Null (SQL)
See also Null reference, Null pointer, Null pointer exception, Nullable type, Null-Safety
TLDR: In SQL, `NULL` represents the absence of a value or unknown data, introduced in the 1980s with the SQL-92 standard. Unlike an empty string or zero, `NULL` signifies that a value has not been assigned or is missing. It is a critical concept for database operations, requiring explicit handling in queries and logical conditions.
https://en.wikipedia.org/wiki/Null_(SQL)
Handling `NULL` in SQL requires special syntax since it does not behave like regular data. For example, comparisons involving `NULL` (e.g., `= NULL`) return unknown, not true or false. To check for `NULL` values, SQL provides the `IS NULL` and `IS NOT NULL` operators. Mismanagement of `NULL` values in queries can lead to logical errors or incorrect results, such as rows being excluded unintentionally from aggregations.
https://docs.microsoft.com/en-us/sql/t-sql/queries/null-and-isnull-transact-sql
NULL also impacts aggregate functions like `SUM` or `AVG`, as they ignore `NULL` values. To handle such scenarios, SQL provides functions like `COALESCE` and `IFNULL` to replace `NULL` with default values. For instance, `COALESCE(column_name, 0)` ensures that null values in a column are treated as zeros in computations, maintaining consistent results.
https://www.postgresql.org/docs/current/functions-conditional.html
Proper use of `NULL` in SQL involves understanding its implications in joins, comparisons, and constraints. For example, a `NOT NULL` constraint ensures that a column cannot have null values, enforcing data integrity. Careful handling of `NULL` is essential for avoiding issues such as data leakage or misinterpretation of results in critical applications.
https://dev.mysql.com/doc/refman/8.0/en/constraint-primary-key.html#constraint-primary-key-null
Null (SQL)