Is this SQL code concurrent safe? - sql

Question

I am pretty sure this code is fine. I wanted to know what you guys think about the insertMediaTags function (2nd func). The things i am worried about is the below concurrent safe? and if insertMediaTags is optimize enough? note it is in a transaction due to first func but it is also in a loop which could mean its bad?
I am open to any coding practice, style or suggestions you guys may have.
(I know someone will ask, i am using sqlite ATM but its prototype code to use with mysql or a version of ms sql or another)
{
long mediaId;
//all the credentials should be verified by this point.
command.CommandText = "SELECT mediaId FROM media " +
"WHERE userId=#userId AND title=#title;";
command.Parameters.Add("#userId", DbType.Int64).Value = m.userid;
command.Parameters.Add("#title", DbType.String).Value = m.title;
if (command.ExecuteScalar() != null)
throw new System.InvalidOperationException("Title already exisit");
using (var dbTrans = connection.BeginTransaction())
{
command.CommandText =
"INSERT INTO " +
"media ( userid, catagory, creation_date, current_media_date, current_desc_date, licence, title, desc, ext) " +
"VALUES(#userid, #catagory, #creation_date, #current_media_date, #current_desc_date, #licence, #title, #desc, #ext); " +
"SELECT last_insert_rowid() AS RecordID;";
DateTime currentDate = m.creation_date;
command.Parameters.Add("#userid", DbType.Int64).Value = m.userid;
command.Parameters.Add("#catagory", DbType.Int64).Value = m.catagory;
command.Parameters.Add("#creation_date", DbType.DateTime).Value = m.creation_date;
command.Parameters.Add("#current_media_date", DbType.DateTime).Value = currentDate;
command.Parameters.Add("#current_desc_date", DbType.DateTime).Value = currentDate;
command.Parameters.Add("#licence", DbType.Int64).Value = m.license;
command.Parameters.Add("#title", DbType.String).Value = m.title;
command.Parameters.Add("#desc", DbType.String).Value = m.desc;
command.Parameters.Add("#ext", DbType.Int64).Value = m.ext;
mediaId = (long)command.ExecuteScalar();
//m.collaborateWith
insertInspired(inspireLinks.external, inspireLinks.internalPair, mediaId);
insertDerived(deriveLinks.external, deriveLinks.internalPair, mediaId);
insertMediaTags(m.listTagString, mediaId);
//command.CommandText = "END TRANSACTION;"; command.ExecuteNonQuery();
updateMediaForWatchers(m.userid, mediaId, m.catagory, currentDate);
dbTrans.Commit();
}
return mediaId;
}
void insertMediaTags(List<string> tags, long mediaId)
{
foreach(string tag in tags)
{
//assure tag exist
long tagId;
command.CommandText = "SELECT tagid FROM tag_name WHERE title=#title;";
command.Parameters.Add("#title", DbType.String).Value = tag;
object o = command.ExecuteScalar();
if (o == null)
{
command.CommandText =
"INSERT INTO tag_name(title) VALUES(#title); " +
"SELECT last_insert_rowid() AS RecordID;";
command.Parameters.Add("#title", DbType.String).Value = tag;
tagId = (long)command.ExecuteScalar();
}
else
tagId = (long)o;
command.CommandText =
"INSERT INTO media_tags(mediaId, tagid) " +
"VALUES(#mediaId, #tagid);";
command.Parameters.Add("#mediaId", DbType.Int64).Value = mediaId;
command.Parameters.Add("#tagid", DbType.Int64).Value = tagId;
command.ExecuteNonQuery();
command.CommandText =
"UPDATE tag_name SET count = count+1 "+
"WHERE tagid=#tagid";
command.Parameters.Add("#tagid", DbType.Int64).Value = tagId;
command.ExecuteNonQuery();
}
}

Answer 1

No it's not concurrent safe. You have a potential race condition between the SELECT to determine whether the tag exists, and the INSERT to create the tag if it does not. Imagine thread A does a SELECT and finds it does not exist, and then thread B does the same before thread A does the INSERT. Thread B will attempt the insert as well and fail.

Answer 2

In SQL Server, it's better to use the SCOPE_IDENTITY() function. Other than that I don't see a problem.