Selecting Distinct Values from Each Column - mysql

Question

Is there a way to get the Distinct values from each column in a MYSQL database without having to do multple SELECT DISTINCT statements for each column?
Right now in my rails controller I'm using .pluck() to run:
#first = User.distinct.pluck(:first_name)
#last = User.distinct.pluck(:last_name)
#city = User.distinct.pluck(:city)
#state = User.distinct.pluck(:state)
#age = User.distinct.pluck(:age)
#info = {
'first' => #first,
'last' => #last,
'city' => #city,
'state' => #state,
'age' => #age
}
respond_with #info
Which creates an object with my two unique arrays in it, however it takes about 7.7 seconds (my table has 3.2 million fully-populated rows) and runs two separate SQL queries.
I tried this method, but this is giving me an array of each unique COMBINATION:
#info = User.distinct.select(:first_name, :last_name, :city, :state, :age)
respond_with #info

Answer 1

Not sure how you want your final data to be output, but if you want to avoid multiple SQL statements, you can do something like this:
SELECT
ARRAY(SELECT DISTINCT(col1) FROM MyTable) AS col1_values,
ARRAY(SELECT DISTINCT(col2) FROM MyTable) AS col2_values
This will give you a single row with all the DISTINCT values for each "colX" you specify. Just add on whatever additional columns you want to include. The DISTINCT values in each column will be returned as an array.
Performance will still be stinky, since I don't think you can avoid doing multiple distinct DISTINCT operations.
Let me know if that's what you're looking for.

Answer 2

You could do this:
#info = User.distinct.select(:first_name, :last_name, :city, :state, :age)
#first, #last, #city, #state, #age = [], [], [], [], []
#info.map{ |i| #first<<i[0]; #last<<i[1]; #city<<i[2]; #state<<i[3], #age<<i[4] }
#info = { 'first' => #first,
'last' => #last,
'city' => #city,
'state' => #state,
'age' => #age }
respond_with #info
Please try benchmarking before implementing this in production.