, will continue to work after the migration.

SQL semantics are not recommended in the following situations:

When you need to use advanced features such as random access and piecewise fetch, you should use LOB APIs.
When performing operations on LOBs that are relatively large in size (greater than 1MB) using SQL semantics can impact performance. Using the LOB APIs is recommended in this situation.

Note:
SQL semantics are used with persistent and temporary LOBs. (SQL semantics do not apply to BFILE columns as BFILE is a read-only datatype.)

SQL Functions and Operators Supported for Use with LOBs

Many SQL operators and functions that take VARCHAR2 columns as arguments also accept LOB columns. The following list summarizes which categories of SQL functions and operators are supported for use with LOBs. Details on individual functions and operators are given in Table 9-1.

The following categories of SQL functions and operators are supported for use with LOBs:

Concatenation
Comparison
(Some comparison functions are not supported for use with LOBs.)
Character functions
Conversion
(Some conversion functions are not supported for use with LOBs.)

The following categories of functions are not supported for use with LOBs:

Aggregate functions
Note that although pre-defined aggregate functions are not supported for use with LOBs, you can create user-defined aggregate functions to use with LOBs. See the Oracle Data Cartridge Developer's Guide for more information on user-defined aggregate functions.
Unicode functions

Details on individual functions and operators are given in Table 9-1. This table lists SQL operators and functions that take VARCHAR2 types as operands or arguments, or return a VARCHAR2 value, and indicates in the "SQL" column which functions and operators are supported for CLOB and NCLOB datatypes. (The LENGTH function is also supported for the BLOB datatype.)

The DBMS_LOB PL/SQL package supplied with Oracle Database supports using LOBs with most of the functions listed in Table 9-1 as indicated in the "PL/SQL" column.

Note:

Operators and functions with "No" indicated in the SQL column of Table 9-1 do not work in SQL queries used in PL/SQL blocks - even though some of these operators and functions are supported for use directly in PL/SQL code.

Implicit Conversion of CLOB to CHAR Types

Functions designated as "CNV" in the SQL or PL/SQL column of Table 9-1 are performed by converting the CLOB to a character datatype, such as VARCHAR2. In the SQL environment, only the first 4K bytes of the CLOB are converted and used in the operation; in the PL/SQL environment, only the first 32K bytes of the CLOB are converted and used in the operation.

Table 9-1 SQL VARCHAR2 Functions and Operators on LOBs

Category	Operator / Function	SQL Example / Comments	SQL	PL/SQL
Concatenation	\|\|, CONCAT()	Select clobCol \|\| clobCol2 from tab;	Yes	Yes
Comparison	= , !=, >, >=, <, <=, <>, ^=	if clobCol=clobCol2 then...	No	Yes
Comparison	IN, NOT IN	if clobCol NOT IN (clob1, clob2, clob3) then...	No	Yes
Comparison	SOME, ANY, ALL	if clobCol < SOME (select clobCol2 from...) then...	No	N/A
Comparison	BETWEEN	if clobCol BETWEEN clobCol2 and clobCol3 then...	No	Yes
Comparison	LIKE [ESCAPE]	if clobCol LIKE '%pattern%' then...	Yes	Yes
Comparison	IS [NOT] NULL	where clobCol IS NOT NULL	Yes	Yes
Character Functions	INITCAP, NLS_INITCAP	select INITCAP(clobCol) from...	CNV	CNV
Character Functions	LOWER, NLS_LOWER, UPPER, NLS_UPPER	...where LOWER(clobCol1) = LOWER(clobCol2)	Yes	Yes
Character Functions	LPAD, RPAD	select RPAD(clobCol, 20, ' La') from...	Yes	Yes
Character Functions	TRIM, LTRIM, RTRIM	...where RTRIM(LTRIM(clobCol,'ab'), 'xy') = 'cd'	Yes	Yes
Character Functions	REPLACE	select REPLACE(clobCol, 'orig','new') from...	Yes	Yes
Character Functions	SOUNDEX	...where SOUNDEX(clobCOl) = SOUNDEX('SMYTHE')	CNV	CNV
Character Functions	SUBSTR	...where substr(clobCol, 1,4) = 'THIS'	Yes	Yes
Character Functions	TRANSLATE	select TRANSLATE(clobCol, '123abc','NC') from...	CNV	CNV
Character Functions	ASCII	select ASCII(clobCol) from...	CNV	CNV
Character Functions	INSTR	...where instr(clobCol, 'book') = 11	Yes	Yes
Character Functions	LENGTH	...where length(clobCol) != 7;	Yes	Yes
Character Functions	NLSSORT	...where NLSSORT (clobCol,'NLS_SORT = German') > NLSSORT ('S','NLS_SORT = German')	CNV	CNV
Character Functions	INSTRB, SUBSTRB, LENGTHB	These functions are supported only for CLOBs that use single-byte character sets. (LENGTHB is supported for BLOBs as well as CLOBs.)	Yes	Yes
Character Functions - Regular Expressions	REGEXP_LIKE	This function searches a character column for a pattern. Use this function in the WHERE clause of a query to return rows matching the regular expression you specify. See the Oracle Database SQL Reference for syntax details on SQL functions for regular expressions. See the Oracle Database Application Developer's Guide - Fundamentals for information on using regular expressions with the database.	Yes	Yes
Character Functions - Regular Expressions	REGEXP_REPLACE	This function searches for a pattern in a character column and replaces each occurrence of that pattern with the pattern you specify.	Yes	Yes
Character Functions - Regular Expressions	REGEXP_INSTR	This function searches a string for a given occurrence of a regular expression pattern. You specify which occurrence you want to find and the start position to search from. This function returns an integer indicating the position in the string where the match is found.	Yes	Yes
Character Functions - Regular Expressions	REGEXP_SUBSTR	This function returns the actual substring matching the regular expression pattern you specify.	Yes	Yes
Conversion	CHARTOROWID	CHARTOROWID(clobCol)	CNV	CNV
Conversion	HEXTORAW	HEXTORAW(CLOB)	No	CNV
Conversion	CONVERT	select CONVERT(clobCol,'WE8DEC','WE8HP') from...	Yes	CNV
Conversion	TO_DATE	TO_DATE(clobCol)	CNV	CNV
Conversion	TO_NUMBER	TO_NUMBER(clobCol)	CNV	CNV
Conversion	TO_TIMESTAMP	TO_TIMESTAMP(clobCol)	No	CNV
Conversion	TO_MULTI_BYTE TO_SINGLE_BYTE	TO_MULTI_BYTE(clobCol) TO_SINGLE_BYTE(clobCol)	CNV	CNV
Conversion	TO_CHAR	TO_CHAR(clobCol)	Yes	Yes
Conversion	TO_NCHAR	TO_NCHAR(clobCol)	Yes	Yes
Conversion	TO_LOB	INSERT INTO... SELECT TO_LOB(longCol)... Note that TO_LOB can only be used to create or insert into a table with LOB columns as SELECT FROM a table with a LONG column.	N/A	N/A
Conversion	TO_CLOB	TO_CLOB(varchar2Col)	Yes	Yes
Conversion	TO_NCLOB	TO_NCLOB(varchar2Clob)	Yes	Yes
Aggregate Functions	COUNT	select count(clobCol) from...	No	N/A
Aggregate Functions	MAX, MIN	select MAX(clobCol) from...	No	N/A
Aggregate Functions	GROUPING	select grouping(clobCol) from... group by cube (clobCol);	No	N/A
Other Functions	GREATEST, LEAST	select GREATEST (clobCol1, clobCol2) from...	No	CNV
Other Functions	DECODE	select DECODE(clobCol, condition1, value1, defaultValue) from...	CNV	CNV
Other Functions	NVL	select NVL(clobCol,'NULL') from...	Yes	Yes
Other Functions	DUMP	select DUMP(clobCol) from...	No	N/A
Other Functions	VSIZE	select VSIZE(clobCol) from...	No	N/A
Unicode	INSTR2, SUBSTR2, LENGTH2, LIKE2	These functions use UCS2 code point semantics.	No	CNV
Unicode	INSTR4, SUBSTR4, LENGTH4, LIKE4	These functions use UCS4 code point semantics.	No	CNV
Unicode	INSTRC, SUBSTRC, LENGTHC, LIKEC	These functions use complete character semantics.	No	CNV

UNICODE Support

Variations on the INSTR, SUBSTR, LENGTH, and LIKE functions are provided for Unicode support. (These variations are indicated as "Unicode" in the "Category" column of Table 9-1.)

See Also:

for a detailed description on the usage of UNICODE functions.

Codepoint Semantics

Codepoint semantics of the INSTR, SUBSTR, LENGTH, and LIKE functions, described in Table 9-1, differ depending on the datatype of the argument passed to the function. These functions use different codepoint semantics depending on whether the argument is a VARCHAR2 or a CLOB type as follows:

When the argument is a CLOB, UCS2 codepoint semantics are used for all character sets.
When the argument is a character type, such as VARCHAR2, the default codepoint semantics are used for the given character set:
- UCS2 codepoint semantics are used for AL16UTF16 and UTF8 character sets.
- UCS4 codepoint semantics are used for all other character sets, such as AL32UTF8.
If you are storing character data in a CLOB, then note that the amount and offset parameters for any APIs that read or write data to the CLOB are specified in codepoints. In some character sets, a full character consists one or more codepoints called surrogate pair. In this scenario, you must ensure that the amount or offset you specify does not cut into a full character. This avoids reading or writing a partial character.

Return Values for SQL Semantics on LOBs

The return type of a function or operator that takes a LOB or VARCHAR2 is the same as the datatype of the argument passed to the function or operator.

Functions that take more than one argument, such as CONCAT, return a LOB datatype if one or more arguments is a LOB. For example, CONCAT(CLOB, VARCHAR2) returns a CLOB.

See Also:

Oracle Database SQL Reference for details on the CONCAT function and the concatenation operator (||).

A LOB instance is always accessed and manipulated through a LOB locator. This is also true for return values: SQL functions and operators return a LOB locator when the return value is a LOB instance.

Any LOB instance returned by a SQL function is a temporary LOB instance. LOB instances in tables (persistent LOBs) are not modified by SQL functions, even when the function is used in the SELECT list of a query.

LENGTH Return Value for LOBs

The return value of the LENGTH function differs depending on whether the argument passed is a LOB or a character string:

If the input is a character string of length zero, then LENGTH returns NULL.
For a CLOB of length zero, or an empty locator such as that returned by EMPTY_CLOB(), the LENGTH and DBMS_LOB.GETLENGTH functions return FALSE.

Implicit Conversion of LOB Datatypes in SQL

Some LOB datatypes support implicit conversion and can be used in operations such as cross-type assignment and parameter passing. These conversions are processed at the SQL layer and can be performed in all client interfaces that use LOB types.

Implicit Conversion Between CLOB and NCLOB Datatypes in SQL

The database enables you to perform operations such as cross-type assignment and cross-type parameter passing between CLOB and NCLOB datatypes. The database performs implicit conversions between these types when necessary to preserve properties such as characterset formatting.

Note that, when implicit conversions occur, each character in the source LOB is changed to the character set of the destination LOB, if needed. In this situation, some degradation of performance may occur if the data size is large. When the character set of the destination and the source are the same, there is no degradation of performance.

After an implicit conversion between CLOB and NCLOB types, the destination LOB is implicitly created as a temporary LOB. This new temporary LOB is independent from the source LOB. If the implicit conversion occurs as part of a define operation in a SELECT statement, then any modifications to the destination LOB do not affect the persistent LOB in the table that the LOB was selected from as shown in the following example:

SQL> -- check lob length before update 
SQL> select dbms_lob.getlength(ad_sourcetext) from Print_media 
  2       where product_id=3106 and ad_id = 13001; 

DBMS_LOB.GETLENGTH(AD_SOURCETEXT) 
--------------------------------- 
         205 

SQL> 
SQL> declare 
  2   clob1 clob; 
  3   amt number:=10; 
  4  begin 
  5    -- select a clob column into a clob, no implicit convesion 
  6    SELECT ad_sourcetext INTO clob1 FROM Print_media 
  7      WHERE product_id=3106 and ad_id=13001 FOR UPDATE; 
  8 
  9    dbms_lob.trim(clob1, amt); -- Trim the selected lob to 10 bytes 
 10  end; 
 11  / 

PL/SQL procedure successfully completed. 

SQL> -- Modification is performed on clob1 which points to the 
SQL> -- clob column in the table 
SQL> select dbms_lob.getlength(ad_sourcetext) from Print_media 
  2       where product_id=3106 and ad_id = 13001; 

DBMS_LOB.GETLENGTH(AD_SOURCETEXT) 
--------------------------------- 
          10 

SQL> 
SQL> rollback; 

Rollback complete. 

SQL> -- check lob length before update 
SQL> select dbms_lob.getlength(ad_sourcetext) from Print_media 
  2       where product_id=3106 and ad_id = 13001; 

DBMS_LOB.GETLENGTH(AD_SOURCETEXT) 
--------------------------------- 
         205 

SQL> 
SQL> declare 
  2   nclob1 nclob; 
  3   amt number:=10; 
  4  begin 
  5 
  6    -- select a clob column into a nclob, implicit convesion occurs 
  7    SELECT ad_sourcetext INTO nclob1 FROM Print_media 
  8      WHERE product_id=3106 and ad_id=13001 FOR UPDATE; 
  9 
 10    dbms_lob.trim(nclob1, amt); -- Trim the selected lob to 10 bytes 
 11  end; 
 12  / 

PL/SQL procedure successfully completed. 

SQL> -- Modification to nclob1 does not affect the clob in the table, 
SQL> -- because nclob1 is a independent temporary LOB 

SQL> select dbms_lob.getlength(ad_sourcetext) from Print_media 
  2       where product_id=3106 and ad_id = 13001; 

DBMS_LOB.GETLENGTH(AD_SOURCETEXT) 
--------------------------------- 
         205

See Also:

"Implicit Conversions Between CLOB and VARCHAR2" for information on PL/SQL semantics support for implicit conversions between CLOB and VARCHAR2 types.
"Implicit Character Set Conversions with LOBs" for more information on implicit character set conversions when loading LOBs from BILEs.
Oracle Database SQL Reference for details on implicit conversions supported for all datatypes.

Unsupported Use of LOBs in SQL

Table 9-2 lists SQL operations that are not supported on LOB columns.

Table 9-2 Unsupported usage of LOBs in SQL

SQL Operations Not Supported	Example of unsupported usage
SELECT DISTINCT	SELECT DISTINCT clobCol from...
SELECT clause ORDER BY	SELECT... ORDER BY clobCol
SELECT clause GROUP BY	SELECT avg(num) FROM... GROUP BY clobCol
UNION, INTERSECT, MINUS (Note that UNION ALL works for LOBs.)	SELECT clobCol1 from tab1 UNION SELECT clobCol2 from tab2;
Join queries	SELECT... FROM... WHERE tab1.clobCol = tab2.clobCol
Index columns	CREATE INDEX clobIndx ON tab(clobCol)...

VARCHAR2 and RAW Semantics for LOBs

The following semantics, used with VARCHAR2 and RAW datatypes, also apply to LOBs:

Defining a CHAR buffer on a CLOB

You can define a VARCHAR2 for a CLOB and RAW for a BLOB column. You can also define CLOB and BLOB types for VARCHAR2 and RAW columns.
Selecting a CLOB column into a CHAR buffer or VARCHAR2

If a CLOB column is selected into a VARCHAR2 variable, then data stored in the CLOB column is retrieved and put into the CHAR buffer. If the buffer is not large enough to contain all the CLOB data, then a truncation error is thrown and no data is written to the buffer. After successful completion of the SELECT operation, the VARCHAR2 variable holds as a regular character buffer.

In contrast, when a CLOB column is selected into a local CLOB variable, the CLOB locator is fetched.
Selecting a BLOB column into a RAW

When a BLOB column is selected into a RAW variable, the BLOB data is copied into the RAW buffer. If the size of the BLOB exceeds the size of the buffer, then a truncation error is thrown and no data is written to the buffer.

LOBs Returned from SQL Functions

When a LOB is returned from a SQL function, the result returned is a temporary LOB. Your application should view the temporary LOB as local storage for the data returned from the SELECT operation as follows:

In PL/SQL, the temporary LOB has the same lifetime (duration) as other local PL/SQL program variables. It can be passed to subsequent SQL or PL/SQL VARCHAR2 functions or queries as a PL/SQL local variable. The temporary LOB will go out of scope at the end of the program block at which time, the LOB is freed. These are the same semantics as those for PL/SQL VARCHAR2 variables. At any time, nonetheless, you can use a
DBMS_LOB.FREETEMPORARY() call to release the resources taken by the local temporary LOBs.
In OCI, the temporary LOBs returned from SQL queries are always in 'session' duration, unless a user-defined duration is present, in which case, the temporary LOBs will be in the user-defined duration.

ALERT:
Ensure that your temporary tablespace is large enough to store all temporary LOB results returned from queries in your program(s).

The following example illustrates selecting out a CLOB column into a VARCHAR2 and returning the result as a CHAR buffer of declared size:

DECLARE
  vc1 VARCHAR2(32000);
  lb1 CLOB;
  lb2 CLOB;
BEGIN
  SELECT clobCol1 INTO vc1 FROM tab WHERE colID=1;
  -- lb1 is a temporary LOB
  SELECT clobCol2 || clobCol3 INTO lb1 FROM tab WHERE colID=2;

  lb2 := vc1|| lb1;
  -- lb2 is a still temporary LOB, so the persistent data in the database 
  -- is not modified. An update is necessary to modify the table data.
  UPDATE tab SET clobCol1 = lb2 WHERE colID = 1;
  
DBMS_LOB.FREETEMPORARY(lb2); -- Free up the space taken by lb2


<... some more queries ...>

END; -- at the end of the block, lb1 is automatically freed

IS NULL and IS [NOT] NULL Usage with VARCHAR2s and CLOBs

You can use the IS NULL and IS [NOT] NULL operators with LOB columns. When used with LOBs, these operators determine whether a LOB locator is stored in the row.

Note::

In the SQL 92 standard, a character string of length zero is distinct from a null string. The return value of IS NULL differs when you pass a LOB compared to a VARCHAR2:

When you pass an initialized LOB of length zero to the IS NULL function, zero (FALSE) is returned. These semantics are compliant with the SQL standard.
When you pass a VARCHAR2 of length zero to the IS NULL function, TRUE is returned.

WHERE Clause Usage with LOBs

SQL functions with LOBs as arguments, except functions that compare LOB values, are allowed in predicates of the WHERE clause. For example, the LENGTH function can be included in the predicate of the WHERE clause:

create table t (n number, c clob);
insert into t values (1, 'abc');

select * from t where c is not null;
select * from t where length(c) > 0;
select * from t where c like '%a%';
select * from t where substr(c, 1, 2) like '%b%';
select * from t where instr(c, 'b') = 2;

9 SQL Semantics and LOBs