fixed build break and fixed doc about exists/not exists.

docs/guide/query-builder.md

@@ -158,6 +158,9 @@ Operator can be one of the following:
   in the generated condition.
 - `or not like`: similar to the `not like` operator except that `OR` is used to concatenate
   the `NOT LIKE` predicates.
+- `exists`: requires one operand which must be an instance of [[Query]] representing the sub-query.
+  It will build a `EXISTS (sub-query)` expression.
+- `not exists`: similar to the `exists` operator and builds a `NOT EXISTS (sub-query)` expression.
 
 If you are building parts of condition dynamically it's very convenient to use `andWhere` and `orWhere`:
 

framework/db/Query.php

@@ -404,6 +404,11 @@ class Query extends Component implements QueryInterface
 	 * - `or not like`: similar to the `not like` operator except that `OR` is used to concatenate
 	 * the `NOT LIKE` predicates.
 	 *
+	 * - `exists`: requires one operand which must be an instance of [[Query]] representing the sub-query.
+	 * It will build a `EXISTS (sub-query)` expression.
+	 *
+	 * - `not exists`: similar to the `exists` operator and builds a `NOT EXISTS (sub-query)` expression.
+	 *
 	 * @param string|array $condition the conditions that should be put in the WHERE part.
 	 * @param array $params the parameters (name => value) to be bound to the query.
 	 * @return static the query object itself

framework/db/QueryBuilder.php

@@ -1077,7 +1077,7 @@ class QueryBuilder extends \yii\base\Object
 	/**
 	 * Creates an SQL expressions with the `EXISTS` operator.
 	 * @param string $operator the operator to use (e.g. `EXISTS` or `NOT EXISTS`)
-	 * @param array $operands contains only the one element. It is sub-query
+	 * @param array $operands contains only one element which is a [[Query]] object representing the sub-query.
 	 * @param array $params the binding parameters to be populated
 	 * @return string the generated SQL expression
 	 */
@@ -1085,8 +1085,10 @@ class QueryBuilder extends \yii\base\Object
 	{
 		$subQuery = $operands[0];
 		list($subQuerySql, $subQueryParams) = $this->build($subQuery);
-		foreach ($subQueryParams as $name => $value) {
-			$params[$name] = $value;
+		if (!empty($subQueryParams)) {
+			foreach ($subQueryParams as $name => $value) {
+				$params[$name] = $value;
+			}
 		}
 		return "$operator ($subQuerySql)";
 	}