Constraint help

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Constraint help

fweymouth

SSC Veteran

Points: 211
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November 22, 2017 at 8:40 am

#326389

I am new to SQL and am playing around with it to familiarize myself with it before taking a class this coming spring. I found a question on another site that was similar to this one but had no soliution and am trying to figure out the answer. I am making a table for shipping packages for a shipping company. I have a table that has many columns, but at the moment I only care about height, width, length, and weight. All dimensions are in inches and all weights are in pounds. I do not care how large the package is in dimensions or weight, but as a company, I strictly enforce that a package may not weigh more than 1/4 lb. per cubic inch.
Write the appropriate constraints that need to be enforced regarding these four columns.
I have this constraint that I have written for the weight column:
CONSTRAINT weight CHECK ( "length" * 'width' * "height" < 0.25 per cubic inch)
I know this is probably wrong. What I'm thinking is that each column must have a separate constraint that restricts the total weight to the 1/4lb per inch.cubic . Just not sure how to script this?

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Thom A SSC Guru Points: 99126 More actions · Answer 1

It seems like, to me, that knowing the weight per cubic inch is probably quite important; thus, it would seem worth while to add the column as a computed column. Then, if you set it to PERSISTED, you can then add a CONSTRAINT to that column.

This is a much simpler example, but should give you the right idea, that you can then apply to your own environment:

USE Sandbox;
GO
CREATE TABLE Shipment (BoxID int IDENTITY(1,1),
        Width decimal(6,2),
         [Length] decimal(6,2),
         Area AS Width * [Length] PERSISTED);
GO
ALTER TABLE Shipment ADD CONSTRAINT SizeRestriction CHECK (Area <= 10);
GO
--box is correct size
INSERT INTO Shipment (Width, [Length])
VALUES (3,2);
--1 row returned!
SELECT *
FROM Shipment;
GO
--Box is too large
INSERT INTO Shipment (Width, [Length])
VALUES (3,4);
GO
--Still only 1 row returned as last row was not inserted
SELECT *
FROM Shipment;
GO
DROP TABLE Shipment;
GO

Thom~

Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk

John Mitchell-245523 SSC Guru Points: 148809 More actions · Answer 2

fweymouth - Wednesday, November 22, 2017 8:40 AM
What I'm thinking is that each column must have a separate constraint that restricts the total weight to the 1/4lb per inch.cubic.

No, you only need a single constraint. You can do it the way Thom showed you, with a computed (persisted) column, or you can put the whole calculation in the constraint itself:

CREATE TABLE Shipment (BoxID int IDENTITY(1,1), Width decimal(6,2), [Length] decimal(6,2));

ALTER TABLE Shipment ADD CONSTRAINT SizeRestriction CHECK (Width * [Length] <= 10);

John

fweymouth SSC Veteran Points: 211 More actions · Answer 3

Thank you both that helps. Is there away to do a constraint for each one that would work?

Thom A SSC Guru Points: 99126 More actions · Answer 4

fweymouth - Wednesday, November 22, 2017 10:51 AM
Thank you both that helps. Is there away to do a constraint for each one that would work?

What do you mean exactly "for each one"? Each value, on it's own, doesn't tell you enough. You can't validate that the weight of the item per cubic inch is above the threshold unless you have know the length, width and height (volume) and the weight of the parcel.

As an example:
I have a parcel measuring 10'' x 6'' x 3''. Is it too heavy?
I also have a parcel weighing 60lbs. It is 4'' wide and 6'' high. Is it too heavy?

You don't know, as you don't know the weight or length of the parcel. (respectively). If, however, I added the final piece of the puzzle:
I have a parcel measuring 10'' x 6'' x 3''. It weight 100lbs. Is it too heavy? - Yes, it weighs 0.27~lbs per square inch.
I also have a parcel weighing 60lbs. It is 4'' wide, 6'' high and 10'' long. Is it too heavy? - No, it weighs exactly 0.25 lbs per square inch.

Only when I add the final piece of the puzzle can you CHECK.

Thom~

Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk