Set column to only accept negative values
CREATE TABLE t ( n INTEGER NOT NULL CHECK (n &lt; 0) );


works in most RDBMS I know. 

Edit: A comment by @IMSop prompts me to specify why I wrote “most RDMS I know:” it is well known (and very unfortunate) that CHECK constraints aren’t honored by MySQL. In MySQL, you have to use triggers instead. Another option is to switch to MariaDB.