Mysql unique constraint allowing single row for a combination

Question

Is it possible to have a unique constraint such that one particular column has a value only once?

For instance

-----------------------    
name | pri         


        

Answer 1

Putting a simple unique constraint over all three columns would not work because it would allow the following:

name | price | default
-----------------------
XYZ  |  20   | TRUE
XYZ  |  30   | TRUE
XYZ  |  40   | FALSE
ABC  |  50   | FALSE

In other SQL engines, you could just create a check constraint that ensures that for every set of name rows, the number of those rows with default = TRUE is <= 1. However, MySQL does not support enforcing check constraints.

In MySQL you could implement this via insert and update triggers instead:

CREATE TRIGGER `before_insert` BEFORE INSERT ON `tableName`
FOR EACH ROW
BEGIN

   DECLARE @sum INT

   SELECT @sum = SUM(CASE WHEN [default] = TRUE THEN 1 ELSE 0 END)
   FROM tableName
   WHERE [name] = new.[name]

   IF (@sum = 0 OR new.default = FALSE)
   BEGIN
      INSERT INTO tableName (name, price, default)
      VALUES (new.[name], new.[price], new.[defaul]t)

   END
END

And similarly for the update.