SQL: How can I update a value on a column only if

问题:

I have an SQL question which may be basic to some but is confusing me.

Here is an example of column names for a table 'Person': PersonalID, FirstName, LastName, Car, HairColour, FavDrink, FavFood

Let's say that I input the row:

121312, Rayna, Pieterson, BMW123d, Brown, NULL, NULL

Now I want to update the values for this person, but only if the new value is not null, Update:

121312, Rayna, Pieterson, NULL, Blonde, Fanta, NULL

The new row needs to be:

121312, Rayna, Pieterson, BMW123d, Blonde, Fanta, NULL

So I was thinking something along the lines of:

Update Person(PersonalID, FirstName, LastName, Car, HairColour, FavDrink, FavFood) set Car = @Car (where @Car is not null), HairColour = @HairColour (where @HairColour...)... etc.

My only concern is that I can't group all the conditions at the end of the query because it will require all the values to have the same condition. Can't i do something like Update HairColour if @HairColour is not Null

回答1:

Id use coalesce for this: http://msdn.microsoft.com/en-us/library/ms190349.aspx

update Person
set Car = coalesce(@Car, Car), HairColour = coalesce(@HairColour, HairColour)

回答2:

The following should work:

UPDATE Person
   SET Car = ISNULL(@Car, Car),
       HairColour = ISNULL(@HairColour, HairColour),
       ...

It uses the SQL Server ISNULL function, which returns

• the first value if it is non-null,
• or, otherwise, the second value (which, in this case, is the current value of the row).

回答3:

You can use the isnull function:

update Person
set
  Car = isnull(@Car, Car),
  HairColour = isnull(@HairColour, HairColour),
  FavDrink = isnull(@FavDrink, FavDrink),
  FavFood = isnull(@FavFood, FavFood)
where PersonalID = @PersonalID

回答4:

Set the column equal to itself with an isnull round it setting it to your parameter.

UPDATE
    YourTable
SET
   YourColumn = ISNULL(YourColumn, @yourParameter)
WHERE
    ID = @id