How to not let sum of child field not exceed parent field in MS Access

问题

I have MS Access database file with following tables

Table In

Id | Quantity
------------------------
1  | 8

Table Out

Id | Parent    | Quantity
-------------------------
1  | 1         | 2
2  | 1         | 5

Table In is a parent table and Table Out is related table.

I need following constraint to be enforced (pseudo formula):

In.Quantity >= Sum(Out.Quantity)

That is: Sum of outgoing quantities cannot be greater than incoming quantity

I am developing a Winforms application. The user will make changes (Insert, Update and delete) to both In and Out

回答1:

You can use a CHECK constraint to validate the condition.

ALTER TABLE Out 
ADD CONSTRAINT NoNegativeTotal
CHECK(
    NOT EXISTS(
        SELECT 1
        FROM In
        INNER JOIN Out ON In.Id = Out.Id
        GROUP BY In.Id, In.Quantity
        HAVING SUM(Out.Quantity) > In.Quantity
    )
)

You might want to add the same constraint to the In table, since if someone changes the In.Quantity field to violate the constraint, that'll lock down all editing in the Out table except edits that make the constraint valid again.

Check constraints need to be executed either in ANSI 92 compatible mode, or using ADO (e.g. using CurrentProject.Connection.Execute). More details here.

回答2:

Use the BeforeUpdate event of the subform to compare the sum of field Quantity with Me.Parent!Quantity.Value.

If exceeding, set Cancel = True and pop a message.

来源：https://stackoverflow.com/questions/57430333/how-to-not-let-sum-of-child-field-not-exceed-parent-field-in-ms-access