SQL NULL keyword | SQL interview questions.
SQL NULL Keyword | SQL Interview Questions
In SQL, the NULL keyword is used to represent a missing, undefined, or unknown value in a database table. It is not the same as an empty string or zero; rather, it signifies the absence of a value. The use of NULL allows for more flexibility in database design, as it enables the storage of incomplete or unknown information without resorting to default values.
Key Points about SQL NULL:
No Value:
NULL represents the absence of a value in a database field.
Compatibility:
NULL is compatible with all data types in SQL.
Three-Valued Logic:
SQL uses a three-valued logic system with NULL values, where a condition can evaluate to true, false, or unknown (due to the presence of NULL).
Arithmetic Operations:
Arithmetic operations involving NULL usually result in a NULL value.
Comparison with NULL:
Comparisons using equality operators like =,(not equal), or IS NULL are used to check for NULL values.
Functions and NULL:
SQL provides functions like ISNULL(), COALESCE(), and NULLIF() to handle NULL values in various ways.
Uses of SQL NULL:
Flexibility:
Allows flexibility in database design by accommodating missing or unknown information.
Marking Uninitialized Data:
Used to mark uninitialized or optional data in a table.
Default Values:
Provides an alternative to using default values, especially when a default value may be a valid data point.
Missing Information:
Ideal for situations where information is incomplete or not yet available.
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