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Oracle® Database Application Developer's Guide - Large Objects
10g Release 2 (10.2)

Part Number B14249-01
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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 contains these 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:

SQL semantics are not recommended in the following situations:

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:

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

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 COMPOSE

COMPOSE('string')

Returns a Unicode string given a string in the datatype CHAR, VARCHAR2,CLOB, NCHAR, NVARCHAR2, NCLOB. An o code point qualified by an umlaut code point will be returned as the o-umlaut code point.
CNV CNV
Conversion DECOMPOSE

DECOMPOSE('str' [CANONICAL | COMPATIBILITY] )

Valid for Unicode character arguments. Returns a Unicode string after decomposition in the same character set as the input. o-umlaut code point will be returned as the o code point followed by the umlaut code point.
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.)

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 or NCLOB, then note that the amount and offset parameters for any APIs that read or write data to the CLOB or NCLOB are specified in UCS2 codepoints. In some character sets, a full character consists one or more UCS2 codepoints called a 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.

  • Starting from 10g, Oracle Database helps to detect half surrogate pair on read/write boundaries in such scenarios. In the case of read, the offset and amount will be adjusted accordingly to avoid returning a half character, in which case the amount returned could be less than what is asked for. In the case of write, an error is raised to prevent from corrupting the existing data caused by overwriting a partial character in the destination CLOB or NCLOB.

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 character set 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:

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:

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.

    Caution:

    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;