MUCH ADO ABOUT MUCH ADO ABOUT NOTHINGNOTHING

Walter SchenkSoluTech Consulting Services Inc

AGENDAAGENDA

What it is and what is notNULL in functions, expressions,

comparisons and conditional controlNULL in IndexesNULL in programming languages

WHAT A NULL IS NOT!WHAT A NULL IS NOT!

A NULL is NOT 0! A NULL is NOT Nothing!A NULL is NOT an empty string (although

Oracle does treat it as such but that may change in the future!)

THEN WHAT IS A NULL?THEN WHAT IS A NULL?

When a column in a row has no valueThe value is not known or meaningful

WHY DO WE NEED NULL WHY DO WE NEED NULL VALUES?VALUES?

Often real-world information is incompleteIt is a way of handling this unknown

NULL IN FUNCTIONSNULL IN FUNCTIONS

“Normal” scalar functions will return NULL when given a NULL argument

All aggregate functions except COUNT(*) and GROUPING ignore nulls.

You can use the NVL in the argument to an aggregate function to substitute a value for a null.

If a query with an aggregate function returns no rows or only rows with nulls for the argument to the aggregate function, the aggregate function returns null.

FUNCTIONS THAT HANDLE FUNCTIONS THAT HANDLE NULLSNULLS

– NVL– CONCAT– REPLACE– NULLIF (9i)– COALESCE (9i)– DECODE

NVLNVL

If expr1 is null, returns expr2; if expr1 is not null, returns expr1. Can be any datatype

NVL ( expr1 , expr2 )

CONCATCONCAT

Returns char1 concatenated with char2.

CONCAT ( char1 , char2 )

SELECT CONCAT(‘job’,NULL) "Job"FROM DUAL;

Job-----------------job

REPLACEREPLACE

Returns char with every occurrence of search_string replaced with replacement_string.

If replacement_string is omitted or null, all occurrences of search_string are removed.

If search_string is null, char is returned.

REPLACE ( char , search_string, replacement_string)

REPLACEREPLACESELECT REPLACE(‘Hello there’,’l’,NULL) "Changes"FROM DUAL;

Changes--------------Heo there

SELECT REPLACE(‘Hello there’,NULL,’xxx’) "Changes"FROM DUAL;

Changes--------------Hello there

NULLIFNULLIF

If the values match, then the result is NULL

A := NULLIF(B,C);

COALESCECOALESCE

COALESCE returns the first non-null expr in the expression list.

At least one expr must not be the literal NULL. If all occurrences of expr evaluate to null, then the function returns null.

COALESCE (expr1, expr2, ..., expr n)

DECODEDECODE

NULL = NULL !!!!

DECODE (deptno, 10, ’ACCOUNTING’, 20, ’RESEARCH’, 30, ’SALES’, NULL, ’OPERATION’, ’NONE’)

NULLS AND COMPARISONSNULLS AND COMPARISONS

Only compare NULLs with IS NULL or IS NOT NULL

Use of any other operator and the result will be NULL!

NULL <> NULL (except in the DECODE expression and compound keys)

NULLS IN CONDITIONSNULLS IN CONDITIONS

Always use: variable IS NULLNever user: variable = NULL

NULLS IN LOGICAL NULLS IN LOGICAL EXPRESSIONSEXPRESSIONS

x y x AND y x OR y NOT xTRUE TRUE TRUE TRUE FALSETRUE FALSE FALSE TRUE FALSETRUE NULL NULL TRUE FALSE

FALSE TRUE FALSE TRUE TRUEFALSE FALSE FALSE FALSE TRUEFALSE TRUE FALSE TRUE TRUENULL TRUE NULL TRUE NULLNULL FALSE FALSE NULL NULLNULL NULL NULL NULL NULL

HANDLING NULLSHANDLING NULLS

Avoid common mistakes by keeping the following in mind:– Comparisons involving NULLs always yield

NULL– Applying the logical operator NOT to a NULL

yields NULL– In conditional control statements, if the

condition yields NULL, its associated sequence of statements is NOT executed!

NULL AND CONDITIONAL NULL AND CONDITIONAL CONTROLCONTROL

MOST COMMON MISTAKE:

In conditional control statements, if the condition yields NULL, its associated sequence of statements is NOT executed!

NULL AND CONDITIONAL NULL AND CONDITIONAL CONTROLCONTROL

IF x > y THEN

high := x;

ELSE

high := y;

END IF;

IF NOT x > y THEN

high := Y;

ELSE

high := x;

END IF;

PROGRAMMING GUIDELINEPROGRAMMING GUIDELINE

Always account for NULL in applications even if the underlying database objects are defined as NOT NULL.

NULL AND INDEX ENTRIESNULL AND INDEX ENTRIES

Oracle does NOT enter an index value if the ENTIRE key is NULL

Consequences:– An index can NOT be used in a search criteria

for NULL values– A UNIQUE constraint on a column that can be

NULL will allow multiple NULL values

NULLS IN PROGRAMMING NULLS IN PROGRAMMING LANGUAGESLANGUAGES

PL/SQLC/C++VB.NET and C#.NET

PL/SQLPL/SQL

Full support of NULL

C/C++C/C++

C/C++ does NOT support NULLVariables are passed on to a C/C++

application through host variables for both input as well as output

Host variables are prefixed with a colon (“:”) to set them apart from Oracle objects

C/C++C/C++

Any host variable can be associated with an indicator variable

An indicator variable is a short integer variable that indicates the condition of its host variable

C/C++ host variable on InputC/C++ host variable on Input

If indicator variable = –1 then the variable is a NULL and Oracle ignores the value of the host variable

If indicator variable >=0 Oracle will assign the value of the host variable to the column

C/C++ host variable on OutputC/C++ host variable on Output

If indicator variable = -1 then the column is NULL and the value of the host variable is indeterminate

If indicator variable = 0 then value of the host variable is assigned

C/C++C/C++EXEC SQL SELECT SAL, COMM

INTO :salary,:commission:ind_com

FROM EMP

WHERE EMPNO = :emp_number;

if (ind_comm == -1)

pay = salary

else

pay=salary + commission;

C/C++C/C++

Set ind_comm = -1;

EXEC SQL INSERT INTO emp (empno,comm)

VALUES (:emp_number,:commision:ind_comm);

VBVB

In VB6 only Variant data types could support NULL

The NULL keyword indicated that a variable contained the NULL value

The IsNull function was used to test for NULL

VB.NETVB.NET

During a migration from VB6 to VB.NET:– Null is converted to DBNull– IsNull is converted to IsDBNull– The Variant data type is converted to Object

In VB6 Null could be used in functions and assignments; DBNull cannot!

Consider using the Nothing keyword in .NET instead of Null.

VB.NET IsDBNull functionVB.NET IsDBNull function

Returns TRUE if the expression evaluates to the DBNull type; otherwise returns FALSE

The System.DBNull value indicates that the object represents missing or nonexistent data

It is NOT the same as Nothing which indicates that a variable has not yet been initialized

VB.NET DBNull classVB.NET DBNull class

The DBNull class is used to indicate the absence of a known value

The class differentiates between a null value and an uninitialized value

PROGRAMMING GUIDELINEPROGRAMMING GUIDELINE

Do not circumvent the use of NULLs by assigning “meaningless” or “out-of-range” values

Example: a column “EndDate” is often assigned a far fetched date in the future to avoid use of NULL

SQL STANDARDS AND SQL STANDARDS AND NULLSNULLS

FIPS 127-2 (1993)

The following features have "preliminary" syntax and semantics available in Working Draft form as part of an on-going ANSI and ISO/IEC standardization effort for further development of the SQL language. Features specified in preliminary form include:

 

17. Multiple null states. A facility that allows user definitions for an arbitrary number of application specific Null values, such as "Unknown", "Missing", "Not Applicable", "Pending", etc. Each such Null value would have a different representation in the database so that they could be distinguished during retrieval or update.

SQL STANDARDS AND SQL STANDARDS AND NULLSNULLS

FIPS 193-7 (1995)

If an SQL/ERI Server implementation at the Minimal SDL level or below chooses not to provide support for null values (see item 4 of Section 4.1), then it may raise an implementation-defined exception in any SQL statement that attempts to process null values.

If an SQL/ERI Server implementation at the Minimal SDL level or below chooses not to provide support for null values (see item 4 of Section 4.1), then it shall provide an implementation-defined conversion of would-be null values in Information Schema tables to an appropriate non-null value.

If an SQL/ERI Server implementation at the Minimal SDL level or below chooses not to provide support for null values (see item 4 of Section 4.1), then it may raise an implementation- defined exception in any SQL statement that attempts to process null values.

SQL STANDARDS AND SQL STANDARDS AND NULLSNULLS

The concept of NULL is subject to change!Various implementations may vary.

DEPARTING WORDSDEPARTING WORDS

Never ignore NULLUse NULL properly

QUESTIONS?QUESTIONS?

NoCOUG, February 20, 2003