SQL之理解：join是把两个table的column合到一个，union是把两个table的row合到一个

join示例：

select the_column1, the_column2
from
  (
    select the_column2 from the_table2
  )a
  join
  (
    select the_column1 from the_table1
  )b
  on 1=1

union示例：

select the_column1, the_column2
from
  (
    select the_column1, the_column2 from the_table1
    union all
    select the_column11 as column1, the_column22 as column2 from the_table2
  )