Question

表1：问题

id  |  question
----------------
1   |  Name
2   |  Age
3   |  Gender
4   |  Position

表2： answer

qid  |  ans  |  record
-----------------------
1    |  Jay  |  1 
2    |  24   |  1 
3    |  M    |  1 
2    |  23   |  2

我想提出一个连接查询，产生下表：

record  |  question  |  ans
-----------------------------
1       |  Name      |  Jay
1       |  Age       |  24
1       |  Gender    |  M
1       |  Position  |  null
2       |  Name      |  null
2       |  Age       |  23
2       |  Gender    |  null
2       |  Position  |  null

我能想到的最接近的是这种联接：

select a.record, q.question, a.ans 
from 
question q left outer join answer a
on q.id = a.qid order by a.record,q.id;

但是，此查询只会产生此问题，但我希望所有问题都显示两次

record  |  question  |  ans
-----------------------------
1       |  Name      |  Jay
1       |  Age       |  24
1       |  Gender    |  M
1       |  Position  |  null
2       |  Age       |  23

Answer 1

您需要交叉连接以生成所需的所有组合，并与左连接配对以检索答案，如：

                    index.html   1.61 KiB                [emitted]  
       static/js/bundle.js.map   32.1 KiB  runtime~main  [emitted]         static/js/main.chunk.js   15.5 KiB          main  [emitted]  main
   static/js/main.chunk.js.map   12.1 KiB          main  [emitted]  main
Entrypoint main = static/js/bundle.js ... blah blah

ERROR in /foo/src/index.tsx
ERROR in /foo/src/index.tsx(14,20):
TS2304: Cannot find name 'nonExistentVar'.

ERROR in /foo/src/index.tsx
ERROR in /foo/src/index.tsx(15,20):
TS2304: Cannot find name 'nonExistentVar2'.

Child HtmlWebpackCompiler:
     1 asset
    Entrypoint HtmlWebpackPlugin_0 = __child-HtmlWebpackPlugin_0
    [./node_modules/html-webpack-plugin/lib/loader.js!./public/index.html] 1.75 KiB {HtmlWebpackPlugin_0} [built]
    [./node_modules/webpack/buildin/global.js] (webpack)/buildin/global.js 475 bytes {HtmlWebpackPlugin_0} [built]
    [./node_modules/webpack/buildin/module.js] (webpack)/buildin/module.js 546 bytes {HtmlWebpackPlugin_0} [built]
        + 1 hidden module
error Command failed with exit code 2.

结果：

select
  r.record,
  q.question,
  a.ans
from question q
cross join (select distinct record from answer) r
left join answer a on a.record = r.record and a.qid = q.id
order by r.record, q.id

作为参考，这是我用来验证情况的测试脚本：

record  question  ans   
------  --------  ------
1       Name      Jay   
1       Age       24    
1       Gender    M     
1       Position  <null>
2       Name      <null>
2       Age       23    
2       Gender    <null>
2       Position  <null>

SQL查询联接两个类似于左外部联接的表

1 个答案: