Implement SELECT DISTINCT [ON <exp>] as a QueryClause.

src/main/java/com/samskivert/depot/Query.java

@@ -178,6 +178,25 @@ public class Query<T extends PersistentRecord>
         return query;
     }
 
+    /**
+     * Configures the query to return only distinct rows.
+     */
+    public Query<T> distinct ()
+    {
+        return distinct(null);
+    }
+
+    /**
+     * Configures the query to return rows for which the given expression evaluates distinctly.
+     */
+    public Query<T> distinct (SQLExpression<?> distinctOn)
+    {
+        checkState(_distinct == null, "Distinct clause is already configured.");
+        Query<T> query = clone();
+        query._distinct = new Distinct(distinctOn);
+        return query;
+    }
+
     /**
      * Configures a {@link GroupBy} clause on the supplied group expressions.
      */
@@ -610,6 +629,7 @@ public class Query<T extends PersistentRecord>
         addIfNotNull(clauses, _where);
         addAll(clauses, _joins);
         addIfNotNull(clauses, _orderBy);
+        addIfNotNull(clauses, _distinct);
         addIfNotNull(clauses, _groupBy);
         addIfNotNull(clauses, _limit);
         addIfNotNull(clauses, _fromOverride);
@@ -636,6 +656,7 @@ public class Query<T extends PersistentRecord>
         checkState(_where != null, "Where clause must be specified for delete.");
         checkState(_joins == null, "Join clauses not supported by delete.");
         checkState(_orderBy == null, "OrderBy clause not applicable for delete.");
+        checkState(_distinct == null, "Distinct clause not applicable for delete.");
         checkState(_groupBy == null, "GroupBy clause not applicable for delete.");
         checkState(_fromOverride == null, "FromOverride clause not applicable for delete.");
         checkState(_fieldDefs == null, "FieldDefinition clauses not applicable for delete.");
@@ -678,6 +699,7 @@ public class Query<T extends PersistentRecord>
 
     protected WhereClause _where;
     protected OrderBy _orderBy;
+    protected Distinct _distinct;
     protected GroupBy _groupBy;
     protected Limit _limit;
     protected FromOverride _fromOverride;

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

@@ -64,6 +64,10 @@ public class SelectClause
             } else if (clause instanceof FieldDefinition) {
                 _disMap.put(((FieldDefinition) clause).getField(), ((FieldDefinition) clause));
 
+            } else if (clause instanceof Distinct) {
+                checkArgument(_orderBy == null, "Query can't contain multiple Distinct clauses.");
+                _distinct = (Distinct) clause;
+
             } else if (clause instanceof OrderBy) {
                 checkArgument(_orderBy == null, "Query can't contain multiple OrderBy clauses.");
                 _orderBy = (OrderBy) clause;
@@ -128,6 +132,11 @@ public class SelectClause
         return _orderBy;
     }
 
+    public Distinct getDistinct ()
+    {
+        return _distinct;
+    }
+
     public GroupBy getGroupBy ()
     {
         return _groupBy;
@@ -182,6 +191,9 @@ public class SelectClause
         if (_orderBy != null) {
             builder.append(", orderBy=").append(_orderBy);
         }
+        if (_distinct != null) {
+            builder.append(", distinct=").append(_distinct);
+        }
         if (_groupBy != null) {
             builder.append(", groupBy=").append(_groupBy);
         }
@@ -215,6 +227,9 @@ public class SelectClause
     /** The order by clause, if any. */
     protected OrderBy _orderBy;
 
+    /** The distinct clause, if any. */
+    protected Distinct _distinct;
+
     /** The group by clause, if any. */
     protected GroupBy _groupBy;
 

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

@@ -14,6 +14,7 @@ import java.util.Set;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 
+import com.samskivert.depot.clause.Distinct;
 import com.samskivert.util.ByteEnum;
 import com.samskivert.util.Tuple;
 
@@ -278,6 +279,17 @@ public abstract class BuildVisitor implements FragmentVisitor<Void>
         return null;
     }
 
+    public Void visit (Distinct distinct)
+    {
+        _builder.append("distinct ");
+        if (distinct.getDistinctOn() != null) {
+            _builder.append("on ");
+            distinct.getDistinctOn().accept(this);
+            _builder.append(" ");
+        }
+        return null;
+    }
+
     public Void visit (Join join)
     {
         switch (join.getType()) {
@@ -345,6 +357,10 @@ public abstract class BuildVisitor implements FragmentVisitor<Void>
         checkArgument(!_definitions.containsKey(pClass),
                       "Can not yet nest SELECTs on the same persistent record.");
 
+        if (selectClause.getDistinct() != null) {
+            selectClause.getDistinct().accept(this);
+        }
+
         Map<String, FieldDefinition> definitionMap = Maps.newHashMap();
         for (FieldDefinition definition : selectClause.getFieldDefinitions()) {
             definitionMap.put(definition.getField(), definition);

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

@@ -9,6 +9,7 @@ import java.lang.reflect.Field;
 import com.samskivert.depot.Key;
 import com.samskivert.depot.PersistentRecord;
 
+import com.samskivert.depot.clause.Distinct;
 import com.samskivert.depot.clause.FieldDefinition;
 import com.samskivert.depot.clause.ForUpdate;
 import com.samskivert.depot.clause.FromOverride;
@@ -252,6 +253,11 @@ public class ExpressionEvaluator
         throw new IllegalArgumentException("Can't evaluate expression: " + exists);
     }
 
+    public Object visit (Distinct distinct)
+    {
+        throw new IllegalArgumentException("Can't evaluate expression: " + distinct);
+    }
+
     public Object visit (GroupBy groupBy)
     {
         throw new IllegalArgumentException("Can't evaluate expression: " + groupBy);

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

@@ -6,6 +6,7 @@ package com.samskivert.depot.impl;
 
 import com.samskivert.depot.Key;
 
+import com.samskivert.depot.clause.Distinct;
 import com.samskivert.depot.clause.FieldDefinition;
 import com.samskivert.depot.clause.ForUpdate;
 import com.samskivert.depot.clause.FromOverride;
@@ -81,6 +82,7 @@ public interface FragmentVisitor<T>
     public T visit (FullText.Rank match);
     public T visit (ColumnExp<?> columnExp);
     public T visit (Not not);
+    public T visit (Distinct distinct);
     public T visit (GroupBy groupBy);
     public T visit (ForUpdate forUpdate);
     public T visit (OrderBy orderBy);

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

@@ -18,6 +18,7 @@ import com.samskivert.depot.Ops;
 import com.samskivert.depot.PersistentRecord;
 import com.samskivert.depot.annotation.FullTextIndex;
 import com.samskivert.depot.annotation.GeneratedValue;
+import com.samskivert.depot.clause.Distinct;
 import com.samskivert.depot.clause.OrderBy;
 import com.samskivert.depot.expression.ColumnExp;
 import com.samskivert.depot.expression.SQLExpression;
@@ -132,6 +133,14 @@ public class HSQLBuilder
             return super.visit(operator);
         }
 
+        public Void visit (Distinct distinct)
+        {
+            if (distinct.getDistinctOn() != null) {
+                throw new IllegalArgumentException("MySQL does not support DISTINCT ON");
+            }
+            return super.visit(distinct);
+        }
+
         @Override public Void visit (DatePart exp) {
 
             if (exp.getPart() == Part.EPOCH) {

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

@@ -10,6 +10,7 @@ import java.sql.SQLException;
 import java.util.Map;
 import java.util.Set;
 
+import com.samskivert.depot.clause.Distinct;
 import com.samskivert.util.StringUtil;
 
 import com.samskivert.depot.PersistentRecord;
@@ -69,6 +70,14 @@ public class MySQLBuilder
             return appendFunctionCall("truncate", exp.getArg());
         }
 
+        public Void visit (Distinct distinct)
+        {
+            if (distinct.getDistinctOn() != null) {
+                throw new IllegalArgumentException("MySQL does not support DISTINCT ON");
+            }
+            return super.visit(distinct);
+        }
+
         @Override public Void visit (DatePart exp) {
             return appendFunctionCall(getDateFunction(exp.getPart()), exp.getArg());
         }