int sqlite3_blob_open( sqlite3*, const char *zDb, const char *zTable, const char *zColumn, sqlite3_int64 iRow, int flags, sqlite3_blob **ppBlob );
This interfaces opens a handle to the BLOB located in row iRow, column zColumn, table zTable in database zDb; in other words, the same BLOB that would be selected by:
SELECT zColumn FROM zDb.zTable WHERE rowid = iRow;
If the flags parameter is non-zero, then the BLOB is opened for read and write access. If it is zero, the BLOB is opened for read access. It is not possible to open a column that is part of an index or primary key for writing. If foreign key constraints are enabled, it is not possible to open a column that is part of a child key for writing.
Note that the database name is not the filename that contains the database but rather the symbolic name of the database that appears after the AS keyword when the database is connected using ATTACH. For the main database file, the database name is "main". For TEMP tables, the database name is "temp".
On success, SQLITE_OK is returned and the new BLOB handle is written to *ppBlob. Otherwise an error code is returned and *ppBlob is set to be a null pointer. This function sets the database connection error code and message accessible via sqlite3_errcode() and sqlite3_errmsg() and related functions. Note that the *ppBlob variable is always initialized in a way that makes it safe to invoke sqlite3_blob_close() on *ppBlob regardless of the success or failure of this routine.
If the row that a BLOB handle points to is modified by an UPDATE, DELETE, or by ON CONFLICT side-effects then the BLOB handle is marked as "expired". This is true if any column of the row is changed, even a column other than the one the BLOB handle is open on. Calls to sqlite3_blob_read() and sqlite3_blob_write() for an expired BLOB handle fail with a return code of SQLITE_ABORT. Changes written into a BLOB prior to the BLOB expiring are not rolled back by the expiration of the BLOB. Such changes will eventually commit if the transaction continues to completion.
The sqlite3_bind_zeroblob() and sqlite3_result_zeroblob() interfaces and the built-in zeroblob SQL function can be used, if desired, to create an empty, zero-filled blob in which to read or write using this interface.