在 T-SQL 中返回 1 或 0 而不是 value 或 null

Question

In my SELECT statement, I join two tables. It's possible that there's no corresponding value in Table2 in which case, my SELECT statement should return NULL for that column. What I want to do is that instead of returning the actual value coming from Table2 I want to return a 1.

I thought I could use ISNULL for this but that function is designed to return the actual value if one exists.

This is what my SELECT statement looks like:

SELECT a.Id, ISNULL(b.PersonId, 0)
FROM Table1 AS a
   LEFT OUTER JOIN Table2 AS b ON a.Id = b.Id

Here the PersonId column is of UNIQUEIDENTIFIER type and I don't want to return either a Guid or a 0. Instead, I'd like to return a 1 or a 0.

How can I handle this scenario?

Answer 1

I think you're looking for a CASE EXPRESSION here.

SELECT a.Id, CASE WHEN b.PersonId IS NOT NULL THEN 1 ELSE 0 END
FROM Table1 AS a
LEFT OUTER JOIN Table2 AS b ON a.Id = b.Id

Answer 2

As well as @DaleK's excellent answer, another option is using EXISTS

SELECT
    a.Id,
    CASE WHEN EXISTS (SELECT 1
        FROM Table2 AS b
        WHERE a.Id = b.Id)
     THEN 1 ELSE 0 END
FROM Table1 AS a

Note that the semantic is different here, as it is a semi-join: the rows are not duplicated if there are multiple matches in Table2