Using a case statement in a check constraint

Question

i'v been learning SQL for the last week but I am unsure how to correctly add a case statement within a check constraint. Can anybody give me any pointers?

I have the following grade table:

CREATE TABLE Grade
(
    salary_grade    char(1) NOT NULL CHECK (salary_grade = UPPER(salary_grade)),
        CONSTRAINT ck_grade_scale CHECK(
        CASE 
            WHEN salary_grade = '[A-D]' 
                THEN salary_scale = 'S1'
            WHEN salary_grade = '[D-G]' 
                THEN salary_scale = 'S2'
        END)

        salary_scale    char(2) DEFAULT 'S1' NOT NULL,

        CONSTRAINT pk_grade PRIMARY KEY (salary_grade),
        CONSTRAINT ck_salary_grade CHECK (REGEXP_LIKE(salary_grade, '[A-G]', 'c')),
        --constraint must be either S1 or S2
        CONSTRAINT ck_salary_scale CHECK (salary_scale IN ('S1', 'S2'))
);

I want to check that if the salary_grade is between A-D then the salary_scale must be 'S1' or if the salary_grade is between E-G then it's 'S2'.

I have tried to research this and come up with the latter but however it does not work.. have I structured the code correctly?

Solution 1

I think you can do the following:

CREATE TABLE Grade
(
  salary_grade    char(1) NOT NULL CHECK (REGEXP_LIKE(salary_grade, '[A-G]', 'c')),
  salary_scale    char(2) DEFAULT 'S1' NOT NULL,
  CONSTRAINT pk_grade PRIMARY KEY (salary_grade),
  CONSTRAINT ck_grade_scale CHECK ( REGEXP_LIKE(salary_grade, '[A-D]', 'c') AND salary_scale = 'S1'
                                 OR REGEXP_LIKE(salary_grade, '[E-G]', 'c') AND salary_scale = 'S2' )
);

Please see SQL Fiddle schema here.

You don't need the UPPER() constraint on salary_grade since the regex check will suffice (you're already checking to make sure it's an uppercase letter between A and G). I don't think the constraint on salary_scale alone is necessary either since it would be contained, logically, in the last constraint.

UPDATE

Here is how you might do it with a CASE statement:

CREATE TABLE Grade
(
  salary_grade    char(1) NOT NULL CHECK (REGEXP_LIKE(salary_grade, '[A-G]', 'c')),
  salary_scale    char(2) DEFAULT 'S1' NOT NULL,  
  CONSTRAINT pk_grade PRIMARY KEY (salary_grade),
  CONSTRAINT ck_grade_scale CHECK ( salary_scale = CASE WHEN REGEXP_LIKE(salary_grade, '[A-D]', 'c') THEN 'S1' ELSE 'S2' END )
);

Please see SQL Fiddle schema here.

Solution 2

A case has to be compared to something, which is why you are getting the missing right parenthesis error. Unless you particularly want a case, you can just check the combination with and/or:

CONSTRAINT ck_grade_scale CHECK(
    (salary_grade BETWEEN 'A' AND 'D' AND salary_scale = 'S1')
    OR (salary_grade BETWEEN 'D' AND 'G' AND salary_scale = 'S2')),

SQL Fiddle demo.

As Parado has said, you can't use constraints to set column values conditionally, only to restrict them. You could potentially use a virtual column for the scale, but it would mean putting part of a look-up table into the DDL rather than the data, which seems a bit strange.

Solution 3

Check Constraints is used to test data before insert to protect data structure from fake data. Actually we use case in select statement. You can't use it for conditional insert. If you want to change data for particular column before insert you need to use trigger or you can also use virtual column but which has some restrictions.

More information you can find here

• check constraints
• triggers
• virtual columns

Comments

[user3414871]

I kinda already know how to do this, as explained in 'Oracle SQL by Example' but the book does not tell me how to use case statements in Check constraints. This is what I want to learn

[user3414871]

Okay. This helps.. Thanks. What I was thinking was if grade is entered then the salary_scale would be automatically put in. Is that possible?

[Robert]

@user3414871 Yes it's possible by using triggers or virtual columns as I mentioned above.

[user3414871]

Okay.. i'm not trying to assign values to the columns, just trying to ensure the values of the columns are either S1 or S2. i.e S1 if grade A

[Robert]

@user3414871 Yes it's Alex Poole``s solution CONSTRAINT ck_grade_scale CHECK((salary_grade BETWEEN 'A' AND 'D' AND salary_scale = 'S1') OR (salary_grade BETWEEN 'D' AND 'G' AND salary_scale = 'S2'))

[Alex Poole]

The first check on salary_grade alone isn't really needed either, since ck_grade_scale covers that too.

[Allan]

@user3414871: Perhaps that book doesn't give you an example of this use because it's not a good use for CASE.

[David Faber]

Yes, you're probably right, but then I would have to change my ELSE in the CASE. Strictly speaking too a regex is probably more expensive than one would like; BETWEEN is better.

[user3414871]

@DavidFaber Thanks dude.. You been very helpful. It's a big learning curve when you just getting into SQL. I thought that with only a few keywords it would be easy to learn but things are never as easy as they look