Using expression string builder to ignore NULL values in QGIS?

Question

Using QGIS 2.2, I would like to create a rule-based filter whereby you add 2 columns together and you get a certain colour as a result. However, some fields in Column_1 may be NULL and those exact same fields may have values in Column_2 and vice-versa.

Is there an expression to add the 2 columns whilst ignoring the NULL values or atleast treating them as zero?

The following is the logic I am trying to achieve:

1 + 2 = 3

1 + NULL = 1

NULL + 2 = 2

Additional info: The data from the columns are JOINED from other layers, I do not want to create any new columns as I am wanting the colours of layers to dynamically change when the user adds or removes the JOINED layers.

Answer 1

Found a link which contained solutions to adding 2 layers or more:

https://stackoverflow.com/questions/1769648/addition-with-null-values

Basic gist is when using 2 columns:

coalesce("Column_1"+"Column_2","Column_1","Column_2")

Using 3 columns or more, I used:

coalesce("Column_1",0.00) + coalesce("Column_2",0.00) + coalesce("Column_3",0.00) + .......