Add the actual Distinct class; fix bugs.

src/main/java/com/samskivert/depot/clause/Distinct.java

@@ -0,0 +1,44 @@
+//
+// Depot library - a Java relational persistence library
+// http://code.google.com/p/depot/source/browse/trunk/LICENSE
+
+package com.samskivert.depot.clause;
+
+import java.util.Collection;
+
+import com.samskivert.depot.PersistentRecord;
+import com.samskivert.depot.expression.SQLExpression;
+import com.samskivert.depot.impl.FragmentVisitor;
+
+/**
+ *  Represents a DISTINCT [ON <exp>] clause.
+ *
+ *  Note: You almost certainly only ever want to use this in a SELECT statement.
+ */
+public class Distinct implements QueryClause
+{
+    public Distinct (SQLExpression<?> on)
+    {
+        _on = on;
+    }
+
+    public SQLExpression<?> getDistinctOn ()
+    {
+        return _on;
+    }
+
+    // from SQLExpression
+    public Object accept (FragmentVisitor<?> builder)
+    {
+        return builder.visit(this);
+    }
+
+    // from SQLExpression
+    public void addClasses (Collection<Class<? extends PersistentRecord>> classSet)
+    {
+    }
+
+    /** The expression to distinguish by, for DISTINCT ON, or null for mere DISTINCT. */
+    protected SQLExpression<?> _on;
+
+}

src/main/java/com/samskivert/depot/clause/SelectClause.java

@@ -65,7 +65,7 @@ public class SelectClause
                 _disMap.put(((FieldDefinition) clause).getField(), ((FieldDefinition) clause));
 
             } else if (clause instanceof Distinct) {
-                checkArgument(_orderBy == null, "Query can't contain multiple Distinct clauses.");
+                checkArgument(_distinct == null, "Query can't contain multiple Distinct clauses.");
                 _distinct = (Distinct) clause;
 
             } else if (clause instanceof OrderBy) {

src/main/java/com/samskivert/depot/impl/HSQLBuilder.java

@@ -136,7 +136,7 @@ public class HSQLBuilder
         public Void visit (Distinct distinct)
         {
             if (distinct.getDistinctOn() != null) {
-                throw new IllegalArgumentException("MySQL does not support DISTINCT ON");
+                throw new IllegalArgumentException("HSQL does not support DISTINCT ON");
             }
             return super.visit(distinct);
         }