Fix missing () in unioned queries.

The original SELECT also needs () for unions to work. While having the
parentheses added in very disparate parts of the code is a bit awkward
I really didn't want to restructure the entire QueryCompiler for this
scenario.

src/Database/QueryCompiler.php

@@ -135,6 +135,9 @@ protected function _buildSelectPart($parts, $query, $generator)
     {
         $driver = $query->connection()->driver();
         $select = 'SELECT %s%s%s';
+        if ($query->clause('union')) {
+            $select = '(SELECT %s%s%s';
+        }
         $distinct = $query->clause('distinct');
         $modifiers = $query->clause('modifier') ?: null;
 
@@ -253,10 +256,10 @@ protected function _buildUnionPart($parts, $query, $generator)
         $parts = array_map(function ($p) use ($generator) {
             $p['query'] = $p['query']->sql($generator);
             $p['query'] = $p['query'][0] === '(' ? trim($p['query'], '()') : $p['query'];
-            $prefix = $p['all'] ? 'ALL' : '';
-            return sprintf('%s (%s)', $prefix, $p['query']);
+            $prefix = $p['all'] ? 'ALL ' : '';
+            return sprintf('%s(%s)', $prefix, $p['query']);
         }, $parts);
-        return sprintf("\nUNION %s", implode("\nUNION ", $parts));
+        return sprintf(")\nUNION %s", implode("\nUNION ", $parts));
     }
 
     /**

src/Database/SqliteCompiler.php

@@ -24,6 +24,25 @@
  */
 class SqliteCompiler extends QueryCompiler
 {
+    /**
+     * Helper function used to build the string representation of a SELECT clause,
+     * it constructs the field list taking care of aliasing and
+     * converting expression objects to string. This function also constructs the
+     * DISTINCT clause for the query.
+     *
+     * @param array $parts list of fields to be transformed to string
+     * @param \Cake\Database\Query $query The query that is being compiled
+     * @param \Cake\Database\ValueBinder $generator the placeholder generator to be used in expressions
+     * @return string
+     */
+    protected function _buildSelectPart($parts, $query, $generator)
+    {
+        $clause = parent::_buildSelectPart($parts, $query, $generator);
+        if ($clause[0] === '(') {
+            return substr($clause, 1);
+        }
+        return $clause;
+    }
 
     /**
      * Builds the SQL string for all the UNION clauses in this query, when dealing