如何从多个独立的select语句构造一个表？

Question

我查看了一些相关问题，但找不到帮助。

我正在尝试创建一个显示已完成注册步骤的公司数量的静态表。我已经完成了所有繁重的工作，我可以使用一个简单的 select 语句来查看有多少完成了，比如说，第 1 步。但我想写一个 select 语句，其中包含 step 1 step 2 step 3 (columns) with number (values) 的公司完成每个步骤.

step table 

----------------------------------------------------
progress_id | label | level | score | is_completed | 
----------------------------------------------------

progress_id：我们正在跟踪进度的公司

标签：步骤级别标签，例如：公司邮箱已验证，

级别：说第一步，

得分：例如：20%，

is_completed：真/假

你看这个，我可以简单地做到：

select count(*) step_one_count from step where level = 1 and is_completed = true;

----------------
step_one_count |
----------------
20

select count(*) step_two_count from step where level = 2 and is_completed = true;

----------------
step_two_count |
----------------
10

然后我需要编写一个选择语句，无论是使用子查询还是在下面实现这个结果

----------------------------------------------------
step_one_count | step_two_count | step_three_count |
----------------------------------------------------
20               10               40

Answer 1

我怀疑你想要条件聚合，在MySQL你可以这样做：

SELECT SUM( (level = 1 and is_completed = true) ) AS step_one_count,
       SUM( (level = 2 and is_completed = true) ) AS step_two_count,
       SUM( (level = 3 and is_completed = true) ) AS step_three_count
FROM step s;

以标准方式，您可以使用 CASE 表达式代替：

SELECT SUM(CASE WHEN level = 1 and is_completed = true THEN 1 ELSE 0 ) AS step_one_count,
       . . . 
FROM step s;

编辑：当您标记DBMS 时，您可以使用FILTER：

SELECT COUNT(*) FILTER (WHERE level = 1 and is_completed = true) AS step_one_count,
       COUNT(*) FILTER (WHERE level = 2 and is_completed = true) AS step_two_count,
       COUNT(*) FILTER (WHERE level = 3 and is_completed = true) AS step_three_count,
FROM step s;

Answer 2

用例当

   select 
   sum(case when level = 1 and is_completed = true then 1 else 0 end) 
   as  step_one_count,
   sum(case when level = 2 and is_completed = true then 1 else 0 end) 
   as step_two_count,
  sum(case when level in (1,2) and is_completed = true then 1 else 0 end)
   as step_three_count        
    from step