How do I perform an IF…THEN in an SQL SELECT?

Question

How do I perform an IF...THEN in an SQL SELECT statement?

For example:

SELECT IF(Obsolete = \'N\' OR InStock = \'Y\' ? 1 :          


        

Answer 1

For the sake of completeness, I would add that SQL uses three-valued logic. The expression:

obsolete = 'N' OR instock = 'Y'

Could produce three distinct results:

| obsolete | instock | saleable |
|----------|---------|----------|
| Y        | Y       | true     |
| Y        | N       | false    |
| Y        | null    | null     |
| N        | Y       | true     |
| N        | N       | true     |
| N        | null    | true     |
| null     | Y       | true     |
| null     | N       | null     |
| null     | null    | null     |

So for example if a product is obsolete but you dont know if product is instock then you dont know if product is saleable. You can write this three-valued logic as follows:

SELECT CASE
           WHEN obsolete = 'N' OR instock = 'Y' THEN 'true'
           WHEN NOT (obsolete = 'N' OR instock = 'Y') THEN 'false'
           ELSE NULL
       END AS saleable

Once you figure out how it works, you can convert three results to two results by deciding the behavior of null. E.g. this would treat null as not saleable:

SELECT CASE
           WHEN obsolete = 'N' OR instock = 'Y' THEN 'true'
           ELSE 'false' -- either false or null
       END AS saleable