Is it possible to create a column in MySQL with an expression as a default value?

问题

How would one implement this in MySQL:

CREATE TABLE employee (
  employeemonthly DECIMAL(10,2), 
  employeeyearly DECIMAL(10,2) DEFAULT employeemonthly*12
);

回答1:

use a insert trigger for that. Something like this

DELIMITER |

CREATE TRIGGER default_yearly BEFORE INSERT ON employee
  FOR EACH ROW BEGIN   
    SET NEW.employeeyearly = NEW.employeemonthly * 12;
  END;
|

DELIMITER ;

回答2:

I would use a view:

CREATE VIEW vemployees AS
  SELECT e.employeemonthly,
         e.employeemonthly * 12 AS employeeyearly
    FROM EMPLOYEE e

...because there's little need to dedicate storage space for an easily calculated value. Otherwise, use a function or simply write the expression into whatever query/stored procedure you need.

What really depends is:

• How often you need to access this data
• How complex the operation is to get the result you need

I recommend starting with not storing the value. If performance gets to be a problem, then dedicate a column for the values storage -- not before. At that, a trigger is a bit overkill to me, when you can use (psuedocode):

INSERT INTO employee 
  (...employeemonthly, employeeyearly, ...)
VALUES
  (...@employeemonthly, @employeemonthly * 12, ...

回答3:

Use a trigger for the insert event, access the new record data using NEW and set the appropiate values.

来源：https://stackoverflow.com/questions/10863263/is-it-possible-to-create-a-column-in-mysql-with-an-expression-as-a-default-value