message: Update design-sql.cpp
diff: -// Time: ctor: O(t), t is the number of tables
  -// insertRow: O(l), l is the number of columns
  +// Time: ctor: O(t * max_m), t is the number of tables, max_m is the max number of columns in all tables
  +// insertRow: O(m), m is the number of columns
  // deleteRow: O(1)
  -// selectCell: O(l)
  -// Space: O(n * l), n is the number of records
  +// selectCell: O(m)
  +// Space: O(d), d is the total size of data
  // hash table
  class SQL {
  