text, unitext, and image columns created with insert and NULL are not initialized and contain no value. They do not use storage space and cannot be accessed with readtext or writetext.
When a NULL value is written in a text, unitext, or image column with update, the column is initialized, a valid text pointer to the column is inserted into the table, and a 2K data page is allocated to the column. Once the column is initialized, it can be accessed by readtext and writetext. See the Reference Manual for more information.
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