Calculate percent of whole

I have rows of client data with 2 fields `weeks` and `recommended_weeks`.  I would simply like to know what percent of rows have `weeks` = `recommended_weeks`.  For some reason I can't seem to make this work.  I tried the following but it's showing me ALL rows, rather than an average.

sum(case when `weeks` = `recommended_weeks` then 1 else 0 end) / count(`id_client`)

It seems like it should be simple enough to separate the data into 2 populations (one where `weeks` = `recommended weeks` and one where `weeks` != `recommended_weeks` but for whatever reason, I cannot seem to get a single number out of the data.