Question

任何人都可以帮助我理解这个SQL查询;我不理解它背后的概念：

SELECT
    t2.*,
    (
        SELECT SUM (salary)
        FROM (
            SELECT
            tb1.*, 
            ROWNUM AS rnk
            FROM ( 
                SELECT *
                FROM employees e
                ORDER BY E.DEPARTMENT_ID, E.EMPLOYEE_ID
            ) tb1
        ) tb2
        WHERE   tb2.DEPARTMENT_ID = t2.DEPARTMENT_ID AND tb2.rnk <= t2.rnk
    ) dept_salary
FROM (
    SELECT   
        t1.*, 
        ROWNUM AS rnk
    FROM (  
        SELECT   
            e.employee_id,
            e.first_name,
            e.department_id,
            e.salary
        FROM employees e
        ORDER BY   E.DEPARTMENT_ID, E.EMPLOYEE_ID
    ) t1
) t2

Answer 1

有不同类型的SubQuery。

标量子查询：完全独立于主查询的查询。

选择empno，（从emp中选择count（*））total_emp 来自emp

这里子查询完全独立于主查询。

核心子查询：子查询引用主查询。

选择empno，（选择deptname 来自dept d 其中e.deptno = d.deptno）deptname 来自emp e
内联视图：用作SQL
中的表或视图的子查询
选择* from（选择empno，deptname 来自emp e，dept d 其中e.deptno = d.deptno）其中sal> 100

来自您的查询：

SELECT
    t2.*,
    (
        SELECT SUM (salary)  -- start Corelated Sub Query
        FROM (
            SELECT
            tb1.*, 
            ROWNUM AS rnk
            FROM ( 
                SELECT *  -- start Scalar Sub Query
                FROM employees e
                ORDER BY E.DEPARTMENT_ID, E.EMPLOYEE_ID -- end Scalar Sub Query
            ) tb1 
        ) tb2
        WHERE   tb2.DEPARTMENT_ID = t2.DEPARTMENT_ID AND tb2.rnk <= t2.rnk -- end Corelated Sub Query
    ) dept_salary
FROM (
    SELECT   -- start inline view
        t1.*, 
        ROWNUM AS rnk
    FROM (  
        SELECT   
            e.employee_id,
            e.first_name,
            e.department_id,
            e.salary
        FROM employees e
        ORDER BY   E.DEPARTMENT_ID, E.EMPLOYEE_ID
    ) t1
) t2  -- end inline view

注意：现在您可以在问题的内联视图中查找子查询和内联视图中的子查询。

如何使用表连接和内联视图

1 个答案: