Use SELECT DISTINCT ON with OrmLite

Question

I tried writing this code to use SELECT DISTINCT ON :

            var q = Db.From<WatchedUrlScanResult>();
            q.Join<WatchedUrlRecord>()
                .Where<WatchedUrlRecord>(x => x.TargetDomainRecordId == request.TargetDomainId)
                .And(x => x.ScanDate < fromEnd)
                .OrderBy(x => x.WatchedUrlRecordId)
                .OrderByDescending(x => x.ScanDate);

            q.SelectExpression = q.SelectExpression.Replace("SELECT",
                    $"SELECT DISTINCT ON ({q.Column<WatchedUrlScanResult>(x => x.WatchedUrlRecordId)})");

            var fromSnapshot = Db.Select<WatchedUrlScanResult>(q);

In debugger it shows the value has set for SelectExpression but the actual SQL that runs is just SELECT without DISTINCT ON .

I tried:

q.Select(q.SelectExpression.Replace("SELECT",
                    $"SELECT DISTINCT ON ({q.Column<WatchedUrlScanResult>(x => x.WatchedUrlRecordId)})"));

But I get:

Potential illegal fragment detected

Is it possible to do this without making full query a string? (using {q.Column<Table>(x => x.ColumnName)} in a full string query is fine but not very readable compared to fluent syntax)

Answer 1

If you want to perform a SELECT DISTINCT query in OrmLite using its SqlExpression<T> you would use the SelectDistinct() API, eg:

var results = Db.Select(q.SelectDistinct());

But as you're trying to perform your own custom SQL distinct Query you'll need to use the Custom SQL APIs , eg:

var sql = q.ToSelectStatement().Replace(what,with);
var results = Db.SqlList<WatchedUrlScanResult>(sql);