A Brief Discussion on NULL Handling in SQL Server WHERE Clauses

TLDR

• To check if a column is NULL in SQL, you must use IS NULL or IS NOT NULL. Using = NULL will lead to unexpected query results.
• SQL logical operations result in one of three states: TRUE, FALSE, or UNKNOWN.
• The WHERE clause only filters for rows where the result is TRUE.
• Any value (including NULL itself) compared with NULL results in UNKNOWN.
• It is recommended to avoid generating UNKNOWN states in query conditions to ensure clear logic.
• SQL Server officially recommends using <> as the not-equal operator, although some environments support !=.

NULL Comparison Logic and the UNKNOWN State

When does this issue occur? It happens when developers attempt to use the = operator to filter columns that are NULL in the database.

In SQL, NULL represents an "unknown value." Therefore, when any value is compared with NULL (e.g., Column = NULL), the result is not TRUE or FALSE, but UNKNOWN. Since the WHERE clause only returns rows where the logical evaluation is TRUE, queries using = NULL will never retrieve the correct data.

Logical Operation Rules for UNKNOWN

To avoid logical errors, it is necessary to understand how UNKNOWN behaves in Boolean operations:

• AND operation:

  • TRUE AND UNKNOWN = UNKNOWN
  • UNKNOWN AND UNKNOWN = UNKNOWN
  • FALSE AND UNKNOWN = FALSE

• OR operation:

  • TRUE OR UNKNOWN = TRUE
  • UNKNOWN OR UNKNOWN = UNKNOWN
  • FALSE OR UNKNOWN = UNKNOWN

Conclusion and Recommendations

• Always use IS NULL or IS NOT NULL to check for empty values.
• Avoid writing complex logic in WHERE conditions that may produce UNKNOWN states, as this may cause query results to be filtered out.

SQL Server Not-Equal Operator

When does this issue occur? It happens when writing SQL queries and being unsure whether to use <> or !=.

Although SQL Server supports the != syntax, according to official documentation, <> is the standard ANSI SQL not-equal operator. To maintain code portability and adhere to standard specifications, it is recommended to prioritize the use of <>.

• Recommended practice: Use <> for not-equal comparisons.
• Note: Some older versions or specific database systems (such as Microsoft Access) may not support !=.

References

• NULL and UNKNOWN (Transact-SQL)

Changelog

• 2024-07-24 Initial version created.