Unioning two tables with different number of columns

Question

I have two tables (Table A and Table B).

These have different number of columns - Say Table A has more columns.

How can I union these two table and get null

Answer 1

Normally you need to have the same number of columns when you're using set based operators so Kangkan's answer is correct.

SAS SQL has specific operator to handle that scenario:

SAS(R) 9.3 SQL Procedure User's Guide

CORRESPONDING (CORR) Keyword

The CORRESPONDING keyword is used only when a set operator is specified. CORR causes PROC SQL to match the columns in table expressions by name and not by ordinal position. Columns that do not match by name are excluded from the result table, except for the OUTER UNION operator.

SELECT * FROM tabA
OUTER UNION CORR
SELECT * FROM tabB;

For:

+---+---+
| a | b |
+---+---+
| 1 | X |
| 2 | Y |
+---+---+

OUTER UNION CORR

+---+---+
| b | d |
+---+---+
| U | 1 |
+---+---+

<=>

+----+----+---+
| a  | b  | d |
+----+----+---+
|  1 | X  |   |
|  2 | Y  |   |
|    | U  | 1 |
+----+----+---+

---

U-SQL supports similar concept:

OUTER UNION BY NAME ON (*)

OUTER

requires the BY NAME clause and the ON list. As opposed to the other set expressions, the output schema of the OUTER UNION includes both the matching columns and the non-matching columns from both sides. This creates a situation where each row coming from one of the sides has "missing columns" that are present only on the other side. For such columns, default values are supplied for the "missing cells". The default values are null for nullable types and the .Net default value for the non-nullable types (e.g., 0 for int).

BY NAME

is required when used with OUTER. The clause indicates that the union is matching up values not based on position but by name of the columns. If the BY NAME clause is not specified, the matching is done positionally.

If the ON clause includes the “*” symbol (it may be specified as the last or the only member of the list), then extra name matches beyond those in the ON clause are allowed, and the result’s columns include all matching columns in the order they are present in the left argument.

And code:

@result =    
    SELECT * FROM @left
    OUTER UNION BY NAME ON (*) 
    SELECT * FROM @right;

---

EDIT:

The concept of outer union is supported by KQL:

kind:

inner - The result has the subset of columns that are common to all of the input tables.

outer - The result has all the columns that occur in any of the inputs. Cells that were not defined by an input row are set to null.

Example:

let t1 = datatable(col1:long, col2:string)  
[1, "a",  
2, "b",
3, "c"];
let t2 = datatable(col3:long)
[1,3];
t1 | union kind=outer t2;

Output:

+------+------+------+
| col1 | col2 | col3 |
+------+------+------+
|    1 | a    |      |
|    2 | b    |      |
|    3 | c    |      |
|      |      |    1 |
|      |      |    3 |
+------+------+------+

demo