Laravel 框架对于分表进行统计合并查询的思路

当对数据表进行水平分表之后，若要进行数据统计，往往需要对所有分表一起进行联合查询。在使用 Laravel 框架时，可以利用 Model 类的 setTable 方法对于多个表进行合并查询。

举个例子，假设 users 表水平拆分为 16 个表，users_0、users_1、users_2……一直到 users_f，这几个表都有相同的结构，使用 SQL 的 UNION ALL 合并查询所有表的结果集，然后调用 Model 类的 setTable 方法，把结果集的别名赋值给 Model 类的 table 属性，这样就能够对 users 所有表进行联合查询操作了。具体请参考下这篇文章的例子。

<?php

class User extends Model
{
    public function setUnionAllTable()
    {
        $sql =<<<'EOT'
SELECT * FROM users_0
UNION ALL
SELECT * FROM users_1
UNION ALL
SELECT * FROM users_2
UNION ALL
SELECT * FROM users_3
UNION ALL
SELECT * FROM users_4
UNION ALL
SELECT * FROM users_5
UNION ALL
SELECT * FROM users_6
UNION ALL
SELECT * FROM users_7
UNION ALL
SELECT * FROM users_8
UNION ALL
SELECT * FROM users_9
UNION ALL
SELECT * FROM users_a
UNION ALL
SELECT * FROM users_b
UNION ALL
SELECT * FROM users_c
UNION ALL
SELECT * FROM users_d
UNION ALL
SELECT * FROM users_e
UNION ALL
SELECT * FROM users_f
EOT;
        return $this->setTable(DB::raw("({$sql}) AS users_all"));
    }
}

除了上面的方法，将合并查询写在 PHP 代码里，也可以写到 SQL 里。具体的做法是新增一个视图，视图的定义就是 UNION ALL 合并查询的 SQL 语句，仍旧以上面的 users 表为例子。

CREATE VIEW users_all AS
SELECT * FROM users_0
UNION ALL
SELECT * FROM users_1
UNION ALL
SELECT * FROM users_2
UNION ALL
SELECT * FROM users_3
UNION ALL
SELECT * FROM users_4
UNION ALL
SELECT * FROM users_5
UNION ALL
SELECT * FROM users_6
UNION ALL
SELECT * FROM users_7
UNION ALL
SELECT * FROM users_8
UNION ALL
SELECT * FROM users_9
UNION ALL
SELECT * FROM users_a
UNION ALL
SELECT * FROM users_b
UNION ALL
SELECT * FROM users_c
UNION ALL
SELECT * FROM users_d
UNION ALL
SELECT * FROM users_e
UNION ALL
SELECT * FROM users_f;