MUCH ADO ABOUT NOTHING Walter Schenk SoluTech Consulting Services Inc

AGENDA What it is and what is not NULL in functions, expressions, comparisons and conditional control NULL in Indexes NULL in programming languages

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

WHY DO WE NEED NULL VALUES? Often real-world information is incomplete It is a way of handling this unknown

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

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

CONCAT Returns char1 concatenated with char2. CONCAT ( char1, char2 ) SELECT CONCAT(‘job’,NULL) "Job" FROM DUAL; Job job

REPLACE 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)

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

NULLIF If the values match, then the result is NULL A := NULLIF(B,C);

COALESCE 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)

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

NULLS 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 CONDITIONS Always use: variable IS NULL Never user: variable = NULL

NULLS IN LOGICAL EXPRESSIONS

HANDLING 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 CONTROL MOST COMMON MISTAKE: In conditional control statements, if the condition yields NULL, its associated sequence of statements is NOT executed!

NULL AND CONDITIONAL CONTROL IF x > y THEN high := x; ELSE high := y; END IF; IF NOT x > y THEN high := Y; ELSE high := x; END IF;

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

NULL 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 LANGUAGES PL/SQL C/C++ VB.NET and C#.NET

PL/SQL Full support of NULL

C/C++ C/C++ does NOT support NULL Variables 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++ 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 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 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++ 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++ Set ind_comm = -1; EXEC SQL INSERT INTO emp (empno,comm) VALUES (:emp_number,:commision:ind_comm);

VB 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.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 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 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 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 NULLS FIPS (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 NULLS FIPS (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 NULLS The concept of NULL is subject to change! Various implementations may vary.

DEPARTING WORDS Never ignore NULL Use NULL properly

QUESTIONS? NoCOUG, February 20, 2003