Unconcretification of QueryClause. I concreted it a while back when it did

something and then those needs went away and now it's just a marker that
differentiates arbitrary SQLExpression instances from those that represent
clauses of a query.

src/java/com/samskivert/depot/clause/DeleteClause.java

@@ -28,7 +28,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  * Builds actual SQL given a main persistent type and some {@link QueryClause} objects.
  */
-public class DeleteClause<T extends PersistentRecord> extends QueryClause
+public class DeleteClause<T extends PersistentRecord> implements QueryClause
 {
     public DeleteClause (Class<? extends PersistentRecord> pClass, WhereClause where)
     {

src/java/com/samskivert/depot/clause/FieldDefinition.java

@@ -37,7 +37,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
  *
  * @see FieldOverride
  */
-public class FieldDefinition extends QueryClause
+public class FieldDefinition implements QueryClause
 {
     public FieldDefinition (String field, String str)
     {

src/java/com/samskivert/depot/clause/ForUpdate.java

@@ -28,7 +28,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  *  Represents a FOR UPDATE clause.
  */
-public class ForUpdate extends QueryClause
+public class ForUpdate implements QueryClause
 {
     // from SQLExpression
     public void accept (ExpressionVisitor builder)

src/java/com/samskivert/depot/clause/FromOverride.java

@@ -32,7 +32,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  *  Completely overrides the FROM clause, if it exists.
  */
-public class FromOverride extends QueryClause
+public class FromOverride implements QueryClause
 {
     public FromOverride (Class<? extends PersistentRecord> fromClass)
     {

src/java/com/samskivert/depot/clause/GroupBy.java

@@ -29,7 +29,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  *  Represents a GROUP BY clause.
  */
-public class GroupBy extends QueryClause
+public class GroupBy implements QueryClause
 {
     public GroupBy (SQLExpression... values)
     {

src/java/com/samskivert/depot/clause/InsertClause.java

@@ -29,7 +29,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  * Builds actual SQL given a main persistent type and some {@link QueryClause} objects.
  */
-public class InsertClause<T extends PersistentRecord> extends QueryClause
+public class InsertClause<T extends PersistentRecord> implements QueryClause
 {
     public InsertClause (
         Class<? extends PersistentRecord> pClass, Object pojo, Set<String> identityFields)

src/java/com/samskivert/depot/clause/Join.java

@@ -32,7 +32,7 @@ import com.samskivert.depot.operator.Conditionals.Equals;
 /**
  *  Represents a JOIN.
  */
-public class Join extends QueryClause
+public class Join implements QueryClause
 {
     /** Indicates the join type to be used. The default is INNER. */
     public static enum Type { INNER, LEFT_OUTER, RIGHT_OUTER };

src/java/com/samskivert/depot/clause/Limit.java

@@ -28,7 +28,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  *  Represents a LIMIT/OFFSET clause, for pagination.
  */
-public class Limit extends QueryClause
+public class Limit implements QueryClause
 {
     public Limit (int offset, int count)
     {

src/java/com/samskivert/depot/clause/OrderBy.java

@@ -30,7 +30,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  *  Represents an ORDER BY clause.
  */
-public class OrderBy extends QueryClause
+public class OrderBy implements QueryClause
 {
     /** Indicates the order of the clause. */
     public enum Order { ASC, DESC };

src/java/com/samskivert/depot/clause/QueryClause.java

@@ -25,6 +25,6 @@ import com.samskivert.depot.expression.SQLExpression;
 /**
  * Represents a piece or modifier of an SQL query.
  */
-public abstract class QueryClause implements SQLExpression
+public interface QueryClause extends SQLExpression
 {
 }

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

@@ -34,7 +34,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  * Builds actual SQL given a main persistent type and some {@link QueryClause} objects.
  */
-public class SelectClause<T extends PersistentRecord> extends QueryClause
+public class SelectClause<T extends PersistentRecord> implements QueryClause
 {
     /**
      * Creates a new Query object to generate one or more instances of the specified persistent

src/java/com/samskivert/depot/clause/UpdateClause.java

@@ -29,7 +29,7 @@ import com.samskivert.depot.impl.ExpressionVisitor;
 /**
  * Builds actual SQL given a main persistent type and some {@link QueryClause} objects.
  */
-public class UpdateClause<T extends PersistentRecord> extends QueryClause
+public class UpdateClause<T extends PersistentRecord> implements QueryClause
 {
     public UpdateClause (Class<? extends PersistentRecord> pClass, WhereClause where,
                          String[] fields, T pojo)

src/java/com/samskivert/depot/clause/WhereClause.java

@@ -25,7 +25,7 @@ import com.samskivert.depot.expression.SQLExpression;
 /**
  * Currently only exists as a type without any functionality of its own.
  */
-public abstract class WhereClause extends QueryClause
+public abstract class WhereClause implements QueryClause
 {
     /**
      * Returns the condition associated with this where clause.