Question

尝试制作一个简单的视图，将总计合为一行。这里的窍门是我想从多个表中获得这些总计。

通过这种方式，我可以只选择该行以获得一堆我想跟踪的东西的总和。

到目前为止，我无法解决这个问题，有关此类事情的大多数帖子都与联接有关。

//OrderContainer.vue

    <template>
        <my-vue-avatar v-if="hasID" class="img-avatar" :id="avatarPicId"></my-vue-avatar>
    ....
    </template>
    <script>
    import MyVueAvatar from '../Avatar'
    ....
    data(){
            return { 
                avatarPicId:'',
                hasID: false
            }
        },
    beforeMount(){
        this.$eventBus.$on('idChange',id => {
            //this fanction will get triggered every time an `idChange` event fires.
            //lets assign the new value to our `avatarPicId`:
            this.avatarPicId = id;
            this.hasID = !!this.avatarPicId; //assign a boolean to this.hasID, wether the accepted id isnt undefined,null,false or empty string.
            //this.avatarPicId is sent as props to `my-vue-avatar` child component.
            //it should re-evaluates itself. if its not happening, you can  
             //cause here a complete destruction of MyVueAvatar 
             //component:
              this.hasID = false;
              //and a re-rendering, after the destruction is completed:
              this.$nextTick(()=> this.hasID = true);
        })
    },
}
</script>

Answer 1

您可以在SELECT中使用子查询：

CREATE VIEW stats AS
  select (select COUNT(DISTINCT p.name) from publisher p) as totalPublisher,
         (select COUNT(DISTINCT s.name) from series s) as totalSeries;

注意：这些表中的名称很可能是不同的。如果是这样，这样做会更有效：

CREATE VIEW stats AS
  select (select count(*) from publisher p) as totalPublisher,
         (select count(*) from series s) as totalSeries;

Answer 2

[…]简单视图，将总计合为一行[…]

不过，您所描述的并不简单；它涉及计算不同记录集的总数。也就是说，必然是一个复杂的查询，它将不同的记录集组合在一起。

您是正确的，联接是执行此操作的好方法（一个原因是，它告诉RDBMS您想要什么结果，而不是如何获得结果）。但是，在您给出的特定示例中，您试图将它们汇总成一行的值之间显然没有关联。

另一种实现方式：

CREATE VIEW stats AS
SELECT
    COUNT (SELECT DISTINCT name FROM publisher) AS total_publisher,
    COUNT (SELECT DISTINCT name FROM series) AS total_series

简单MySQL视图由不同表中的聚合数据组成

2 个答案: