I have an SQLite table that contains a BLOB I need to do a size/length check on. How do I do that?
According to documentation length(blob) only works on texts and will stop counting after the first NULL. My tests confirmed this. I'm using SQLite 3.4.2.
I 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 with both text and binary data.
for me length(blob) works just fine, gives the same results like the other.
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.
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;
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
Related
I try to read BLOB data from a database file with Octave 6.2.0 and the mex-sqlite3-master package.
I am able to select and read any other data from my database file. For the column containing BLOB data it gives me the following:
octave> x=sqlite3('file.db', 'SELECT column FROM list');
error: sqlite3: unsupported column type
octave> x=sqlite3('file.db', 'SELECT column FROM list WHERE column=CAST(column AS TEXT)');
gives no error, however x with a dimension of 1x0.
The BLOB-data contains hexadecimal numbers. I am fine having them as string (and work my way further, no probs).
What can I do to extract the BLOB-data in a processable format?
Thanks for any hint!
thanks a lot for your answers! I tried the "select CAST(column AS TEXT) from bl;" hint. It created an array of the expected size, however with empty cells.
I think you might want:
SELECT hex(column) FROM list
instead of doing a CAST to TEXT.
See Sqlite: How to cast(data as TEXT) for BLOB.
I have a column C of type REAL in table F in SQLite. I want to join this everywhere where in another table the negative value of F exists (along with some other fields).
However -C or 0-C etc.. all return the rounded value of C e.g. when C contains "123,456" then -C returns "-123".
Should I cast this via a string first or is the syntax differently?
Looks like the , in 123,456 is meant to be a decimal separator but SQLite treats the whole thing as a string (i.e. '123,456' rather than 123.456). Keep in mind that SQLite's type system is a little different than SQL's as values have types but columns don't:
[...] In SQLite, the datatype of a value is associated with the value itself, not with its container. [...]
So you can quietly put a string (that looks like a real number in some locales) into a real column and nothing bad happens until later.
You could fix the import process to interpret the decimal separator as desired before the data gets into SQLite or you could use replace to fix them up as needed:
sqlite> select -'123,45';
-123
sqlite> select -replace('123,45', ',', '.');
-123.45
What is wrong with my code:
ExecSql('DELETE FROM STLac WHERE RegN=99 AND BegDate>= 2016-12-14');
This runs, but deletes ALL the rows in STLac for RegN, not just the rows with BegDate on or after 2016-12-14.
Originally I had:
ExecSql('DELETE FROM STLac WHERE RegN=99 AND BegDate>= :myDdate,[myDate]);
which has the advantage I hoped of not being particular to the date format. So I tried the literal date should in the format SQLite likes. Either way I get all rows deleted, not just those on or after the specified date.
Scott S.
Try double quote while putting date. As any value must be provided in between quotes until and unless that column is not int
ExecSql('DELETE FROM STLac WHERE RegN=99 AND BegDate>= "2016-12-14"');
SQLite does not have datetime format as such, so you have to figure out how date is actually represented in the table and change your query to provide the same format. First execute the "select" statement in some kind of management tool,
select * from STLac where RegN = 99 and BegDate >= '2016-12-14' --(or '2016.12.04' or something else)
which displays the result in the grid; when you see the expected rows, change it to "delete" query and copy into your Delphi program.
I need to use cast function with length of column in teradata.
say I have a table with following data ,
id | name
1|dhawal
2|bhaskar
I need to use cast operation something like
select cast(name as CHAR(<length of column>) from table
how can i do that?
thanks
Dhawal
You have to find the length by looking at the table definition - either manually (show table) or by writing dynamic SQL that queries dbc.ColumnsV.
update
You can find the maximum length of the actual data using
select max(length(cast(... as varchar(<large enough value>))) from TABLE
But if this is for FastExport I think casting as varchar(large-enough-value) and postprocessing to remove the 2-byte length info FastExport includes is a better solution (since exporting a CHAR() will results in a fixed-length output file with lots of spaces in it).
You may know this already, but just in case: Teradata usually recommends switching to TPT instead of the legacy fexp.
I am trying to convert from a string representing hex data to the textual encoding (ASCII, UTF-8, etc.) of the hex data using purely SQLite language. Essentially I want the functionality of the X'[hex]' syntax, but applied to a programmatically derived hex string.
I want, for example, select X(hex_data_string) from ..., which is not legal SQLite syntax.
Obviously in the above snippet, I would not necessarily be able to output the data if it was not in a valid textual encoding. That is, if hex_data_string contains control chars, etc., the X() should fail in some way. If this is possible, there would have to be a default character encoding or the desired character encoding would have to be specified somehow.
I am not asking about how to retrieve the hex data string value from the SQLite database and then use C or some other facility to convert it. I am trying to perform this conversion in pure SQLite because I have queries that I check which return a text representation of hex characters representing binary data. Most of the binary data is ASCII, so I want to be able to quickly view the content of the binary data in my query output when applicable.
Intuitively, I figured this could be accomplished by casting the hex data string to a blob and using hex() but that still returns the hex data string.
Any ideas?
Possible duplicates:
SQLite X'...' notation with column data
sqlite char, ascii function
This is quite old but I was looking for the same thing so I thought I'd post:
It seems you can just cast hex data as a varchar to convert it to ascii.
ie:
select cast(data as varchar) from some_table will return a string representation of a binary field (data).
This is not possible in pure SQLite.
As an embedded database, SQLite is designed to provide only pure database functions, and leave the program logic to the the application.
You are supposed to retrieve the hex data string value from the SQLite database and then use C or some other facility to convert it.
If you control the program you're running the queries in, you could install a user-defined function that does this conversion.
This will not be fully useful since it can't really be used inline, but I have done this in the past when I just wanted to convert one row of data and not create/find another utility.
WITH RECURSIVE test(c,cur) as (
select '','686F77647921'
UNION ALL
select c || char((case substr(cur,1,1) when 'A' then 10 when 'B' then 11 when 'C' then 12 when 'D' then 13 when 'E' then 14 when 'F' then 15 else substr(cur,1,1) end)*16
+ (case substr(cur,2,1) when 'A' then 10 when 'B' then 11 when 'C' then 12 when 'D' then 13 when 'E' then 14 when 'F' then 15 else substr(cur,2,1) end)),
substr(cur,3)
from test where length(cur)>0
)
select * from test