How do I find the length (size) of a binary blob in sqlite

Question

I have an sqlite table that contains a BLOB file, but need to do a size/length check on the blob, how do I do that?

According to some documentation I did find, using

Answer 1

As an additional answer, a common problem is that sqlite effectively ignores the column type of a table, so if you store a string in a blob column, it becomes a string column for that row. As length works different on strings, it will then only return the number of characters before the final 0 octet. It's easy to store strings in blob columns because you normally have to cast explicitly to insert a blob:

insert into table values ('xxxx'); // string insert
insert into table values(cast('xxxx' as blob)); // blob insert

to get the correct length for values stored as string, you can cast the length argument to blob:

select length(string-value-from-blob-column); // treast blob column as string
select length(cast(blob-column as blob)); // correctly returns blob length

The reason why length(hex(blob-column))/2 works is that hex doesn't stop at internal 0 octets, and the generated hex string doesn't contain 0 octets anymore, so length returns the correct (full) length.

Answer 2

Example of a select query that does this, getting the length of the blob in column myblob, in table mytable, in row 3:

select length(myblob) from mytable where rowid=3;

Answer 3

for me length(blob) works just fine, gives the same results like the other.

Answer 4

LENGTH() function in sqlite 3.7.13 on Debian 7 does not work, but LENGTH(HEX())/2 works fine.

# sqlite --version
3.7.13 2012-06-11 02:05:22 f5b5a13f7394dc143aa136f1d4faba6839eaa6dc

# sqlite xxx.db "SELECT docid, LENGTH(doccontent), LENGTH(HEX(doccontent))/2 AS b FROM cr_doc LIMIT 10;"
1|6|77824
2|5|176251
3|5|176251
4|6|39936
5|6|43520
6|494|101447
7|6|41472
8|6|61440
9|6|41984
10|6|41472

Answer 5

haven't had this problem, but you could try length(hex(glob))/2

Update (Aug-2012): For SQLite 3.7.6 (released April 12, 2011) and later, length(blob_column) works as expected both both text and binary data.