Retrieve ID along with existence status in a single SQL statement

Data and Databases
1

I have two tables, where table A has a primary key (A.Id) and table B contains a foreign key (EquipmentKey) linking back to A. My goal is to obtain results showing A.Id along with an indication of whether or not each ID exists in table B.

I am attempting to use the following SQL query, but I encounter an error stating that T.Id cannot be located. Is it possible to accomplish this in a single query without errors?

SELECT 
    A.Id, 
    ExistsInB = CASE 
                    WHEN EXISTS(SELECT 1 FROM B WHERE EquipmentKey = A.Id) THEN 1 
                    ELSE 0 
                END 
FROM A
2

Have you tried using a LEFT JOIN to achieve this? I’m curious if joining the tables directly might simplify the query. Maybe playin’ around with that could help locate the issue? How have others approached similar scenarios with primary and foreign keys?

3

You can also consider using a subquery approach to achieve this in one step. Here’s an alternative solution where a subquery checks the existence status of each ID from table A in table B:

SELECT 
    A.Id, 
    (SELECT CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END FROM B WHERE B.EquipmentKey = A.Id) AS ExistsInB 
FROM A;

This query will effectively give you the Ids from table A and a 1 or 0 depending on whether that Id exists in table B. Using a subquery can help streamline the logic when EXISTS might be causing issues.

4

You might try using COALESCE with a LEFT JOIN too. It can check if EquipmentKey doesn’t match any rows, then you’ll know the Id is not in table B. Sometimes going for basic joins does the trick better than EXISTS or subqueries. Worth a shot!

5

Hey folks! What do you all think about using an OUTER APPLY in this scenario? Maybe it could add flexibility by dynamically working on each row from table A and checking its presence in table B. Anyone tried this approach and found it useful in similar situations? Curious to hear your thoughts!

6

You might want to consider using a CTE (Common Table Expression) for this problem. This can help in making the query more readable while maintaining efficiency. Here’s a suggestion:

WITH ExistenceCheck AS (
    SELECT A.Id, CASE WHEN B.EquipmentKey IS NOT NULL THEN 1 ELSE 0 END AS ExistsInB
    FROM A 
    LEFT JOIN B ON B.EquipmentKey = A.Id
)
SELECT * FROM ExistenceCheck;

The CTE allows you to compute the existence status beforehand and fetch it cleanly, combining both tables’ information.