Let's go step by step.

CREATE TABLE #TEST(A varchar,B varchar(30)); ---> Field named A is of size 1 with type Varchar.

ISNULL(A,B) ---> means:evaluate the field A, (the first parameter), if has something other than a NULL then return it to me otherwise return me the value converted to Varchar size 1 (As A is Varchar (1)) from varchar size 30 in field B for current record.

So if you run

SELECT A, B, ISNULL(A,B),LEN(ISNULL(A,B)) FROM #Test

A B

---- ------------------------------ ---- -----------

A NULL A 1

NULL Some text S 1

B NULL B 1

NULL Some more text S 1

so if you now run

SELECT COUNT(*),LEN(ISNULL(A,B))

FROM #TEST

GROUP BY LEN(ISNULL(A,B));

you'll see

----------- -----------

4 1

And

SELECT A, B, ISNULL(B,A),LEN(ISNULL(B,A)) FROM #Test

A B

---- ------------------------------ ------------------------------ -----------

A NULL A 1

NULL Some text Some text 9

B NULL B 1

NULL Some more text Some more text 14

and run

SELECT COUNT(*),LEN(ISNULL(B,A))

FROM #TEST

GROUP BY LEN(ISNULL(B,A));

----------- -----------

2 1

1 9

1 14

Now let's talk about COALESCE(A,B)

COALESCE returns the first non-null expression among its arguments,

If you run

SELECT LEN(COALESCE(A,B)),COALESCE(A,B)

FROM #TEST

then

----------- ------------------------------

1 A

9 Some text

1 B

14 Some more text

so

SELECT COUNT(*),LEN(COALESCE(A,B))

FROM #TEST

GROUP BY LEN(COALESCE(A,B));

----------- -----------

2 1

1 9

1 14

.

I hope this has been useful.

Abdul Rehman