如何在选择时根据它们的值将一列分成多列？

Question

我有一个名为 assignment_answers 的 table，它具有以下属性： assignment_answers_id，question_id 和订单。 order 是一个属性，可以取 0 到 9 之间的值。我希望每个值都可以显示在不同的列中。例如，当订单值为 0 时，我希望它显示在名为 number0 的列中。当它的值为 1 时，我希望它显示在名为 number1 的列中。

有人可以帮我吗？到目前为止，我已经尝试过了，但它不起作用：

SELECT (CASE WHEN assessment_answers.order = 0 
             THEN(
              select aq.order as number0
              from assessment_answers)
              END)
       (CASE WHEN assessment_answers.order = 1 
             THEN(
              select aq.order as number1
              from assessment_answers)
              END)
FROM assessment_answers

我收到一条错误消息：错误：“（”处或附近的语法错误第 6 行：（当 assessment_questions."order" = 1

时的情况

示例数据

assignment_answers_id     question_id    order
 1                            1           0
 2                            1           0
 3                            2           1

期望的输出：

assignment_answers_id     question_id    order0  order1
 1                            1           0       null
 2                            1           0       null
 3                            2           null     1

Answer 1

这是你想要的吗？

select (aa.order = 0)::int as order_0, 
       (aa.order = 1)::int as order_1, 
       (aa.order = 2)::int as order_2, 
       . . .
from  assessment_answers aa;

Answer 2

你可以尝试使用普通的CASE WHEN

查询 1:

SELECT assignment_answers_id,
       question_id,
       (CASE WHEN order = 0 THEN order END) order0,
       (CASE WHEN order = 1 THEN order END) order1
FROM assessment_answers

Results:

| assignment_answers_id | question_id | order0 | order1 |
|-----------------------|-------------|--------|--------|
|                     1 |           1 |      0 | (null) |
|                     2 |           1 |      0 | (null) |
|                     3 |           2 | (null) |      1 |

如何在选择时根据它们的值将一列分成多列？

How can I seperate one column into multiple columns depending on their value when selecting it?

sql

postgresql

multiple-columns

sql-manager