SQL Nulls: A Guide to SQL Data Integrity

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In SQL, NULL is a special marker used to indicate that a data value in a database table does not exist in the database. It's not the same as an empty string or a zero value; rather, it represents the absence of a value in a field.

Here are some key points about NULL in SQL:

1. NULLability:

• Columns in SQL tables can be defined as nullable or non-nullable.
• If a column is nullable, it means that it can contain NULL values.
• If a column is non-nullable (also known as NOT NULL), it must always have a value, and NULL is not allowed in that column.

CREATE TABLE example (
    nullable_column INT,
    non_nullable_column INT NOT NULL
);

2. Usage:

• NULL can be used in columns of various data types. For example, a numeric column, a string column, or a date/time column can all allow NULL values.
• NULL is useful for representing unknown or missing information in a database, allowing a clear distinction between an empty value and a value that is undefined or unknown.

3. Comparison:

• When comparing values in SQL, you typically use the IS NULL or IS NOT NULL conditions, as the standard comparison operators like = and…