<p>What's the best way to create a non-NULL constraint in MySQL such that fieldA and fieldB can't both be NULL. I don't care if either one is NULL by itself, just as long as the other field has a non-NULL value. And if they both have non-NULL values, then it's even better.</p>

I've done something similar in SQL Server, I'm not sure if it will work directly in MySQL, but:
```
```
ALTER TABLE tableName ADD CONSTRAINT constraintName CHECK ( (fieldA IS NOT NULL) OR (fieldB IS NOT NULL) );
```
```
At least I believe that's the syntax.
However, keep in mind that you cannot create check constraints across tables, you can only check the columns within one table.