Subquery

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SELECT * 
FROM EmployeeDemographics
WHERE EmployeeID IN
 (SELECT EmployeeID
  FROM EmployeeSalary
   WHERE JobTitle = 'Accountant')
;

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SELECT JobTitle,
(SELECT AVG(Salary)
FROM EmployeeSalary)
FROM EmployeeSalary
;

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SELECT AVG(avg_age), AVG(min_age), AVG(max_age)
FROM 
(SELECT Gender,
 AVG(Age) AS avg_age,
 MIN(Age) AS min_age,
 MAX(Age) AS max_age,
 COUNT(Age) AS count_age
 FROM EmployeeDemographics
 GROUP BY Gender) AS age_table
;