# SQL SERVER – Tips from the SQL Joes 2 Pros Development Series – All about SQL Constraints – Day 22 of 35

Answer simple quiz at the end of the blog post and –

Every day one winner from India will get Joes 2 Pros Volume 4.

Every day one winner from United States will get Joes 2 Pros Volume 4.

## Check Constraints

My old track coach would tell us to give 110% effort. However, had my math teacher heard this, he would have explained that a percentage value exceeding 100% in this context is not possible. For the coach it was a fun way that implies that you will give all you have, but then somehow you will give 10% more than “all.” Does that mean 110% is always an incorrect value? No, it depends on the situation. For example, this year’s sales could be 110% of last year’s sales. The range of acceptable values defined for a field depends upon the context.

Let’s consider negative numbers for a moment. If a medical form asked for my height in inches, and I wrote negative -67.5, you would think this was either a typo or a joke.  -67.5 would be easily accepted by a decimal or float data type, but in the context of height, a negative number simply isn’t acceptable. A person’s height will always be a positive number, never a negative number. For that reason, a “Height” field in a database should be restricted to only positive values. If you want to add your own logic on top of a data type, you can use what is known as a check constraint.

In the StateList the “District of Columbia” record should consist of 68 square miles of land. However, we see a negative LandMass value for that record and a few other records.

Similar to the concept of height, there really is no such thing as a negative land mass (coastal erosion anomalies aside).  We know that the absolute value for each LandMass number in our table is correct, so we essentially just need to clean up any negative figures to positive numbers (e.g., -68 should be 68).  The UPDATE statement below multiplies the LandMass value by -1 where the existing LandMass value is < 0 (less than zero). The SELECT query in the below Figure shows that no negative LandMass values remain following the update.

### Creating Check Constraints

Our LandMass data has been scrubbed and now we want to create the check constraint to maintain this field’s data integrity during future inserts or updates. We want SQL Server to say “Yes” to legitimate LandMass values (even if 2 states are the same size) but “No” to any update or insert resulting in a negative LandMass values. The code below will check to see if the value is positive before accepting the transaction. The preferred naming convention for the check constraint is CK for check (constraint) plus the table name and the fieldname, with each part separated by an underscore: CK_StateList_LandMass.

`ALTER TABLE StateList ADD CONSTRAINT CK_StateList_LandMass CHECK(LandMass >=0)`

To prove that our check constraint will prevent DML statements which try to introduce negative LandMass numbers, we will run a quick test. Below you see an UPDATE statement which attempts to change some of the LandMass values into negative numbers. The CK_StateList_LandMass check constraint guards against updates which would result in invalid LandMass values.

In the figure below we see the CK_StateList_LandMass check constraint that we just added. In Object Explorer, expand the “Tables” folder of the JProCo database and expand Constraints.  Note: You may need to right-click to refresh the Constraints folder and see newly created items.

Let’s consider other errors which could occur in our LandMass data. Our largest current LandMass is Alaska at just over 650,000 square miles. A typo adding one or two extra zeros would really be a problem (i.e., distorting Alaska’s LandMass to either 6 million or 65 million miles).  Let’s attempt a bad UPDATE to help us recognize that our check constraint isn’t currently protecting our table from these kinds of mistakes.

`UPDATE StateList SET LandMass = 65642500 WHERE StateID = 'AK' `

Since the UPDATE succeeded, our StateList table now incorrectly indicates that Alaska’s LandMass exceeds 65 million square miles. While constraints can’t guard against all potential data integrity problems, we should constrain the upper limit of the LandMass field in this example to disallow such glaringly large, erroneous values. Before proceeding, let’s reset Alaska’s LandMass back to its proper value:

`UPDATE StateList SET LandMass = 656425 WHERE StateID = 'AK' `

### Changing Existing Check Constraints

We want to add another condition to CK_StateList_LandMass, our existing check constraint for the LandMass field. There isn’t a T-SQL statement which allows us to directly modify our existing check constraint.  But we can easily accomplish our purpose by dropping the current constraint object and then rebuilding it with both conditions (values for LandMass must be non-negative and less than 2 million square miles).  Re-create CK_StateList_LandMass with expressions covering both of the conditions which we want the constraint to prevent (LandMass >= 0 and LandMass < 2000000) with the following formula.

`ALTER TABLE StateList DROP CONSTRAINT CK_StateList_LandMass GO ALTER TABLE StateList ADD CONSTRAINT CK_StateList_LandMass CHECK(LandMass >=0 AND LandMass < 2000000) GO`

To test whether our revised constraint works, let’s reattempt our earlier UPDATE statement. Our second requirement (LandMass < 2000000) is similarly enforced by the revised check constraint. Values in excess of 2 million (square miles) are disallowed for the LandMass field. You may also wish to perform additional testing to observe the check constraint allows valid transactions into the StateList table.

Note: If you want to setup the sample JProCo database on your system you can watch this video. For this post you will want to run the SQLProgrammingChapter1.0Setup.sql script from Volume 4.

## Question 22

You have a table named Feedback that contains every record of how a customer felt about their purchase. One field is called Complaint, where 0 is no complaint and 1 is a complaint. You also have a field called Rating that ranges from 0 to 100. If a customer complains they should not be giving a perfect rating of 100. If they complain then they can enter a score between 0 and 90. If they don’t then it can be between 1 and 100. Which check constraint would you use?

1. CHECK (Rating BETWEEN 1 and 100)
2. CHECK (Rating <=90 AND Complaint = 1)
3. CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )
4. CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1)
AND ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

### Rules:

Every day one winner will be announced from United States.
Every day one winner will be announced from India.
Every day one winner from India will get Joes 2 Pros Volume 4.
Every day one winner from United States will get Joes 2 Pros Volume 4.
The contest is open till next blog post shows up at which is next day GTM+2.5.

Reference:  Pinal Dave (https://blog.sqlauthority.com)

## SQL SERVER – Tips from the SQL Joes 2 Pros Development Series – What is XML? – Day 29 of 35

• HI ,

Option 3 : CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

this is correct option.

I am from Gujarat,India

• Hi,

None of the options are Correct.

CHECK ( (Rating BETWEEN 0 and 90 AND Complaint = 1)
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Because,
Rating should allow values between 0 and 90 When there is a complaint(i.e., complaint=1).

OR

Rating Should allow values between 1 and 100 When there is No Complaint(i.e., Complaint=0)

• Hi,

None of the options are Correct.

CHECK ( (Rating BETWEEN 0 and 90 AND Complaint = 1)
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Because,
Rating should allow values between 0 and 90 When there is a complaint(i.e., complaint=1).

OR

Rating Should allow values between 1 and 100 When there is No Complaint(i.e., Complaint=0)

Raju,
India.

• Answer 3 is the closest, but it does not quite meet the specifications of the question.
CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

The first line should read “Rating BETWEEN 0 AND 90 AND Complaint = 1” because if the customer complains, they could enter a rating of 0. Answer 3 would exclude the 0 rating by starting with a 1.

Answer 3 is also better than Answer 4 due to the “OR” criteria between the two check constraints. Answer 4 would always be wrong because Complaint can not always be 1 AND 0.

Matt Nelson, USA

• Mike Michalicek
August 22, 2011 6:14 pm

None of the options will create the check constraint we need here.

Question 22 stated: If a customer complains they should not be giving a perfect rating of 100. If they complain then they can enter a score between 0 and 90. If they don’t then it can be between 1 and 100. Which check constraint would you use?

Option #3 is close but not correct. It shoud look something like this:
CHECK ( (Rating BETWEEN 0 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

USA

Mike Michalicek

• Correct Option is 3
CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0)

(Sale, Nigeria)

• Hi Pinal,

Challenge:
Question 22
You have a table named Feedback that contains every record of how a customer felt about their purchase. One field is called Complaint, where 0 is no complaint and 1 is a complaint. You also have a field called Rating that ranges from 0 to 100. If a customer complains they should not be giving a perfect rating of 100. If they complain then they can enter a score between 0 and 90. If they don’t then it can be between 1 and 100. Which check constraint would you use?

1. CHECK (Rating BETWEEN 1 and 100)
2. CHECK (Rating <=90 AND Complaint = 1)
3. CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )
4. CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1)
AND ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

The choice above that is closest to being correct is choice #2, although it falls short of the requirement.

Explanation:
Choice #1 is incorrect because it will allow a rating up to 100 even if the complaint column is 1.
Choice #3 is also incorrect becuase the requirement is to allow a user to enter a score between 0 and 90 when they mark the complaint column with a 1 (not between 1 and 90).
Choice #4 is also incorrect because it will never be possible. The Complaint column can have only one value per customer experience. It can be zero or 1, but not both. The 4th choice uses an "AND" to try to force both values, which isn't possible.
In Choice #2, if the complaint column has a value of 1, then the Rating score must be 90 or less. If the Complaint column is zero, then this constraint will not enforce a range of the Rating score.

Choice #2 should be enhanced to also constrain the range of values when the Complaint column is zero. In that case, you would want to allow a value from zero to 100. You would also want to enforce that if there is a
complaint, the value should be between zero and 90, not just less than 90. You wouldn't want a negative value to be entered and stored in the database.

It would like like this:
CHECK ( (Rating BETWEEN 0 and 90 AND Complaint = 1 ) OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Country:
United States

Thanks for the knowledge!

Regards,

Bill Pepping

• The answer is number 3.

3. CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Answers 1 and 2 only check one scenario. Answer 4 uses the AND clause; it is not possible to have a complaint and no complaint at the same time.

Answer 3 is the only one to account for both scenarios and accept only 1.

Country: USA

• CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Chetan – USA

• Nikhil Mahajan
August 22, 2011 7:56 pm

the correct answer option 3 i.e.

3.CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

India

• The closest option is Option#3.

3.CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Maybe it is a typo, but if Option 3 has the range from “0 to 90” then it would fir the requirement spec. Or else the spec needs to be modified to “1 to 90″

Spec – ” If a customer complains they should not be giving a perfect rating of 100. If they complain then they can enter a score between 0 and 90. If they don’t then it can be between 1 and 100. Which check constraint would you use? ”

Hima
USA

• Diljeet Kumari
August 22, 2011 8:02 pm

Hi Pinal Sir,

Good Question In fact very good:-

Here None of the given answer satisfies.

Here None of the answers given satisfies the given condition, Because all check constraints shows the values condition between 1 to 100 except for the #2) Option.

But even that doesn’t contain value ranges from 1 to 100 if complaint=0 condition comes.

Here None of the given answer satisfies.

Diljeet Kumari
Country : India

• Abraham Forchue
August 22, 2011 9:00 pm

CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

–where 0 is no complaint and 1 is a complaint
–You also have a field called Rating that ranges from 0 to 100
–If a customer complains they should not be giving a perfect rating of 100, should be 0-90
CHECK (Rating BETWEEN 1 and 90 AND Complaint = 1)

If they don’t then it can be between 1 and 100
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

Abraham Forchue
Dominican Republic

• Option #3 is very close to the problem:
CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

However, in case they complain, we want to be able to enter score of 0 also, so the first part should be Rating between 0 and 90

All other options are not even close to the correct answer (option 4 is close, but incorrectly uses AND so it never will be true).

I am from USA

• Option 3 is the only answer that will satify both required conditions.

Country: United States

• If they complain then they can enter a score between 0 and 90. If they don’t then it can be between 1 and 100. Which check constraint would you use?

option 3

CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) )
OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )

David

• Correct Answer is: Option 3

Thanks,
Basavaraj
India

• A. Arul Prakash
August 22, 2011 11:40 pm

This will check that if complaint =1 , then rating is between 1 and 90(coded and an AND operation) or if Complaint =0 , rating is between 1 and 100 (again coded with AND operation)
Both these conditions are the possible conditions.

• A. Arul Prakash
August 22, 2011 11:40 pm

This will check that if complaint =1 , then rating is between 1 and 90(coded and an AND operation) or if Complaint =0 , rating is between 1 and 100 (again coded with AND operation)
Both these conditions are the possible conditions.
Country : USA

• dilipkumarjena
August 22, 2011 11:45 pm

Hi pinal Sir,

In the above question: none of the above answers are correct.

Explanation:

Given : Feedback table complaint and Rating two columns.

Requirement: if complaint = 0 (No complaint ) then Enter 1 to 100 the third query vary nicely provides.

But when we go for if complaint = 1 (complaint) then we will be able to enter 90 but also we are allowed to enter 1 but not 0 but as per our requirement we can enter 0 hence none of the above answers are correct.

Create statement :

create table Feedback
(
complaint int,
Rating int, CHECK ( (Rating BETWEEN 1 and 90 AND Complaint = 1) OR ( Rating BETWEEN 1 and 100 AND Complaint = 0) )
)

also there is a typo in option 3 you have included one bracket extra that leads to syntax error rectified in above query.

when no complaint works fine

insert into Feedback values(0,100)
insert into Feedback values(0,1)

when complaint works fine
insert into Feedback values(1,90)

but not allowed
insert into Feedback values(1,0)

Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint “CK__Feedback__4A18FC72”. The conflict occurred in database table “dbo.Feedback”.
The statement has been terminated.

Hence no option provide with the exact solution..

Dilip Kumar Jena
Country : INDIA