Breaking changes in DataFrames master

[amellnik]

As you probably know, there's lots of breaking changes in the DF master (see JuliaData/DataFrames.jl#1092). I'm opening this issue to track compatibility with MySQL master. There's tons of stuff that will need to change, but a typical use case is:

using DataFrames, MySQL #Both at master
conn = mysql_connect("my.server", "username", "pass", schema")
q = """
    Select * from Model_Data
    """
mysql_execute(conn, q)

LoadError: MethodError: no method matching mysql_interpret_field(::String, ::Type{Nullable{AbstractString}})
Closest candidates are:
  mysql_interpret_field(::AbstractString, !Matched::Type{UInt8}) at C:\Users\amellnik\.julia\v0.5\MySQL\src\results.jl:95
  mysql_interpret_field{T<:Number}(::AbstractString, !Matched::Type{T<:Number}) at C:\Users\amellnik\.julia\v0.5\MySQL\src\results.jl:97
  mysql_interpret_field{T<:AbstractString}(::AbstractString, !Matched::Type{T<:AbstractString}) at C:\Users\amellnik\.julia\v0.5\MySQL\src\results.jl:100
  ...
while loading In[4], in expression starting on line 4

 in populate_row!(::DataFrames.DataFrame, ::Int64, ::Ptr{Ptr{Int8}}, ::Int64) at C:\Users\amellnik\.julia\v0.5\MySQL\src\results.jl:244
 in mysql_result_to_dataframe(::MySQL.MySQLResult) at C:\Users\amellnik\.julia\v0.5\MySQL\src\results.jl:257
 in #mysql_execute#5(::Int64, ::Function, ::MySQL.MySQLHandle, ::String) at C:\Users\amellnik\.julia\v0.5\MySQL\src\handy.jl:160
 in mysql_execute(::MySQL.MySQLHandle, ::String) at C:\Users\amellnik\.julia\v0.5\MySQL\src\handy.jl:144

[nkottary]

The type is wrapped in a Nullable. It needs to be extracted and the null case handled.

[amellnik]

Closing -- DataFrames is sticking with the existing DataArrays API and will not be migrating to Nullables as per JuliaData/DataFrames.jl#1154.