Using subquery after the from clause

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I'm a little confused, e.g say that We have a query and it looks like in the following:

select * from (select c1,c2 from table_name where condition) table_name;

I want to understand how it works? I mean I never use in my during sql time subquery after the from clause, but I want to understand how it works?

What I have tried:

I readed a book about sql and there is a sentence that subquery means that it generate derived table. and as I understand that's why we can use subquery after the from clause and sql generate sucessfully syntax because of this rule, isn't it?

Posted 23-Oct-22 10:35am

vor_zakon_developer

Updated 23-Oct-22 19:05pm

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OriginalGriff · Answer 1 · 2022-10-23T19:05:00

The example you give isn't too useful: it doesn't need to include a subquery because it just selects everything from it anyway.

Subqueries are useful when you want to extract specific data, and then use it to match other data together: for example in a WHERE clause.

Suppose you have a table of employees, and you want to find all the employees in your department whose salary is higher than yours - you use a subquery.
First you write a query to return your salary:

SQL

SELECT Salary FROM Employees 
 WHERE EmpName = 'Proqramlaşdırma Dünyası'

That returns a single value - your salary.
You can then use that as a subquery to find the info you want:

SQL

SELECT * FROM Employees
 WHERE Salary > (SELECT Salary FROM Employees 
                  WHERE EmpName = 'Proqramlaşdırma Dünyası')
   AND Department = 'Software'

Make sense?