sql 查询从两个相关表中获取数据

Question

我有两张桌子，员工和库存。 一名员工可以拥有零个或多个库存。 我想列出员工信息以及最多一个库存信息，并且库存计数属于一名员工。

员工表

emp_num  last_name  first_name
-----------------------------------
100      john       smith

101      mike       pet

102      jes        lyoid

库存表

inv_num  emp_num
---------------------------
12       100

13       100

15       100

30       102

所需 Output

emp_num     last_name       invnum  count(inv_num)
-------------------------------------------------------------------------- 
100         john            12      3

101         mike            -       0

102         jes             30      1

在这种情况下，我可以使用什么 sql 查询？

Answer 1

尝试这个：

SELECT emp_num, last_name, MAX(inv_num) AS invnum, COUNT(inv_num) AS inv_count
FROM employee e LEFT OUTER JOIN inventory i ON e.emp_num = i.emp_num
GROUP BY e.emp_num, e.last_name

Answer 2

你可以做这样的事情

Select E.Emp_Num,
       e.Last_name,
       MIN(Inv_Num) AS OldestInv,
       COUNT(Inv_Num) AS TotalInv
FROM Employee E
    LEFT OUTER JOIN Inventory I
        (E.Emp_Num = I.Emp_Num)
GROUP BY E.Emp_Num, E.Last_Name

这将为您提供最小发票编号和总计数。 左外连接是关键

Answer 3

SELECT 
    e.emp_num,
    e.last_name,
    IFNULL(MAX(i.inv_num),'-') AS 'invnum',
    COUNT(i.inv_num) AS 'count(inv_num)'
FROM
    employee e LEFT JOIN inventory i
    ON e.emp_num = i.emp_num
GROUP BY
    e.emp_num, e.last_name