Question

我正在使用How to do a FULL OUTER JOIN in MySQL?的答案在MySQL 5.7中进行完全外部联接。

但是，我不确定如何为子表添加别名，以便查询为右联接使用与左联接相同的表（甚至不确定MySQL是否会在后台进行优化）

代码如下：

List<int> filtered(List<Abc> list1, List<Abc> list2) {
    Set<String> dataStrings = list2.stream()
        .map(x -> x.dataString))
        .collect(Collectors.toSet());

    return list1.stream()
        .filter(x -> dataStrings.contains(x.dataString))
        .map(x -> x.rowNum)
        .collect(Collectors.toList());
}

查询本身将返回错误：

select *

from

(select cell as hdi_social_cell, count(*) as hdi_social_cell_count
from Detector_Social_Events where streamtype='social' and source = 'hdi'
group by cell having hdi_social_cell_count > 5) social_hdi

left join

(select cell as ml_social_cell, count(*)*0.1 as ml_social_cell_count
from Detector_Social_Events where streamtype='social' and source = 'ml'
group by cell having ml_social_cell_count > 1) social_ml

on social_hdi.cell = social_ml.cell

union select * from

(select cell as hdi_social_cell, count(*) as hdi_social_cell_count
from Detector_Social_Events where streamtype='social' and source = 'hdi'
group by cell having hdi_social_cell_count > 5)  social_hdi 

right join 

(select cell as ml_social_cell, count(*)*0.1 as ml_social_cell_count
from Detector_Social_Events where streamtype='social' and source = 'ml'
group by cell having ml_social_cell_count > 1)  social_ml

on social_hdi.cell = social_ml.cell;

Answer 1

因为您查询select cell as hdi_social_cell

所以你必须编码

on social_hdi.hdi_social_cell = social_ml.hdi_social_cell

在完全外部联接中选择时使用select的MySQL别名

1 个答案: