Empty character strings

Informix®

Informix SQL consisders empty strings ( '' or "" ) as a non-NULL string with a length of zero.

Note:

In Genero BDL, when setting a variable with an empty string constant, it is automatically set to a NULL value:

DEFINE x char(10)
LET x = ""
IF x IS NULL THEN -- evaluates to TRUE
   ...
END IF

ORACLE

Oracle® SQL considers empty string literals ('') as NULL.

Using literal string values that are empty ('') for INSERT or UPDATE statements will result in the storage of NULL values with Oracle, while Informix stores the value as a string with a length of zero:

INSERT INTO tab1 ( col1, col2 ) VALUES ( NULL, '' )

Using comparison operators (col='') with Oracle makes no sense, because an empty string is equivalent to NULL: The correct SQL expression is (col IS NULL).

SELECT * FROM tab1 WHERE col2 IS NULL

With Oracle, we must also distinguish CHAR and VARCHAR2. CHARs are blank padded. In SQL, '' empty string literals are NULL, but in PL/SQL, assigning a CHAR variable with '' results as NOT NULL, because CHAR is blank-padded. However, a VARCHAR2 variable set with '' becomes NULL:

SET SERVEROUTPUT ON;

DROP TABLE tab1;
CREATE TABLE tab1 ( pk NUMBER, c1 CHAR(10), c2 VARCHAR2(10) );

DECLARE
    v1 CHAR(10);
    v2 VARCHAR2(10);
BEGIN
    v1 := '';
    v2 := '';
    IF v1 IS NULL THEN
       dbms_output.put_line('v1 is null');
    ELSE
       dbms_output.put_line('v1 = ' || v1);
    END IF;
    IF v2 IS NULL THEN
       dbms_output.put_line('v2 is null');
    ELSE
       dbms_output.put_line('v2 = ' || v2);
    END IF;
    INSERT INTO tab1 VALUES ( 101, '', '' );
    INSERT INTO tab1 VALUES ( 102, v1, v2 );
END;
/

SELECT pk, NVL(c1,'NULL'), NVL(c2,'NULL') FROM tab1 ORDER BY pk;

QUIT

Output:

v1 =
v2 is null
PL/SQL procedure successfully completed.
 PK NVL(C1,'NULL') NVL(C2,'NULL')
---------- ------------------------------ ------------------------------
 101 NULL NULL
 102 NULL

Solution

To increase portability, it is recommended that you avoid the usage of literal string values with a length of zero in SQL statements. Instead, use the NULL constant, or program variables.