如何设置MySQL约束以禁止单行的某些列值？

Question

I have a Table.

word      singular_only      plural_only
homework       1                  0       #homework has no plural
dog            0                  0       #dog and dogs are both real words
scissors       0                  1       #scissor is not a valid singular noun 
 x             1                  1       #should not be insertable

I want to allow the first three rows but not the fourth. How do I add a constraint for this. The values in singular_only and plural_only are boolean. The columns mean whether a word only has a plural form or singular form or both (if both booleans are false). Obviously if something can only be singular it cannot also be plural. What is the best way to model this in MySQL.

Is it better to add this constraint in MySQL or to ensure that the overlying code never tries to insert these values.

In other words

singular_only NAND plural_only should also be true

Answer 1

You can't add a constraint on one column based on another column except to constrain matching values, which you do not want to do. You should be using a single column to set "singular/plural" status. There are several solutions including using an integer (0,1,2 for the three possible states), an enum, and even another table. I'll leave the choice up to you, but I would favor the flexibility and clarity of another table.