local skinID = exports.fs_sql:_QuerySingle("select * from users where pSkin = ? and username = ?", SkinTable, username)
or
local skinID = exports.fs_sql:_QuerySingle("select * from users where username = ? and pSkin = ?", username, SkinTable)
or more simple
local check = exports.fs_sql:_QuerySingle("select * from users where username = ? ", username)
if check then
local checkPass = check.password
if (checkPass == password) then
local pSkin = check.pSkin
...
simple