[英]Update a column using Subquery in oracle
I have an EMPLOYEE table below: 我下面有一个EMPLOYEE表:
EMP_ID DEPT_ID
101 1
102 2
103 3
104 1
And a DEPARTMENT table as: 和一个DEPARTMENT表为:
DEPT_ID COUNTS
1
2
3
I want to write a query which would count the number of Employee that belong to a department and store it into Department column table so the Department table will look like: 我想编写一个查询,该查询将计算属于某个部门的Employee的数量并将其存储到Department列表中,因此Department表将如下所示:
DEPT_ID COUNTS
1 2
2 1
3 1
The solution is 解决方案是
update department p
set counts = (select count(*) from EMPLOYEE e where p.dept_id = e.dept_id);
But i really dont understand how it works internally How does it know which dept ids in DEPARTMENT it has to set counts to. 但是我真的不明白它是如何在内部工作的。它如何知道它必须将计数设置到DEPARTMENT中的哪个部门ID。 what exactly does this subquery return "select count(*) from EMPLOYEE e where p.dept_id = e.dept_id"
该子查询返回的确切内容是“从EMPLOYEE e中选择count(*),其中p.dept_id = e.dept_id”
This is called correlated subquery: 这称为相关子查询:
This is your query: 这是您的查询:
update department p
set counts = (select count(*) from EMPLOYEE e where p.dept_id = e.dept_id);
So, the inner query get executed based on the outer query (ie has department
table). 因此,基于外部查询(即具有
department
表)执行内部查询。 If, your outer query has total 3 records then, inner query (ie has EMPLOYEE
table) will get execute three times. 如果您的外部查询总共有3条记录,那么内部查询(即具有
EMPLOYEE
表)将被执行3次。
In your scenario, you have mapped the queries with dept_id
from outer query, so this will look up into inner query (ie EMPLOYEE
table) and get the counts based on dept_id
. 在您的方案中,您已将外部查询中具有
dept_id
的查询映射到外部查询,因此这将查询内部查询(即EMPLOYEE
表)并基于dept_id
获取计数。
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