Conditionally select from one table or another in MySQL

Question

In MySQL, how do I select from another table if a foreign key is set?

What I'm trying to do is select Fields.value if Fields.value_id isn't set, otherwise select Values.value from Values where Value.id is equal to Fields.value_id .

My tables:

Fields:

id | value | value_id

Values:

id | value

What's wrong with my code here?

Code:

SELECT CASE WHEN Field.value_id = NULL OR Field.value_id = "" THEN Field.value ELSE Value.value FROM values as Value WHERE (Field.value_id = Value.id)

Answer 1

One syntax error is that you are missing the end in the case . I also think you want a left join between the tables. My best guess given the available information is this:

SELECT (CASE WHEN f.value_id = NULL OR f.value_id = '' 
             THEN f.value 
             ELSE v.value 
        END)
FROM fields f left join
     values v
     on f.value_id = v.id;