Implement the SQL 'exists' operator. I think this is our first serious subquery clause. Seems to work well.

src/java/com/samskivert/jdbc/depot/BindVisitor.java

@@ -43,6 +43,7 @@ import com.samskivert.jdbc.depot.expression.FunctionExp;
 import com.samskivert.jdbc.depot.expression.LiteralExp;
 import com.samskivert.jdbc.depot.expression.SQLExpression;
 import com.samskivert.jdbc.depot.expression.ValueExp;
+import com.samskivert.jdbc.depot.operator.Conditionals.Exists;
 import com.samskivert.jdbc.depot.operator.Conditionals.In;
 import com.samskivert.jdbc.depot.operator.Conditionals.IsNull;
 import com.samskivert.jdbc.depot.operator.Conditionals.FullTextMatch;
@@ -191,6 +192,12 @@ public class BindVisitor implements ExpressionVisitor
         }
     }
 
+    public void visit (Exists<? extends PersistentRecord> exists)
+        throws Exception
+    {
+        exists.getSubClause().accept(this);
+    }
+
     public void visit (SelectClause<? extends PersistentRecord> selectClause)
         throws Exception
     {

src/java/com/samskivert/jdbc/depot/BuildVisitor.java

@@ -47,6 +47,7 @@ import com.samskivert.jdbc.depot.expression.FunctionExp;
 import com.samskivert.jdbc.depot.expression.LiteralExp;
 import com.samskivert.jdbc.depot.expression.SQLExpression;
 import com.samskivert.jdbc.depot.expression.ValueExp;
+import com.samskivert.jdbc.depot.operator.Conditionals.Exists;
 import com.samskivert.jdbc.depot.operator.Conditionals.In;
 import com.samskivert.jdbc.depot.operator.Conditionals.IsNull;
 import com.samskivert.jdbc.depot.operator.Conditionals.FullTextMatch;
@@ -312,6 +313,13 @@ public abstract class BuildVisitor implements ExpressionVisitor
         _builder.append("?");
     }
 
+    public void visit (Exists<? extends PersistentRecord> exists)
+        throws Exception
+    {
+        _builder.append("exists ");
+        exists.getSubClause().accept(this);
+    }
+
     public void visit (SelectClause<? extends PersistentRecord> selectClause)
         throws Exception
     {

src/java/com/samskivert/jdbc/depot/expression/ExpressionVisitor.java

@@ -38,6 +38,7 @@ import com.samskivert.jdbc.depot.clause.SelectClause;
 import com.samskivert.jdbc.depot.clause.UpdateClause;
 import com.samskivert.jdbc.depot.clause.Where;
 
+import com.samskivert.jdbc.depot.operator.Conditionals.Exists;
 import com.samskivert.jdbc.depot.operator.Conditionals.In;
 import com.samskivert.jdbc.depot.operator.Conditionals.IsNull;
 import com.samskivert.jdbc.depot.operator.Conditionals.FullTextMatch;
@@ -92,6 +93,8 @@ public interface ExpressionVisitor
         throws Exception;
     public void visit (ValueExp valueExp)
         throws Exception;
+    public void visit (Exists<? extends PersistentRecord> exists)
+        throws Exception;
     public void visit (SelectClause<? extends PersistentRecord> selectClause)
         throws Exception;
     public void visit (UpdateClause<? extends PersistentRecord> updateClause)

src/java/com/samskivert/jdbc/depot/operator/Conditionals.java

@@ -23,6 +23,7 @@ package com.samskivert.jdbc.depot.operator;
 import java.util.Collection;
 
 import com.samskivert.jdbc.depot.PersistentRecord;
+import com.samskivert.jdbc.depot.clause.SelectClause;
 import com.samskivert.jdbc.depot.expression.ColumnExp;
 import com.samskivert.jdbc.depot.expression.ExpressionVisitor;
 import com.samskivert.jdbc.depot.expression.SQLExpression;
@@ -247,6 +248,32 @@ public abstract class Conditionals
         }
     }
 
+    /** The SQL ' exists' operator. */
+    public static class Exists<T extends PersistentRecord> implements SQLOperator
+    {
+        public Exists (SelectClause<T> clause)
+        {
+            _clause = clause;
+        }
+
+        public void accept (ExpressionVisitor builder) throws Exception
+        {
+            builder.visit(this);
+        }
+
+        public void addClasses (Collection<Class<? extends PersistentRecord>> classSet)
+        {
+            _clause.addClasses(classSet);
+        }
+
+        public SelectClause<T> getSubClause ()
+        {
+            return _clause;
+        }
+
+        protected SelectClause<T> _clause;
+    }
+
     /**
      * An attempt at a dialect-agnostic full-text search condition, such as MySQL's MATCH() and
      * PostgreSQL's @@ TO_TSQUERY(...) abilities.
@@ -265,7 +292,7 @@ public abstract class Conditionals
         {
             return _pClass;
         }
-        
+
         public String getQuery ()
         {
             return _query;