将CONSTRAINT添加到部分主键

Question

I am relatively new to using SQL, and MySQL in specific.

I have the following table definition, and need to add a general constraint to it which I will explain shortly.

    CREATE TABLE `ProjInvestigat` (
    `projectID` INT,
    `principalInvestigator` NUMERIC(8,0) ZEROFILL,
    `coInvestigator` NUMERIC(8,0) ZEROFILL,
    PRIMARY KEY (projectID, principalInvestigator, coInvestigator)
);

I can have multiple instances of a projectID and principalInvesigator in the table, as long as the coInvesigator is different. ie

     projID principID   coInvestID  
     15     17          27      OKAY
     15     17          37      OKAY
     15     27          47      NOT OKAY, I need to prevent having two principal investigators on the same project.

I know to add a CONSTRAINT on the table definition, or in an ALTER TABLE statement, but what do I state to enforce this policy? How do I approach this?

Thanks for your help.

Answer 1

Before filling your base, you have to write a script which allows you to verify if a project has two main investigators or not. You can also look at this link , maybe it's helpful.