My problem statement is bit unique.
I am trying to generate a sql string recursively that would contain UNION and JOIN using python.

Below are the conditions:

1. Number of columns would be determined from a variable. If v = 2 the number of columns would be 3, with 1st one would always be fixed. If v = 3 the number of columns would be 4 with 1st one would always be fixed.

2. In each UNION block the number of join : number of columns – 1. And this would be increasing in numbers in each UNION block till the time the loop reaches v-1.

3. Number of UNION : v-1

4. Number of columns that would take NULL value (i.e. null as) in each UNION block : v-1. And this would keep on decreasing like v-2,v-3 in each subsequent UNION block till it reaches 0

With the above two conditions let me show the desired output.

v = 2
tables = ['t1','t2'] #<---number of tables would be fixed

select 
   a.col_1    #<---This would be fixed
  ,a0.col_2   
  ,null as col_3
from t1 a 
  left join t2 a0
  on a.col_1 = a0.col_1 and a0.level = 1
union all
select 
   a.col_1
  ,a0.col_2
  ,a1.col_3   #<----the null is being replaced by actual column col_3
from table1 a
  left join t2 a0 
  on a.col_1=a0.col_1 and a0.level = 1
left join t2 a1   #<----please observe that same table t1 is being aliased differently
  on a.col_1 = a1.col_1  and a1.level = 2

Now what I have worked:

v=2
sql_b = 'select a.col_1'
tables = ['t1','t2']
join ='LEFT Join t2'
col = ''

for i in range(v):
   col = col + 'NULL AS col_{0}'.format(i+1)
   sql+=col + ' ' + 'FROM final_h c ' + join_s + ' a{0}'.format(i)+ 
   ' ON c.col_+1 = a{}.col_1 AND a{}.level = 1 '.format(i,i)
print(sql)

But this approach is not working.

1. Comma is not coming properly

2. Not able to include UNION part

Any help would be appreciated.