Add sub() to the query function builder

Signed-off-by: Robin Appelman <robin@icewind.nl>

lib/private/DB/QueryBuilder/FunctionBuilder/FunctionBuilder.php

@@ -53,4 +53,8 @@ class FunctionBuilder implements IFunctionBuilder {
 			return new QueryFunction('SUBSTR(' . $this->helper->quoteColumnName($input) . ', ' . $this->helper->quoteColumnName($start) . ')');
 		}
 	}
+
+	public function sum($field) {
+		return new QueryFunction('SUM(' . $this->helper->quoteColumnName($field) . ')');
+	}
 }

lib/public/DB/QueryBuilder/IFunctionBuilder.php

@@ -59,4 +59,14 @@ interface IFunctionBuilder {
 	 * @since 12.0.0
 	 */
 	public function substring($input, $start, $length = null);
+
+	/**
+	 * Takes the sum of all rows in a column
+	 *
+	 * @param mixed $field the column to sum
+	 *
+	 * @return IQueryFunction
+	 * @since 12.0.0
+	 */
+	public function sum($field);
 }