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Column comparison breaks when a NULL value is involved because NULL represents an unknown or missing value, and therefore, its outcome cannot be compared to other values. In SQL, comparisons involving NULL values always result in "unknown" or "NULL." As a result, a comparison of a NULL value to any other value (including another NULL value) always returns false. This behavior can cause unexpected results when using operators such as =, <, >, <=, >=, and <>. That's why it's important to handle NULL values with caution in SQL queries.