分组后查出count()为零的字段

1.需求：

想要统计出camille1，camille2，camille3，camille4每个人都有几件衣服，没有衣服的就为0件。

数据表(clothes)

id	name	color
1	camille1	blue
2	camille2	blue
3	camille2	red
4	camille4	yellow
5	camille4	blue

数据表(user)

id	name
1	camille1
2	camille2
3	camille3
4	camille4

2.实现

• a.一般情况我们会这样写SQL语句：

SELECT `name`, count(*) AS clothes_count FROM `clothes` WHERE `name` IN (‘camille1’, ‘camille2’, ‘camille3’,‘camille4’) GROUP BY `name`

但是结果为

从输出结果看少了这条数据

name	clothes_count
camille3	0

• b.实际SQL应当这样写：
  以原来查询出来的结果作为一个子表然后leftjoin这个子表查出我们想要的结果

SELECT DISTINCT a. name, IFNULL(b.clothes_count, 0) AS clothes_count FROM `user` AS a LEFT JOIN(
SELECT `name`, count(*) AS clothes_count FROM `clothes` WHERE`name` IN (‘camille1’, ‘camille2’, ‘camille3’,‘camille4’) GROUP BY `name`
) AS b ON `a`.`name` = `b`.`name` WHERE `a`.`name` IN (‘camille1’, ‘camille2’, ‘camille3’,‘camille4’)

3.用Laravel框架来实现上述SQL语句

//查询出计数为零的字段
   public function index(){
       $name = ['camille1','camille2','camille3','camille4'];
       $clothesQuery = new Clothes();
       $clothesCountObj = $clothesQuery->selectRaw('count(*) as clothes_count, name')
           ->whereIn('name', $name)
           ->groupBy('name');
       //为了返回结果有计数为零的字段
       $clothes = DB::table('user as a')
           ->select(DB::raw('IFNULL( b.clothes_count, 0 ) AS clothes_count,a.name'))
           ->distinct()
           ->whereIn('a.name', $name)
           ->leftJoin(\DB::raw("({$clothesCountObj->toSql()}) as b"), function ($join) use ($clothesCountObj) {
               $join->mergeBindings($clothesCountObj->getQuery())->on('a.name', '=', 'b.name');
           })
           ->get();
       return $clothes;
   }

4.解惑