如何简化这个子查询？

Question

我从http://www.programmerinterview.com/index.php/database-sql/derived-table-vs-subquery/ 看到了这个子查询，它以下面的子查询为例，但说它可以写得更简单。谁能告诉我该怎么做？谢谢！

一个名为employee 的表，其中包含employee_name、last_name、employee_salary 和employee_number 列。我们希望找到所有薪水高于平均水平的员工。

   select employee_name 
    from employee
    where employee_salary >
    (select avg(employee_salary)
            from employee)

Answer 1

试试这个：

SELECT employee_name 
FROM employee e 
INNER JOIN (SELECT AVG(employee_salary) salary FROM employee) e1 ON e.employee_salary > e1.salary;

或

SELECT employee_name 
FROM employee e 
WHERE EXISTS (SELECT 1 FROM employee e1 WHERE e.employee_salary > AVG(e1.employee_salary)); 

Answer 2

我认为您的查询设计得很好。 使用 JOIN 效率会降低，并且 HAVING 子句将在检索后过滤数据。您将获取所有员工，然后过滤平均工资。

Answer 3

希望这会有所帮助

--DECLARE A TABLE VARIABLE
DECLARE @Emp AS TABLE

(
     ID                 INT
    ,EMPLOYEE_NAME      VARCHAR(50)
    ,EMPLOYEE_SALARY    NUMERIC(18,2)

)
-- ADD SOME VALUES TO THE TABLE
INSERT INTO @Emp
VALUES (1,'DANNY',7000),(2,'JOHN',10000),(3,'DAVID',1000)

--AVERAGE SALARY IS 6000..

--USING COMMON TABLE EXPRESSIONS
    ; WITH CTE AS
     (
        SELECT 
            AVG(EMPLOYEE_SALARY)  AS AVGSALARY
        FROM @EMP       
    )
        SELECT 
            EMPLOYEE_NAME
        FROM 
            @Emp    EMPLOYEE    -- MY TABLE
        INNER JOIN
            CTE
        ON
            EMPLOYEE_SALARY>CTE.AVGSALARY

Answer 4

您可以使用OVER 子句计算average 的薪水以及当前员工的薪水是否大于它。很遗憾，window 函数不能在 where 子句中使用，所以你也应该使用命令表表达式。

DECLARE @employee TABLE
(
     [employee_name] VARCHAR(12)
    ,[employee_salary] INT
)

INSERT INTO @employee ([employee_name], [employee_salary])
VALUES ('Empl1', 100)
      ,('Empl2', 200)
      ,('Empl3', 300)
      ,('Empl4', 400)
      ,('Empl5', 500)
      ,('Empl6', 600)
      ,('Empl7', 700)
      ,('Empl8', 800)

-- your query
 select employee_name 
    from @employee
    where employee_salary >
    (select avg(employee_salary)
            from @employee)

-- alternative query
;WITH DataSource AS
(
     select employee_name
           ,employee_salary - AVG(employee_salary) OVER () AS diff
     from @employee
 )
 SELECT employee_name
 FROM DataSource
 WHERE diff > 0

Answer 5

为sql server 2012

    select iif(((employee_salary) > (SELECT avg(employee_salary) FROM employee)),
            employee_name ,null) employee_name 
    FROM employee
    group by employee_name,employee_salary