SQL/MS Access：在 SQL 表中添加一个自动更新的字段，其中包含另一个表中的值的总和

Question

I am building a database with MS Access, and I am quite new to Access and SQL language.
I would like to add a field in a table which will automatically update its value based on the sum of values listed from another table.

---

Example:

There is a staff and a payroll table, and I would like to add a total field to the staff table, which is equal to the total amount of salary listed in the payroll table for each staff.
With a step-by-step approach, I have managed to build a query which do the summation of salary for each staff.

staff

| id |   name   |
|----|----------|
| A1 | staff A1 |
| A2 | staff A2 |
| B1 | staff B1 |

payroll

| id | month | salary |
|----|-------|--------|
| A1 |  JAN  |  10000 |
| A2 |  JAN  |  15000 |
| A1 |  FEB  |  10000 |

summation

SELECT staff.id, SUM([payroll]![salary]) AS total
FROM payroll 
INNER JOIN staff ON staff.id = payroll.id
GROUP BY staff.id;

| id | total |
|----|-------|
| A1 | 20000 |
| A2 | 15000 |

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Expected result: staff

| id |   name   | total |
|----|----------| ----- |
| A1 | staff A1 | 20000 |
| A2 | staff A2 | 15000 |
| B1 | staff B1 |     0 |

Questions:

On MS Access, I tried to add a lookup query to the total field in staff to automatic update the value, but if only gives me a combo box options for selection.
Any suggestions and solutions for this problem will be appreciated, thanks.

SELECT total
FROM summation
WHERE staff.id = summation.id

Answer 1

using your main table from STAFF table might work, like ;

SELECT staff.id, SUM([payroll]![salary]) AS total
FROM staff 
INNER JOIN payroll ON staff.id = payroll.id
GROUP BY staff.id;

Answer 2

INNER JOIN means, you're summing two tables with common values which means If B1 doesn't exist into the payroll tables, it also will not appear in the relationship table.

FULL OUTER JOIN will help you to query all the data from two tables.

Maybe this chart will explain what I mean;

Maybe you need to use FULL OUTER JOIN . So, It has to go like that;

SELECT staff.id, SUM([payroll]![salary]) AS total
FROM payroll 
FULL OUTER JOIN staff ON staff.id = payroll.id
GROUP BY staff.id;

OUTPUT

| id |   name   | total |
|----|----------| ----- |
| A1 | staff A1 | 20000 |
| A2 | staff A2 | 15000 |
| B1 | staff B1 |     0 |

Reference: SQL Joins

Answer 3

You want a LEFT JOIN and NZ() :

SELECT s.id, NZ(SUM(p.salary)) AS total
FROM staff as s LEFT JOIN
     payroll as p
     ON s.id = p.id
GROUP BY s.id;

You can also write this using a correlated subquery. This makes it easier to keep more columns from staff :

select s.*,
       (select nz(sum(p.salary))
        from payroll as p
        where p.id = s.id
       ) as total
from staff as s;

If you have a total column in staff you can update it using the last approach:

update staff
    set total = (select nz(sum(p.salary))
                 from payroll as p
                 where p.id = staff.id
                );