Question

with a as (
select a.*, row_number() over (partition by department order by attributeID) rn
  from attributes a),
e as (
select employeeId, department, attribute1, 1 rn from employees union all
select employeeId, department, attribute2, 2 rn from employees union all
select employeeId, department, attribute3, 3 rn from employees
)
select e.employeeId, a.attributeid, e.department, a.attribute, a.meaning, 
   e.attribute1 as value 
 from e join a on a.department=e.department and a.rn=e.rn 
order by e.employeeId, a.attributeid

此查询由Ponder Stibbons撰写，用于this问题的答案。但我对它太晕了，因为我完全不明白这里发生了什么。我是SQL新手。所以如果有人能解释这个查询发生了什么，我将不胜感激。谢谢

Answer 1

基本上，他使用3个select语句（每个属性1个）和UNION将它们组合在一起，以形成一个公共表表达式，以便为每个employees属性获取行。

select employeeId, department, attribute1, 1 rn from employees union all
select employeeId, department, attribute2, 2 rn from employees union all
select employeeId, department, attribute3, 3 rn from employees

另一张表他使用窗口函数为属性，部门分配编号。他稍后使用这个数字加入他的不透明数据。他发布了他的代码示例。

select a.*, row_number() over (partition by department order by attributeID) rn
  from attributes a

我建议您使用他提供的示例数据并执行以下操作。这将向您展示CTE。我想一旦你看到这些数据就会更有意义。

with a as (
select a.*, row_number() over (partition by department order by attributeID) rn
  from attributes a),
e as (
select employeeId, department, attribute1, 1 rn from employees union all
select employeeId, department, attribute2, 2 rn from employees union all
select employeeId, department, attribute3, 3 rn from employees
)

SELECT * from a
SELECT * from e

谁能解释一下这个查询？

1 个答案: