CHAR versus VARCHAR2 Semantics

This appendix explains the semantic differences between the CHAR and VARCHAR2 base types. These subtle but important differences come into play when you assign, compare, insert, update, select, or fetch character values.

This appendix discusses the following topics:

Assigning Character Values

Comparing Character Values

Inserting Character Values

Selecting Character Values

Assigning Character Values

When you assign a character value to a CHAR variable, if the value is shorter than the declared length of the variable, PL/SQL blank-pads the value to the declared length. So, information about trailing blanks is lost. In the following example, the value assigned to last_name includes six trailing blanks, not just one:

last_name CHAR(10) := 'CHEN ';  -- note trailing blank

If the character value is longer than the declared length of the CHAR variable, PL/SQL aborts the assignment and raises the predefined exception VALUE_ERROR. PL/SQL neither truncates the value nor tries to trim trailing blanks. For example, given the declaration

acronym CHAR(4);

the following assignment raises VALUE_ERROR:

acronym := 'SPCA ';  -- note trailing blank

When you assign a character value to a VARCHAR2 variable, if the value is shorter than the declared length of the variable, PL/SQL neither blank-pads the value nor strips trailing blanks. Character values are assigned intact, so no information is lost. If the character value is longer than the declared length of the VARCHAR2 variable, PL/SQL aborts the assignment and raises VALUE_ERROR. PL/SQL neither truncates the value nor tries to trim trailing blanks.

Comparing Character Values

You can use the relational operators to compare character values for equality or inequality. Comparisons are based on the collating sequence used for the database character set. One character value is greater than another if it follows it in the collating sequence. For example, given the declarations

last_name1 VARCHAR2(10) := 'COLES';
last_name2 VARCHAR2(10) := 'COLEMAN';

the following IF condition is true:

IF last_name1 > last_name2 THEN ...

ANSI/ISO SQL requires that two character values being compared have equal lengths. So, if both values in a comparison have datatype CHAR, blank-padding semantics are used. That is, before comparing character values of unequal length, PL/SQL blank-pads the shorter value to the length of the longer value. For example, given the declarations

last_name1 CHAR(5) := 'BELLO';
last_name2 CHAR(10) := 'BELLO   ';  -- note trailing blanks

the following IF condition is true:

IF last_name1 = last_name2 THEN ...

If either value in a comparison has datatype VARCHAR2, non-blank-padding semantics are used. That is, when comparing character values of unequal length, PL/SQL makes no adjustments and uses the exact lengths. For example, given the declarations

last_name1 VARCHAR2(10) := 'DOW';
last_name2 VARCHAR2(10) := 'DOW   ';  -- note trailing blanks

the following IF condition is false:

IF last_name1 = last_name2 THEN ...

If one value in a comparison has datatype VARCHAR2 and the other value has datatype CHAR, non-blank-padding semantics are used. But, remember, when you assign a character value to a CHAR variable, if the value is shorter than the declared length of the variable, PL/SQL blank-pads the value to the declared length. So, given the declarations

last_name1 VARCHAR2(10) := 'STAUB';
last_name2 CHAR(10)     := 'STAUB';  -- PL/SQL blank-pads value

the following IF condition is false because the value of last_name2 includes five trailing blanks:

IF last_name1 = last_name2 THEN ...

All string literals have datatype CHAR. So, if both values in a comparison are literals, blank-padding semantics are used. If one value is a literal, blank-padding semantics are used only if the other value has datatype CHAR.

Inserting Character Values

When you insert the value of a PL/SQL character variable into an Oracle database column, whether the value is blank-padded or not depends on the column type, not on the variable type.

When you insert a character value into a CHAR database column, Oracle does not strip trailing blanks. If the value is shorter than the defined width of the column, Oracle blank-pads the value to the defined width. As a result, information about trailing blanks is lost. If the character value is longer than the defined width of the column, Oracle aborts the insert and generates an error.

When you insert a character value into a VARCHAR2 database column, Oracle does not strip trailing blanks. If the value is shorter than the defined width of the column, Oracle does not blank-pad the value. Character values are stored intact, so no information is lost. If the character value is longer than the defined width of the column, Oracle aborts the insert and generates an error.

Note: The same rules apply when updating.

When inserting character values, to ensure that no trailing blanks are stored, use the function RTRIM, which trims trailing blanks. An example follows:

DECLARE
   ...
   my_name VARCHAR2(15);
BEGIN
   ...
   my_ename := 'LEE   ';  -- note trailing blanks
   INSERT INTO emp
      VALUES (my_empno, RTRIM(my_ename), ...);  -- inserts 'LEE'
END;

Selecting Character Values

When you select a value from an Oracle database column into a PL/SQL character variable, whether the value is blank-padded or not depends on the variable type, not on the column type.

When you select a column value into a CHAR variable, if the value is shorter than the declared length of the variable, PL/SQL blank-pads the value to the declared length. As a result, information about trailing blanks is lost. If the character value is longer than the declared length of the variable, PL/SQL aborts the assignment and raises VALUE_ERROR.

When you select a column value into a VARCHAR2 variable, if the value is shorter than the declared length of the variable, PL/SQL neither blank-pads the value nor strips trailing blanks. Character values are stored intact, so no information is lost.

For example, when you select a blank-padded CHAR column value into a VARCHAR2 variable, the trailing blanks are not stripped. If the character value is longer than the declared length of the VARCHAR2 variable, PL/SQL aborts the assignment and raises VALUE_ERROR.

Note: The same rules apply when fetching.