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:
You can access
NCLOB datatypes using SQL
VARCHAR2 semantics, such as SQL string operators and functions. (
LENGTH functions can be used with
BLOB datatypes as well as
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
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:
(Some comparison functions are not supported for use with LOBs.)
(Some conversion functions are not supported for use with LOBs.)
The following categories of functions are not supported for use with LOBs:
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 Database Data Cartridge Developer's Guide for more information on user-defined aggregate 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
NCLOB datatypes. (The
LENGTH function is also supported for 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.
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.
|Category||Operator / Function||SQL Example / Comments||SQL||PL/SQL|
Select clobCol || clobCol2 from tab;
|Comparison||= , !=, >, >=, <, <=, <>, ^=||
if clobCol=clobCol2 then...
|Comparison||IN, NOT IN||
if clobCol NOT IN (clob1, clob2, clob3) then...
|Comparison||SOME, ANY, ALL||
if clobCol < SOME (select clobCol2 from...) then...
if clobCol BETWEEN clobCol2 and clobCol3 then...
if clobCol LIKE '%pattern%' then...
|Comparison||IS [NOT] NULL||
where clobCol IS NOT NULL
|Character Functions||INITCAP, NLS_INITCAP||
select INITCAP(clobCol) from...
|Character Functions||LOWER, NLS_LOWER, UPPER, NLS_UPPER||
...where LOWER(clobCol1) = LOWER(clobCol2)
|Character Functions||LPAD, RPAD||
select RPAD(clobCol, 20, ' La') from...
|Character Functions||TRIM, LTRIM, RTRIM||
...where RTRIM(LTRIM(clobCol,'ab'), 'xy') = 'cd'
select REPLACE(clobCol, 'orig','new') from...
...where SOUNDEX(clobCOl) = SOUNDEX('SMYTHE')
...where substr(clobCol, 1,4) = 'THIS'
select TRANSLATE(clobCol, '123abc','NC') from...
select ASCII(clobCol) from...
...where instr(clobCol, 'book') = 11
...where length(clobCol) != 7;
...where NLSSORT (clobCol,'NLS_SORT = German') > NLSSORT ('S','NLS_SORT = German')
|Character Functions||INSTRB, SUBSTRB, LENGTHB||These functions are supported only for
|Character Functions - Regular Expressions||REGEXP_LIKE||This function searches a character column for a pattern. Use this function in the
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.
|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|
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.
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.
select CONVERT(clobCol,'WE8DEC','WE8HP') from...
INSERT INTO... SELECT TO_LOB(longCol)...Note that
select count(clobCol) from...
|Aggregate Functions||MAX, MIN||
select MAX(clobCol) from...
select grouping(clobCol) from... group by cube (clobCol);
|Other Functions||GREATEST, LEAST||
select GREATEST (clobCol1, clobCol2) from...
select DECODE(clobCol, condition1, value1, defaultValue) from...
select NVL(clobCol,'NULL') from...
select DUMP(clobCol) from...
select VSIZE(clobCol) from...
|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|
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.)
for a detailed description on the usage of UNICODE functions.
Codepoint semantics of the
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
NCLOB, then note that the amount and offset parameters for any APIs that read or write data to the
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
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
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.
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
For a CLOB of length zero, or an empty locator such as that returned by
DBMS_LOB.GETLENGTH functions return
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.
The database enables you to perform operations such as cross-type assignment and cross-type parameter passing between
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
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
Table 9-2 lists SQL operations that are not supported on LOB columns.
|SQL Operations Not Supported||Example of unsupported usage|
SELECT DISTINCT clobCol from...
SELECT... ORDER BY clobCol
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;
SELECT... FROM... WHERE tab1.clobCol = tab2.clobCol
CREATE INDEX clobIndx ON tab(clobCol)...
The following semantics, used with
RAW datatypes, also apply to LOBs:
CHAR buffer on a
You can define a
VARCHAR2 for a
RAW for a
BLOB column. You can also define
BLOB types for
CLOB column into a
CHAR buffer or
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.
BLOB column into 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.
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.
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
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
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
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;