说明

    用于合并两个或多个 SELECT 语句的结果集

示例

Db::field('name')      ->table('think_user_0')      ->union('SELECT name FROM think_user_1')      ->union('SELECT name FROM think_user_2')      ->select();

Db::field('name')      ->table('think_user_0')      ->union(function($query){          $query->field('name')->table('think_user_1');        })      ->union(function($query){          $query->field('name')->table('think_user_2');        })      ->select();

Db::field('name')      ->table('think_user_0')      ->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'])      ->select();

union all

Db::field('name')      ->table('think_user_0')            ->union('SELECT name FROM think_user_1',true)            ->union('SELECT name FROM think_user_2',true)            ->select();

Db::field('name')      ->table('think_user_0')      ->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'],true)      ->select();