COALESCE function won't return CHAR(1) - sql

Question

Using COALESCE function but getting the following error:
Conversion failed when converting the varchar value 'X' to data type int.
I have to join two tables on two conditions. I want that if the second condition doesn't hold but there is a blank cell (not null but blank '') in Table 1 then to join to that row. If the second condition doesn't hold then to return a zero.
Join Table 1 and Table 2 - return Table 2 and column 3 from Table 1.
Table 1
(A, 1, X),
(A, 2, Y),
(A, 3, Z),
(A, , X),
(B, 1, X),
(B, 2, Z),
(B, 3, Y),
Table 2
(A, 1),
(A, 2),
(A, 3),
(A, 5),
(B, 1),
(B, 2),
(B, 3),
(B, 5)
I want to get a return of
(A, 1, X),
(A, 2, Y),
(A, 3, Z),
(A, 5, X),
(B, 1, X),
(B, 2, Z),
(B, 3, Y),
(B, 5, NULL)
Code:
DECLARE #table1 TABLE (letter1 CHAR(1), num1 INT, letter2 CHAR(1))
DECLARE #table2 TABLE (letter1 CHAR(1), num1 INT)
INSERT INTO #table1 VALUES
('A', 1, 'X'),
('A', 2, 'Y'),
('A', 3, 'Z'),
('A', null, 'X'),
('B', 1, 'X'),
('B', 2, 'Y'),
('B', 3, 'Z')
INSERT INTO #table2 VALUES
('A', 1),
('A', 2),
('A', 3),
('A', 5),
('B', 1),
('B', 2),
('B', 3),
('B', 5)
SELECT t2.*,
COALESCE(
(SELECT TOP 1 letter2 FROM #table1 WHERE letter1 = t2.letter1 AND num1 = t2.num1),
(SELECT TOP 1 letter2 FROM #table1 WHERE letter1 = t2.letter1 AND num1 IS NULL),
0
) AS missing_letter
FROM #table2 t2

Answer 1

Perhaps you need :
select t1.*, t2.*
from table1 t1 outer apply
( select top (1) t2.*
from table2 t2
where t1.col1 = t.col1 and t1.col2 in ('', t2.col2)
order by t2.col2 desc
) t2;

Answer 2

If I understand correctly, this has less to do with coalesce() and more to do with the joins:
select t2.*, coalesce(t1.letter2, t1def.letter2) as letter2
from table2 t2 left join
table1 t1
on t2.letter1 = t1.letter1 and t2.num1 = t1.num1 left join
table1 t1def
on t2.letter1 = t1def.letter1 and t1def.num1 is null;

Answer 3

The problem here is your datatype. COALESCE is short hand for a CASE expression. For example. COALESCE('a',1,'c') would be short hand for:
CASE WHEN 'a' IS NOT NULL THEN 'a'
WHEN 1 IS NOT NULL THEN 1
ELSE 'c'
END
The Documentation (COALESCE (Transact-SQL) describes this as well:
The COALESCE expression is a syntactic shortcut for the CASE
expression. That is, the code COALESCE(expression1,...n) is
rewritten by the query optimizer as the following CASE expression:
CASE
WHEN (expression1 IS NOT NULL) THEN expression1
WHEN (expression2 IS NOT NULL) THEN expression2
...
ELSE expressionN
END
A CASE expression follows Data type precedence, and int has a higher datatype precedence than varchar; thus everything will implicit cast to an int. This is why both the COALESCE and CASE expression will fail, because neither 'a' or 'c' can be converted to an int.
You'll need to therefore explicitly CONVERT your int to a varchar:
COALESCE('a',CONVERT(char(1),1),'c')
The documentation (cited above), however, also goes to state:
This means that the input values (expression1, expression2,
expressionN, etc.) are evaluated multiple times. Also, in compliance
with the SQL standard, a value expression that contains a subquery is
considered non-deterministic and the subquery is evaluated twice. In
either case, different results can be returned between the first
evaluation and subsequent evaluations.
For example, when the code COALESCE((subquery), 1) is executed, the
subquery is evaluated twice. As a result, you can get different
results depending on the isolation level of the query. For example,
the code can return NULL under the READ COMMITTED isolation level in a
multi-user environment. To ensure stable results are returned, use the
SNAPSHOT ISOLATION isolation level, or replace COALESCE with the
ISNULL function.
Considering you are using a subquery, a (nested) ISNULL might be the better choice here.
It's worth noting, as people seem to confuse them as they are functionally similar, but COALESCE and ISNULL do not behave the same. COALESCE uses Data Type precedence, however, ISNULL implicitly casts the second value to whatever the datatype of the first paramter is. Thus ISNULL('a',1) works fine, but COALESCE('a',1) does not.

Answer 4

Just change the zero to a null. You can't mix datatypes in a coalesce:
SELECT t2.*,
COALESCE(
(SELECT TOP 1 letter2 FROM #table1 WHERE letter1 = t2.letter1 AND num1 = t2.num1),
(SELECT TOP 1 letter2 FROM #table1 WHERE letter1 = t2.letter1 AND num1 IS NULL),
null
) AS missing_letter
FROM #table2 t2

Answer 5

The query works if the 0 in the COALESCE is replaced by '0'.
That way the COALESCE doesn't contain mixed data types.
SELECT t2.*,
COALESCE(
(SELECT TOP 1 letter2 FROM #table1 t1 WHERE t1.letter1 = t2.letter1 AND t1.num1 = t2.num1),
(SELECT TOP 1 letter2 FROM #table1 t1 WHERE t1.letter1 = t2.letter1 AND t1.num1 IS NULL),
'0'
) AS missing_letter
FROM #table2 t2
ORDER BY t2.letter1, t2.num1;
And you can avoid having to retrieve data from table1 twice.
By using an OUTER APPLY.
Since the expected results has a NULL for ('B',5), the COALESCE isn't even needed this way.
SELECT t2.letter1, t2.num1, t1.letter2 AS missing_letter
FROM #table2 AS t2
OUTER APPLY (
select top 1 t.letter2
from #table1 AS t
where t.letter1 = t2.letter1
and (t.num1 is null or t.num1 = t2.num1)
order by t.num1 desc
) AS t1
ORDER BY t2.letter1, t2.num1;
Result:
letter1 num1 missing_letter
------- ---- --------------
A 1 X
A 2 Y
A 3 Z
A 5 X
B 1 X
B 2 Y
B 3 Z
B 5 NULL