Problem with union casting integer to ceiling(decimal)
Looks like a bug to me and I can confirm this puzzling behaviour in:

10.2.14-MariaDB


If possible you can cast the integer value to a double:

SELECT cast(20 as double) UNION SELECT null UNION SELECT 2.2;


or make sure you have the double value first:

SELECT 2.2 UNION SELECT null UNION SELECT 22;


Further observations after reading the comments in @Evan Carroll's answer

select 20 union select null union select 2;
+------+
| 20   |
+------+
|   20 |
| NULL |
|    2 |
+------+


Ok, using int values does not seem to produce the error.

select 20 union select null union select 9.0;
+------+
| 20   |
+------+
| 9.9  |
| NULL |
| 9.0  |
+------+


ERROR: Seems like output is decimal(2,1)

create table tmp as select * from (select 20 as x 
                                   union 
                                   select null 
                                   union 
                                   select 9.0) as t

describe tmp;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| x     | decimal(2,1) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+


The error is not isolated to the command line interface, it exists for python2-mysql-1.3.12-1.fc27.x86_64 as well:

&gt;&gt;&gt; import MySQLdb
&gt;&gt;&gt; db = MySQLdb.connect(host="localhost", user="*****", passwd="*****", db="test") 
&gt;&gt;&gt; cur = db.cursor()
&gt;&gt;&gt; cur.execute("SELECT 20 union select null union select 2.2")
3L
&gt;&gt;&gt; for row in cur.fetchall() :
...     print row
... 
(Decimal('9.9'),)
(None,)
(Decimal('2.2'),)


Oddly enough the error disappears if null is moved first or last:

select null union select 20 union select 9.0;
select 20 union select 9.0 union select null;

+------+
| NULL |
+------+
| NULL |
| 20.0 |
| 9.0  |
+------+


If null is placed first, resulting type is decimal(20,1). If null is placed last resulting type is decimal(3,1)

The error also disappears if another leg is added to the union:

select 20 union select 6 union select null union select 9.0;
+------+
| 20   |
+------+
| 20.0 |
| 6.0  |
| NULL |
| 9.0  |
+------+


resulting type decimal(20,1)

adding another null in the middle preserves the error:

select 20 union select null union select null union select 9.0;
+------+
| 20   |
+------+
| 9.9  |
| NULL |
| 9.0  |
+------+


But adding a null at the beginning fixes it:

select null union select 20 union select null union select null union select 9.0;
+------+
| NULL |
+------+
| NULL |
| 20.0 |
| 9.0  |
+------+


As expected casting first value to decimal(3,1) works.

Finally, explicitly casting to decimal(2,1) produces the same error but with a warning:

select cast(20 as decimal(2,1));
+--------------------------+
| cast(20 as decimal(2,1)) |
+--------------------------+
| 9.9                      |
+--------------------------+
1 row in set, 1 warning (0.00 sec)