Blobs are just binary data that require special processing. Since blobs
can contain NULL characters and other special characters that affect MySQL and
Rapid-Q, you'll need to do some special processing with your binary data.
Please note that blob sizes vary, depending on the field type. A BLOB,
by default can hold up to 65535 bytes (a LONGBLOB should be able
to handle 2^32-1 bytes). If you want to change the buffer size,
you'll have to read up on it (max_allowed_packet).
To store binary data, like images, you'll
have to preprocess certain characters before you can send it over to your MySQL
database.
$INCLUDE "RAPIDQ.INC"
DIM MySQL AS QMYSQL
''-- Connect to database code, etc...
DIM File AS QFILESTREAM
File.Open("test.bmp", fmOpenRead)
''-- Read file as binary, preserves NULL chars
Buffer$ = File.ReadBinStr(File.Size)
''-- Here's the special processing
''-- Converts NULLs to \0 and others
ProcessedBuffer$ = MySQL.EscapeString(Buffer$, File.Size)
''-- Insert blob in our table
MySQL.Query("insert into MyImageTable values('Label 1', '" + _
ProcessedBuffer$ + "')")
File.Close
MySQL.Close
Since we're dealing with binary data, you have to be aware that we don't use LEN()
or any other STRING processing functions with our binary data. Note that we can
store text data in blob fields, in which case you can use any STRING processing
functions you want. You may notice that MySQL.EscapeString
converts the binary data to a usable format that can be passed to the MySQL database.
Basically all it does is convert NULLs to '\0', ' to \', " to \", and CRLF to \r and \n
respectively. Once parsed, the new string is returned which you can use in a
query. So far so good, now we consider retrieving our blobs:
$INCLUDE "RAPIDQ.INC"
$INCLUDE "MYSQL.INC"
DIM MySQL AS QMYSQL
''-- Connect to database code, etc...
DIM File AS QFILESTREAM
File.Open("out.bmp", fmCreate) ''-- Create new file
''-- Your query here
MySQL.Query("select * from MyImageTable")
WHILE MySQL.FetchRow
MySQL.FieldSeek(0)
MySQL.FetchLengths ''-- Required for binary data
FOR I = 0 TO MySQL.NumField-1
MySQL.FetchField
IF MySQL.Field.Type = FIELD_TYPE_BLOB THEN
''-- Read binary data from table
Buffer$ = MySQL.RowBlob(I, MySQL.Length(I))
File.WriteBinStr(Buffer$, MySQL.Length(I))
ELSE
Buffer$ = MySQL.Row(I)
END IF
NEXT
WEND
File.Close
MySQL.Close
Retrieving blobs from the database is a little more confusing than storing blobs.
The first thing you have to note is that reading binary data and text strings are 2
different operations. Since text strings are NULL terminated, you don't have to
worry about how many bytes to read, this is automatically calculated for you.
However, since binary data can contain NULL characters, this makes reading
blobs a little bit tricky. You have to fetch the lengths of each field, since
we have blobs in our table, this is done by calling MySQL.FetchLengths
for each row. Then to find out the length of each field, you use the internal array
MySQL.Length(index%) which returns the length for field index%.
Why do we have to do this? Because we need to know the size of our blob before we can read it in.
To read binary data from our database, you use the specialized function
MySQL.RowBlob(Row%, Bytes%) which returns a binary
string. With this binary string, you can write the data to a file or memory, or
manipulate the data directly. In the above example, we're just writing the data
to a file.
14.6 Using LOADBLOB and SAVEBLOB
In the previous section, we uncovered the natural way of storing and
retrieving blobs (binary data). However, there is an easier and faster way of
doing the samething.
''-- Recall the natural way
ProcessedBuffer$ = MySQL.EscapeString(Buffer$, File.Size)
MySQL.Query("insert into MyImageTable values('Label 1', '" + _
ProcessedBuffer$ + "')")
''-- Using LOADBLOB instead
ProcessedBuffer$ = MySQL.LoadBlob("test.bmp")
MySQL.Query("insert into MyImageTable values('Label 1', '" + _
ProcessedBuffer$ + "')")
Using LOADBLOB eliminates the use of MySQL.EscapeString
since it is just implemented internally for you. It simply takes a single argument
(the name of the file to include), and returns the processed string.
To retrieve a blob from the database and save it to a file, you can try this:
WHILE MySQL.FetchRow
MySQL.FieldSeek(0)
'' MySQL.FetchLengths
FOR I = 0 TO MySQL.NumField-1
MySQL.FetchField
IF MySQL.Field.Type = FIELD_TYPE_BLOB THEN
''-- Save data to file
MySQL.SaveBlob(I, "temp.bmp")
ELSE
Buffer$ = MySQL.Row(I)
END IF
NEXT
WEND
In this instance we can ignore using MySQL.FetchLengths
since it is, again, implemented internally for you. MySQL.SaveBlob takes two
arguments. The first is the index (or field number if you will), and the second argument is the
name of the file to save to. There are no return values associated with this function.
14.7 In conclusion
Using MySQL can save you a lot of time and effort, since you
don't need to design your own database file structure, and searching is quite fast.
Not to mention that MySQL is multiplatform, so you can use MySQL with Windows
and Linux. If you already have prior knowledge with programming in MySQL,
you'll probably notice some differences. For example,
I = MySQL.SelectDB("mysql")
Under Rapid-Q, this function returns 0 if database cannot be opened, and a non-zero number
otherwise. However, if you've used LIBMYSQL.DLL before, this function (called
mysql_select_db) returns 0 on success and -1 on failure.
So basically, just reverse your way of thinking when using Rapid-Q.
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