Question

对于我的数据库，有这四个表

第一个，DEPARTMENT

//DEPARTMENT
D#        DNAME
------------------
1        RESEARCH
2          IT
3        SCIENCE

第二个，EMPLOYEE

//Employee
E#     ENAME         D#
-----------------------
1      ALI           1
2      SITI          2
3      JOHN          2
4      MARY          3
5      CHIRS         3

第三，项目

//PROJECT
P#     PNAME        D#
-----------------------
1     Computing     1
2     Coding        3
3     Researching   3

第四，WORKSON

//WORKSON
E#     P#     Hours
--------------------
1      1       3
1      2       5
4      3       6

所以我的输出应该是

E#       ENAME      D#       TOTAL HOURS/W
--------------------------------------------
1         ALI       1              8
2        SITI       2              0
3        JOHN       2              0
4         MAY       3              6
5        CHIRS      3              0

显示0，因为员工没有可以处理的项目。

我目前使用

的陈述

SELECT E#,ENAME,D# and sum(Hours) as TOTAL HOURS/W
    FROM EMPLOYEE,PROJECT,WORKSON
    WHERE EMPLOYEE.P#

不知道应该如何选择

Answer 1

您需要使用GROUP BY和JOINS才能实现输出

Answer 2

您应该像这样使用左连接。您只需要2个表employee和workson。

尝试此查询：

SELECT e_tbl.E#, e_tbl.ENAME, e_tbl.D#, 
coalesce(SUM(w_tbl.Hours), 0) as "Total Hours/W"
FROM 
EMPLOYEE e_tbl LEFT JOIN WORKSON w_tbl
ON e_tbl.E# = w_tbl.E#
GROUP BY e_tbl.E#

Answer 3

SELECT E.E#,
       E.ENAME,
       E.D#,
       sum(Hours) AS TOTAL HOURS/W
FROM Employee AS E
JOIN WORKSON AS W ON E.E# = W.E#
GROUP BY E.E#,
         E.ENAME,E.D#

使用此：）

Answer 4

使用给定的输出，您不需要加入所有表，这可以通过加入员工并以

工作来完成

select
e.`E#`,
e.ENAME,
e.`D#`,
coalesce(tot,0) as `TOTAL HOURS/W`
from Employee e
left join
(
  select `E#`,
  sum(Hours) as tot
  from WORKSON
  group by `E#`
)w
on w.`E#` = e.`E#`
group by e.`E#`

<强> DEMO

SELECT sql包含四个具有主键和外键的不同表

4 个答案: