Computed column does not include one of the values - sql

Question

I'm trying to create a computed column in order to have a unique index on a nullable column that ignores NULL rows1. I've composed this test case:
SELECT TEST_ID, CODE, UNIQUE_CODE, CAST(UNIQUE_CODE AS VARBINARY(4000)) AS HEX
FROM (
SELECT TEST_ID, CODE,
ISNULL(CODE, CONVERT(VARCHAR, SPACE(10)) + CONVERT(VARCHAR, TEST_ID)) AS UNIQUE_CODE
FROM (
SELECT 1 AS TEST_ID, 'ABCDEFGHIJ' AS CODE
UNION ALL
SELECT 2, 'XYZ'
UNION ALL
SELECT 3, NULL
) TEST
) X;
It works as expected when CODE is not null but I only get a string of whitespaces when CODE is null (i.e., the trailing TEST_ID is missing):
TEST_ID | CODE | UNIQUE_CODE | HEX
--------+------------+-------------+-----------------------
1 | ABCDEFGHIJ | ABCDEFGHIJ | 0x4142434445464748494A
2 | XYZ | XYZ | 0x58595A
3 | NULL | | 0x20202020202020202020
The funny thing is that I already use this technique successfully in another table and I can't spot the difference:
CREATE TABLE SOME_OTHER_TABLE (
SOME_OTHER_TABLE_ID INT IDENTITY(1, 1) NOT NULL,
NOMBRE VARCHAR(50),
-- This works just fine:
NOMBRE_UNICO AS ISNULL(NOMBRE, CONVERT(VARCHAR, SPACE(50)) + CONVERT(VARCHAR, SOME_OTHER_TABLE_ID)),
CONSTRAINT SOME_OTHER_TABLE_PK PRIMARY KEY (SOME_OTHER_TABLE_ID)
);
What am I missing?
(1) This was a workaround for SQL Server 2005 that's no longer necessary in later versions thanks to filtered indexes.

Answer 1

There you go with " 3"
SELECT TEST_ID, CODE, UNIQUE_CODE, CAST(UNIQUE_CODE AS VARBINARY(4000)) AS HEX
FROM (
SELECT TEST_ID, CODE,
ISNULL(CODE, CONVERT(VARCHAR(20), SPACE(10)) + CONVERT(VARCHAR(20), TEST_ID)) AS UNIQUE_CODE
FROM (
SELECT 1 AS TEST_ID, cast('ABCDEFGHIJ' as varchar(20)) AS CODE
UNION ALL
SELECT 2, 'XYZ'
UNION ALL
SELECT 3, NULL
) TEST
) X;
'ABCDEFGHIJ' (first value in the union list) is exactly 10 characters and this column is a first argument of IsNull. So it takes 10 characters as size for IsNull result. Which is enough only for spaces. Replacing this constant with 'ABCDEFGHIJKLMNOPQR' would do the trick also.

Answer 2

It looks like SQL is trying to help out to define the column length in your inner query. By casting/converting to a specific size this fixes the problem. Once your UNIQUE_CODE field exceeds this value, the returned value is limited to the size of the column.
SELECT TEST_ID, CODE, UNIQUE_CODE, CAST(UNIQUE_CODE AS VARBINARY(4000)) AS HEX
FROM (
SELECT TEST_ID, CODE,
ISNULL(CODE, CONVERT(VARCHAR, SPACE(10)) + CONVERT(VARCHAR, TEST_ID)) AS UNIQUE_CODE
FROM (
SELECT 1 AS TEST_ID, CONVERT(VARCHAR(50), 'ABCDEFGHIJ') AS CODE
UNION ALL
SELECT 2, 'XYZ'
UNION ALL
SELECT 3, NULL
) TEST
) X;

Answer 3

You can run below piece of code to find out why ?
this fails :
declare #a char(20)
set #a=null
declare #b char(10)
set #b='aaaaaa'
select isnull(#a,convert(char(10),space(10)+#b))
This works:
declare #a char(20)
set #a=null
declare #b char(10)
set #b='aaaaaa'
select isnull(#a,convert(char(30),space(10)+#b))