check constraint with if logic oracle sql

Question

Is there a way to use if logic in an oracle sql check constraints?

This is my pseudo table:

create table child
(
    name,
    behaviour,
    treat,
);

Now what i want is that if the behaviour of child = 'bad' then treat != 'lollies' .

Answer 1

Sometimes it is easier to express using NOT to define what is not allowed:

CHECK (NOT (behaviour = 'bad' AND treat = 'lollies'))

... which means same as:

CHECK (behaviour != 'bad' OR treat != 'lollies')

It is easy to get this wrong, as other answers have shown!

Answer 2

ALTER TABLE child 
  ADD CONSTRAINT bad_behaviour_CK 
    CHECK (NOT (behaviour = 'bad' AND treat = 'lollies')) ;

Tesed in SQL-Fiddle

Answer 3

If you have learned propositional logic before, we can apply it in this situation and explain how the top voted answer is derived.

We let A be the proposition of behavior = 'bad' and B be the proposition of treat != 'lollies' . The statement of "If behavior = 'bad' , then treat != 'lollies' " can be written as " A → B ".

Note that " A → B " is logically equivalent to "¬ A ∨ B " (¬ means NOT and ∨ means OR). Therefore, we can translate the statement "¬ A ∨ B " back to " behavior != 'bad' OR 'treat != 'lollies' ".

In terms of SQL, we can write this as:

CHECK (behaviour <> 'bad' OR treat <> 'lollies')

This is equivalent to:

CHECK (NOT (behaviour = 'bad' AND treat = 'lollies'))

Answer 4

Try this

create table children ( 
        childname VARCHAR2(50), behaviour VARCHAR2(50),treats VARCHAR2(50),
        constraint behave_treat check (behaviour = 'bad' AND treats != 'lollies')
        )