Add support for distinct queries in Query class

src/main/java/org/javawebstack/orm/query/Query.java

@@ -5,7 +5,6 @@
 import org.javawebstack.orm.SQLMapper;
 import org.javawebstack.orm.connection.pool.PooledSQL;
 import org.javawebstack.orm.exception.ORMQueryException;
-import org.javawebstack.orm.connection.SQL;
 import org.javawebstack.orm.renderer.SQLQueryString;
 
 import java.sql.ResultSet;
@@ -31,6 +30,8 @@ public class Query<T extends Model> {
     private QueryGroup<T> having;
     private boolean applyAccessible = false;
     private Object accessor;
+    private boolean distinct = false;
+    private String distinctColumn = null;
 
     public Query(Class<T> model) {
         this(Repo.get(model), model);
@@ -45,6 +46,14 @@ public boolean isWithDeleted() {
         return withDeleted;
     }
 
+    public boolean isDistinct() {
+        return distinct;
+    }
+
+    public String getDistinctColumn() {
+        return distinctColumn;
+    }
+
     public boolean shouldApplyAccessible() {
         return applyAccessible;
     }
@@ -94,6 +103,25 @@ public Query<T> select(String... columns) {
         return this;
     }
 
+    public Query<T> distinct() {
+        this.distinct = true;
+        this.distinctColumn = null;
+        return this;
+    }
+
+    public Query<T> distinct(String column) {
+        this.distinct = true;
+        this.distinctColumn = column;
+        return this;
+    }
+
+    public Query<T> distinct(boolean distinct) {
+        this.distinct = distinct;
+        if (!distinct)
+            this.distinctColumn = null;
+        return this;
+    }
+
     public Query<T> and(Function<QueryGroup<T>, QueryGroup<T>> group) {
         where.and(group);
         return this;

src/main/java/org/javawebstack/orm/renderer/MySQLQueryStringRenderer.java

@@ -49,7 +49,16 @@ public SQLQueryString buildQuery(Query<?> query) {
         Repo<?> repo = query.getRepo();
         List<Object> parameters = new ArrayList<>();
         StringBuilder sb = new StringBuilder("SELECT ");
-        if(query.getSelect().size() == 0)
+        if (query.isDistinct()) {
+            sb.append("DISTINCT ");
+            // Prepend distinctColumn only when no explicit select list is set (i.e. SELECT *).
+            // With an explicit select list (e.g. count(*)) the column position must not shift.
+            if (query.getDistinctColumn() != null && query.getSelect().isEmpty()) {
+                String col = new QueryColumn(query.getDistinctColumn()).toString(repo.getInfo());
+                sb.append(col).append(", ");
+            }
+        }
+        if(query.getSelect().isEmpty())
             sb.append("*");
         else
             sb.append(String.join(",", query.getSelect()));